20 changed files with 4633 additions and 5 deletions
--- a/compose.yaml
+++ b/compose.yaml
@ -20,7 +20,7 @@ services:
        max-size: "20m"
        max-file: "10"
    ports:
-      - "3000:3000"
+      - "3221:3000"
    volumes:
      - ./src:/app/src
      - ./scripts:/app/scripts
@ -64,7 +64,7 @@ services:
      api:
        condition: service_healthy
    ports:
-      - "3001:3001"
+      - "3220:3001"
    volumes:
      - ./src:/app/src
      - ./next.config.mjs:/app/next.config.mjs:ro
@ -80,3 +80,9 @@ services:
      timeout: 3s
      retries: 12
      start_period: 20s
+
+networks:
+  default:
+    ipam:
+      config:
+        - subnet: 172.16.61.0/24
--- a/docs/cable-sizing-module.md
+++ b/docs/cable-sizing-module.md
@ -0,0 +1,243 @@
+# Cable Sizing Module (Proposal)
+
+**Status: proposal, not yet reviewed or merged by the project owner.** This
+document and the accompanying `src/cable-sizing/`, `src/db/schema/cable-sizing-calculations.ts`,
+`src/server/{controllers,routes}/cable-sizing.*` and `src/frontend/components/cable-sizing-*`
+files were written by a third party (see git history/authorship) against the
+project's own stated direction in `docs/spec/06-future-sizing-and-calculations.md`
+("The app should later support rule-based protection and cable sizing") and
+`AGENTS.md` ("... and later electrical sizing logic"). Nothing here has been
+pushed to the project's own repository; it exists as a local branch for
+review, testing and discussion.
+
+## Why a separate module instead of extending core domain code
+
+`AGENTS.md` is explicit that critical multi-write commands, the revision/undo
+system and the Circuit-First domain model are the supported architecture, and
+that changes should be small and reviewable. A cable-sizing calculation is
+**not** a project mutation - it doesn't need `expectedRevision`, doesn't
+belong in the undo/redo stack, and shouldn't grow the `circuit.update`
+command's `switch` statement or the DTOs it doesn't already have.
+
+So this module is built exactly like the existing `src/external-model/`
+adapter (the Revit/CSV import foundation): a one-way dependency boundary.
+
+```
+src/cable-sizing/domain/     <- pure functions and types, zero imports from
+                                 db/, server/ or frontend/
+src/db/schema/                <- one new, fully additive table
+src/db/repositories/          <- one plain repository (no revision semantics)
+src/server/{controllers,routes}/cable-sizing.*  <- one new, isolated route
+                                 group, mounted with a single app.use() line
+src/frontend/components/cable-sizing-*          <- one new modal + one small
+                                 API client, following the existing
+                                 FormModal / CircuitProtectionModal pattern
+```
+
+Nothing outside these files imports from them except the two required
+one-line hooks described below. If this module is rejected or needs to move
+out again, removing it is a matter of deleting these files, the two
+one-line hooks, and running a `DROP TABLE` migration - it never touches
+`circuits`, `project_revisions` or any command/history table.
+
+## The two required hooks into existing code
+
+1. `src/server/index.ts`: one import + one `app.use("/api/cable-sizing", cableSizingRouter)`
+   line, next to the existing `app.use("/api/projects", ...)` etc.
+2. `src/frontend/components/circuit-tree-editor.tsx`: one new `useState` for
+   the open modal, one new trigger condition (mirroring the existing
+   `isProtectionTrigger` / `protectionEditorCircuit` pattern almost exactly),
+   and one new conditionally-rendered `<CableSizingModal />` alongside the
+   existing `<CircuitProtectionModal />`. See the diff for the exact lines.
+
+No changes to `circuit-project-command.model.ts`, `project-command.service.ts`,
+any Zod command schema, any migration for `circuits`/`project_revisions`, or
+any existing test.
+
+## Coupling to the grid UI (the one fragile point)
+
+Everything described above - the calculation, the audit table, applying a
+result via `circuit.update` - depends only on stable, explicitly documented
+domain fields (`cableType`/`cableCrossSection`/`cableLength`/
+`circuitTotalPower`/`voltage`/`protectionDevice.ratedCurrentA`), the same
+ones `AGENTS.md` already treats as protected ("Protection and cable data
+belong to the circuit"). None of that breaks if the grid UI changes.
+
+The one place that *is* coupled to a UI implementation detail: the click
+trigger in `circuit-tree-editor.tsx` matches grid **column keys**
+(`cableSummary` and `cableCrossSection`, defined in
+`circuit-grid-model.ts`) to decide where to show the calculator icon.
+Column visibility and order in this app are a per-browser user preference,
+not fixed - so this is checked against both known cable-related columns to
+reduce (not eliminate) the chance a user's personal column layout hides the
+trigger entirely. If a future column rename removes both keys, the failure
+mode is purely cosmetic: the calculator icon/click stops appearing on that
+cell, nothing crashes, no data is affected, and the rest of the app
+(including manual cable-field editing) is completely unaffected - the
+module's actual logic and persistence never depend on this column key.
+
+## Feature overview
+
+- **Calculate a recommendation**: laying method, insulation, conductor
+  material, ambient temperature, grouping, cos phi and max voltage drop -
+  same VDE 0298-4 reference-method model as the sibling Kabelliste tool
+  (see Verification status below).
+- **Breaker-aware sizing**: if the circuit already has a protection device
+  (`circuit.protectionDevice`), the cross-section is selected against
+  `designCurrentA = max(operatingCurrentA, protectionDevice.ratedCurrentA)`,
+  not the load current alone - the standard `In <= Iz` rule (a breaker only
+  trips at its own rated current, so a cable sized for the actual load
+  alone could overheat under sustained current below that threshold). If no
+  cross-section can cover an oversized breaker, that is reported as the
+  limiting factor by name, not a generic capacity error.
+- **Maximum length for the voltage-drop limit**: each cross-section option
+  also reports the longest single run that still meets the requested
+  `maxVoltageDropPercent` at the given load - the inverse of the
+  voltage-drop check, shown for the recommended cross-section.
+- **Practical minimum for socket circuits**: `single_phase`-category
+  circuits are raised to at least 2.5 mm² if the calculation alone would
+  recommend less - see the dedicated note under Verification status. Never
+  lowers a calculation that already needs more.
+- **Manual entry, always available**: the modal has a plain text Kabeltyp/
+  Querschnitt field. Running a calculation pre-fills it as a suggestion,
+  but the field is the single source of truth for what gets applied, and
+  stays freely editable - satisfying
+  `docs/spec/06-future-sizing-and-calculations.md`'s explicit requirement
+  that "users must remain able to manually override suggestions" without
+  requiring a calculation to have been run first. A non-blocking warning
+  appears if the typed value is not a standard cross-section, is smaller
+  than the last calculation, or is below the practical minimum above -
+  informational only, never forced, matching the same spec doc's "shown as
+  a warning or status indicator, not as an automatic forced change."
+
+## How a suggestion or manual entry is applied
+
+The module never writes to `circuits` directly. The modal's "Übernehmen"
+button calls the **existing, unmodified** frontend helper with whatever is
+currently in the Kabeltyp/Querschnitt fields, calculated or hand-typed:
+
+```ts
+updateCircuitById(projectId, expectedRevision, circuitId, {
+  cableCrossSection: "4 mm²",
+  cableType: "NYM-J 3x2.5",
+  cableLength: 30,
+});
+```
+
+which is the same `circuit.update` command the grid already uses for manual
+cell edits. This means:
+
+- optimistic concurrency (`expectedRevision`) is respected automatically
+- the change appears in the project's revision history and is undoable/
+  redoable exactly like a manual edit
+- nothing is ever written automatically - calculating only fills the modal's
+  own fields, applying is always an explicit, separate click
+
+## API contract
+
+`POST /api/cable-sizing/calculate`
+
+```jsonc
+{
+  "phase": 1,                    // 1 | 3
+  "mode": "power",                // "power" | "current"
+  "powerKw": 4.2,                  // circuit.circuitTotalPower, when mode="power"
+  "cosPhi": 1,
+  "voltage": 230,                  // circuit.voltage (already project-derived, read-only)
+  "lengthM": 23.5,                 // circuit.cableLength
+  "layingMethod": "C",             // DIN VDE 0298-4 reference method, A1|A2|B1|B2|C|D1|D2|E|F|G
+  "conductorMaterial": "copper",   // "copper" | "aluminum"
+  "insulation": "pvc",             // "pvc" | "xlpe"
+  "ambientTemperatureC": 30,
+  "groupingCircuits": 1,
+  "maxVoltageDropPercent": 3,
+  "harmonicNeutralLoad": "none",   // "none" | "15to33Percent" | "over33Percent", three-phase only
+  "existingProtectionRatedCurrentA": 16,  // optional, from circuit.protectionDevice
+  "circuitCategory": "single_phase",      // optional, from the circuit's section.category -
+                                            // enables the practical-minimum convention below
+  "context": { "projectId": "...", "circuitId": "...", "equipmentIdentifier": "-1F1.1" }
+}
+```
+
+Response: `{ calculationId, result: CableSizingResult, alerts: CableSizingAlert[] }`
+(see `src/cable-sizing/domain/cable-sizing-calculation.ts` for the exact
+shape). `GET /api/cable-sizing/laying-methods` and `.../insulation-materials`
+expose the pick-list metadata (including which combinations are verified) so
+a future non-modal UI could build its own form without hard-coding the enum.
+
+## Verification status - important
+
+The reference current-carrying-capacity tables are ported from a sibling
+project's Kabelliste module, which was itself only verified against ~8
+public sources for **six** of the ten DIN VDE 0298-4 reference laying
+methods (A1, B2, C, E, D1, D2) and **PVC insulation only**. The other four
+methods (A2, B1, F, G) and XLPE/VPE insulation could not be verified without
+contradiction across sources during that earlier work, so
+`calculateCableSizing` deliberately returns `dataVerified: false` and no
+numeric recommendation for those combinations, rather than a guessed value.
+The UI surfaces this as a plain critical alert. Anyone with access to the
+actual norm text can extend `LAYING_METHOD_VERIFIED` and
+`CURRENT_CAPACITY_A` in `src/cable-sizing/domain/cable-sizing-calculation.ts`
+once the missing tables are confirmed.
+
+The protection-coordination check (`existingProtectionRatedCurrentA` vs. the
+recommended cross-section's corrected capacity) is a **simplified** `In <=
+Iz` check only - it is not a full IEC 60364-4-43 overload (`I2 <= 1.45 x
+Iz`) or short-circuit withstand check.
+
+`PRACTICAL_MINIMUM_CROSS_SECTION_MM2` (2.5 mm² for `single_phase`) is
+**not** a verified norm value at all - it is a named planning convention,
+sourced directly from this project's own
+`docs/spec/06-future-sizing-and-calculations.md` ("Standard Single-Phase
+Circuits ... usually use ... cable cross-section: 2.5 mm²"), applied as a
+floor on top of the calculated recommendation. It intentionally only
+covers the one category and one convention that document already states;
+it is not a general substitute for norm-compliant calculation.
+
+## Relationship to the project's own future-sizing spec
+
+`docs/spec/06-future-sizing-and-calculations.md` separately describes simple
+category-based defaults (e.g. lighting circuits -> 10 A / 1.5 mm²) as
+"common planning defaults, not a replacement for full norm-compliant
+calculation". This module is the latter: an on-demand, norm-referenced
+calculation for one circuit at a time, not a bulk default-filling tool. The
+two are complementary and could later be wired together (e.g. the category
+defaults pre-fill this module's laying-method/insulation fields), but that
+integration is out of scope here.
+
+The spec's `isPublicBuilding` halogen-free rule and the `Control Requirement`
+(DALI/KNX/core-count) field are **not** implemented by this module - both
+concern cable *type* selection, not cross-section sizing, and the latter
+isn't in the current schema yet. They're natural follow-ups once this module
+is reviewed.
+
+## Maintenance plan
+
+The `src/cable-sizing/domain/` folder has zero imports from the rest of the
+app by design (see the dependency-direction note above), so it can be
+extracted into an independently versioned/published package later (e.g. a
+private npm package or a git subtree) without touching anything outside the
+five files/folders listed above - "separately maintained but part of the
+app for now", per the intent of this proposal.
+
+## Local testing
+
+Deployed and manually exercised end-to-end on an internal test host at
+`http://192.168.0.133:3220` (own docker deployment, own subnet/ports to
+avoid clashing with ~50 other containers on that host - see the two
+commits marked "Local-only" in this branch, which are not part of this
+proposal and should not be carried over if it is ever proposed upstream).
+Exercised against seeded real project/circuit data created through the
+actual command API (not direct DB writes), including a deliberately long
+(85 m) circuit to trigger the voltage-drop-critical path.
+
+`npm test`, `npm run build:api`, `npm run build:web` and
+`npm run typecheck:scripts` all pass with this module included at every
+commit in this branch.
+
+This branch (`feature/cable-sizing-module`) is pushed to a mirror of this
+repository under this homelab's own Forgejo instance, not to
+`git.jappel.io` - no write access to the upstream repository was available
+or used. See the branch's commit history for the incremental history of
+this module, including fixes made during manual testing (trigger column,
+icon rendering, breaker-aware sizing).
--- a/next.config.mjs
+++ b/next.config.mjs
@ -5,7 +5,8 @@ const nextConfig = {
  allowedDevOrigins: [
    "192.168.3.13",
    "docker01.int.jappel.io",
-    "lb.jappel.io"
+    "lb.jappel.io",
+    "192.168.0.133"
  ],

  typescript: {
--- a/src/app/globals.css
+++ b/src/app/globals.css
@ -1209,6 +1209,48 @@ a.kpi:hover {
  box-shadow: inset 0 0 0 1px var(--color-signal);
 }

+.tree-grid .cell-cable-sizing-trigger {
+  color: var(--color-primary);
+  cursor: pointer;
+  font-weight: 600;
+  transition:
+    background-color 0.12s ease,
+    box-shadow 0.12s ease;
+}
+
+:root[data-bs-theme="dark"] .tree-grid .cell-cable-sizing-trigger {
+  color: var(--color-accent-pale);
+}
+
+.tree-grid .cell-cable-sizing-trigger:hover {
+  background: rgba(63, 130, 166, 0.15);
+  box-shadow: inset 0 0 0 1px var(--color-primary);
+}
+
+.tree-grid .cell-cable-sizing-trigger {
+  position: relative;
+  padding-left: 1.4em;
+}
+
+.tree-grid .cell-cable-sizing-trigger::before {
+  content: "";
+  position: absolute;
+  left: 0.2em;
+  top: 50%;
+  transform: translateY(-50%);
+  width: 0.9em;
+  height: 0.9em;
+  opacity: 0.85;
+  background-repeat: no-repeat;
+  background-position: center;
+  background-size: contain;
+  background-image: url("data:image/svg+xml;utf8,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 24 24' fill='none' stroke='%233f82a6' stroke-width='2' stroke-linecap='round' stroke-linejoin='round'%3E%3Crect x='4' y='2' width='16' height='20' rx='2'/%3E%3Cline x1='8' y1='6' x2='16' y2='6'/%3E%3Cline x1='8' y1='10' x2='8' y2='10.01'/%3E%3Cline x1='12' y1='10' x2='12' y2='10.01'/%3E%3Cline x1='16' y1='10' x2='16' y2='10.01'/%3E%3Cline x1='8' y1='14' x2='8' y2='14.01'/%3E%3Cline x1='12' y1='14' x2='12' y2='14.01'/%3E%3Cline x1='16' y1='14' x2='16' y2='14.01'/%3E%3Cline x1='8' y1='18' x2='8' y2='18.01'/%3E%3Cline x1='12' y1='18' x2='12' y2='18.01'/%3E%3Cline x1='16' y1='18' x2='16' y2='18.01'/%3E%3C/svg%3E");
+}
+
+:root[data-bs-theme="dark"] .tree-grid .cell-cable-sizing-trigger::before {
+  background-image: url("data:image/svg+xml;utf8,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 24 24' fill='none' stroke='%23a9d2e6' stroke-width='2' stroke-linecap='round' stroke-linejoin='round'%3E%3Crect x='4' y='2' width='16' height='20' rx='2'/%3E%3Cline x1='8' y1='6' x2='16' y2='6'/%3E%3Cline x1='8' y1='10' x2='8' y2='10.01'/%3E%3Cline x1='12' y1='10' x2='12' y2='10.01'/%3E%3Cline x1='16' y1='10' x2='16' y2='10.01'/%3E%3Cline x1='8' y1='14' x2='8' y2='14.01'/%3E%3Cline x1='12' y1='14' x2='12' y2='14.01'/%3E%3Cline x1='16' y1='14' x2='16' y2='14.01'/%3E%3Cline x1='8' y1='18' x2='8' y2='18.01'/%3E%3Cline x1='12' y1='18' x2='12' y2='18.01'/%3E%3Cline x1='16' y1='18' x2='16' y2='18.01'/%3E%3C/svg%3E");
+}
+
 .tree-grid .device-drag-handle {
  cursor: grab;
 }
--- a/src/cable-sizing/domain/cable-sizing-calculation.ts
+++ b/src/cable-sizing/domain/cable-sizing-calculation.ts
@ -0,0 +1,547 @@
+// Pure cable sizing domain logic. No imports from db/, server/ or frontend/ -
+// mirrors the dependency direction of src/external-model/ (see
+// docs/cable-sizing-module.md). Everything here is a stateless function on
+// plain data; persistence and the versioned circuit.update command live
+// outside this module.
+//
+// The numeric reference tables below (cross-section current-carrying
+// capacity, temperature/grouping correction factors) are ported from a
+// sibling project's already-verified Kabelliste module
+// (elt-planung-suite/src/tools/kabel/index.ts), not re-derived. See
+// docs/cable-sizing-module.md for the verification history and the explicit
+// "not yet verified" methods, which intentionally return no numeric result
+// instead of a guessed one (dataVerified: false).
+
+// -- Laying methods (DIN VDE 0298-4 reference installation methods) --------
+
+export const LAYING_METHODS = [
+  "A1",
+  "A2",
+  "B1",
+  "B2",
+  "C",
+  "D1",
+  "D2",
+  "E",
+  "F",
+  "G",
+] as const;
+export type LayingMethod = (typeof LAYING_METHODS)[number];
+
+export const LAYING_METHOD_GROUP: Record<LayingMethod, "air" | "ground"> = {
+  A1: "air",
+  A2: "air",
+  B1: "air",
+  B2: "air",
+  C: "air",
+  E: "air",
+  F: "air",
+  D1: "ground",
+  D2: "ground",
+  G: "ground",
+};
+
+// A1, B2, C, E, D1, D2: verified against elt-planung-suite's ported original
+// tool. A2, B1, F, G: no verified capacity table available yet (see
+// docs/cable-sizing-module.md) - calculateCableSizing returns
+// dataVerified: false for these instead of a guessed number.
+export const LAYING_METHOD_VERIFIED: Record<LayingMethod, boolean> = {
+  A1: true,
+  B2: true,
+  C: true,
+  E: true,
+  D1: true,
+  D2: true,
+  A2: false,
+  B1: false,
+  F: false,
+  G: false,
+};
+
+export const LAYING_METHOD_LABELS: Record<LayingMethod, string> = {
+  A1: "A1 - Rohr in wärmegedämmter Wand",
+  A2: "A2 - Mehradriges Kabel im Rohr in wärmegedämmter Wand (Werte nicht hinterlegt)",
+  B1: "B1 - Einzeladern im Rohr auf/unter Putz (Werte nicht hinterlegt)",
+  B2: "B2 - Rohr auf/unter Putz",
+  C: "C - Kabel direkt auf Wand verlegt",
+  E: "E - Kabelpritsche / frei in Luft",
+  F: "F - Kabel frei in Luft, einzeln mit Abstand (Werte nicht hinterlegt)",
+  D1: "D1 - Kabel direkt im Erdreich",
+  D2: "D2 - Kabel im Rohr im Erdreich",
+  G: "G - Erdverlegung, Sonderfall (Werte nicht hinterlegt)",
+};
+
+// -- Insulation / conductor material ----------------------------------------
+
+export const INSULATION_MATERIALS = ["pvc", "xlpe"] as const;
+export type InsulationMaterial = (typeof INSULATION_MATERIALS)[number];
+
+export const INSULATION_MATERIAL_LABELS: Record<InsulationMaterial, string> = {
+  pvc: "PVC (Grenztemperatur 70 °C)",
+  xlpe: "VPE/XLPE (Grenztemperatur 90 °C) - Strombelastbarkeitswerte nicht hinterlegt",
+};
+
+export const CONDUCTOR_MATERIALS = ["copper", "aluminum"] as const;
+export type ConductorMaterial = (typeof CONDUCTOR_MATERIALS)[number];
+
+// Only PVC combined with one of the six verified laying methods returns a
+// numeric result. XLPE has no verified capacity table for any method yet.
+export function isCableSizingDataVerified(
+  layingMethod: LayingMethod,
+  insulation: InsulationMaterial
+): boolean {
+  return insulation === "pvc" && LAYING_METHOD_VERIFIED[layingMethod];
+}
+
+// -- Harmonic neutral loading (IEC 60364-5-52, three-phase only) -----------
+
+export const HARMONIC_NEUTRAL_LOAD_OPTIONS = [
+  "none",
+  "15to33Percent",
+  "over33Percent",
+] as const;
+export type HarmonicNeutralLoad = (typeof HARMONIC_NEUTRAL_LOAD_OPTIONS)[number];
+
+export const HARMONIC_NEUTRAL_LOAD_LABELS: Record<HarmonicNeutralLoad, string> = {
+  none: "Keine nennenswerte 3. Harmonische (< 15 %) - Standardfall",
+  "15to33Percent":
+    "3. Harmonische 15-33 % - Reduktionsfaktor 0,86 (IEC 60364-5-52)",
+  over33Percent:
+    "3. Harmonische > 33 % - Neutralleiter wie Außenleiter dimensionieren",
+};
+
+export const HARMONIC_REDUCTION_FACTOR = 0.86;
+
+// -- Reference tables (ported, verified subset only) ------------------------
+
+export const CROSS_SECTIONS_MM2 = [
+  1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240,
+] as const;
+
+type CoreCount = 2 | 3;
+
+export const CURRENT_CAPACITY_A: Partial<
+  Record<LayingMethod, Record<CoreCount, (number | null)[]>>
+> = {
+  A1: {
+    2: [15.5, 21, 28, 36, 50, 68, 89, 110, 134, 171, 207, 239, null, null, null],
+    3: [13.5, 18, 24, 31, 42, 56, 73, 89, 108, 136, 164, 188, null, null, null],
+  },
+  B2: {
+    2: [16.5, 23, 30, 38, 52, 69, 90, 111, 134, 171, 207, 239, null, null, null],
+    3: [15, 20, 27, 34, 46, 62, 80, 99, 118, 149, 179, 206, null, null, null],
+  },
+  C: {
+    2: [19.5, 27, 36, 46, 63, 85, 112, 138, 168, 213, 258, 299, 344, 392, 461],
+    3: [17.5, 24, 32, 41, 57, 76, 96, 119, 144, 184, 223, 259, 299, 341, 403],
+  },
+  E: {
+    2: [22, 30, 40, 51, 70, 94, 119, 148, 180, 232, 282, 328, 379, 434, 514],
+    3: [18.5, 25, 34, 43, 60, 80, 101, 126, 153, 196, 238, 276, 319, 364, 430],
+  },
+  D1: {
+    2: [26, 34, 44, 56, 74, 96, 123, 147, 174, 216, 256, 290, 328, 367, 424],
+    3: [22, 29, 38, 47, 63, 81, 104, 125, 148, 183, 216, 246, 278, 312, 361],
+  },
+  D2: {
+    2: [22, 29, 38, 47, 63, 81, 104, 125, 148, 183, 216, 246, 278, 312, 360],
+    3: [18.5, 24, 31, 39, 52, 67, 86, 103, 122, 151, 179, 203, 230, 258, 297],
+  },
+};
+
+export const TEMPERATURE_FACTOR_AIR: Record<number, number> = {
+  10: 1.29,
+  15: 1.22,
+  20: 1.15,
+  25: 1.08,
+  30: 1.0,
+  35: 0.91,
+  40: 0.82,
+  45: 0.71,
+  50: 0.58,
+};
+
+export const TEMPERATURE_FACTOR_GROUND: Record<number, number> = {
+  10: 1.1,
+  15: 1.05,
+  20: 1.0,
+  25: 0.95,
+  30: 0.89,
+  35: 0.84,
+  40: 0.77,
+};
+
+export const GROUPING_FACTOR_AIR: Record<number, number> = {
+  1: 1.0,
+  2: 0.8,
+  3: 0.7,
+  4: 0.65,
+  5: 0.6,
+  6: 0.57,
+  8: 0.54,
+  10: 0.5,
+};
+
+export const GROUPING_FACTOR_GROUND: Record<number, number> = {
+  1: 1.0,
+  2: 0.75,
+  3: 0.65,
+  4: 0.6,
+  5: 0.55,
+  6: 0.52,
+  8: 0.49,
+  10: 0.46,
+};
+
+export const ALUMINUM_CAPACITY_FACTOR = 0.78;
+export const KAPPA: Record<ConductorMaterial, number> = { copper: 48, aluminum: 30 };
+
+export const GROUPING_OPTIONS = [1, 2, 3, 4, 5, 6, 8, 10] as const;
+
+export const GROUPING_LABELS: Record<number, string> = {
+  1: "1 (keine Häufung)",
+  2: "2",
+  3: "3",
+  4: "4",
+  5: "5",
+  6: "6",
+  8: "7-9",
+  10: "≥10",
+};
+
+// -- Practical minimum cross-sections by circuit category ------------------
+// Not a thermal/voltage-drop calculation result - a widely used planning
+// convention for margin against future load growth, mechanical robustness
+// and fault-current withstand, beyond what a pure Ib-based calculation
+// would give. Matches this project's own docs/spec/06-future-sizing-and-
+// calculations.md ("Standard Single-Phase Circuits ... usually use ...
+// cable cross-section: 2.5 mm²"). Only applied for the "single_phase"
+// circuit category (general sockets and similar loads); lighting and
+// three-phase circuits are not covered by this specific convention.
+export const PRACTICAL_MINIMUM_CROSS_SECTION_MM2: Partial<Record<CircuitCategory, number>> = {
+  single_phase: 2.5,
+};
+
+export type CircuitCategory = "lighting" | "single_phase" | "three_phase";
+
+// -- Input / result -----------------------------------------------------
+
+export interface CableSizingInput {
+  phase: 1 | 3;
+  mode: "power" | "current";
+  /** kW, only used when mode === "power" */
+  powerKw?: number;
+  /** A, only used when mode === "current" */
+  currentA?: number;
+  cosPhi: number;
+  /** Line voltage in V. The circuit-list app derives this from project
+   * settings (singlePhaseVoltageV / threePhaseVoltageV); pass it through
+   * rather than hard-coding 230/400 here. */
+  voltage: number;
+  /** Single-run cable length in meters (circuits.cableLength). */
+  lengthM: number;
+  layingMethod: LayingMethod;
+  conductorMaterial: ConductorMaterial;
+  insulation: InsulationMaterial;
+  ambientTemperatureC: number;
+  groupingCircuits: number;
+  maxVoltageDropPercent: number;
+  harmonicNeutralLoad: HarmonicNeutralLoad;
+  /** Optional: existing protection device rated current (LS/circuit
+   * breaker), for the simplified coordination check below. */
+  existingProtectionRatedCurrentA?: number;
+  /** Optional: enables the practical-minimum-cross-section convention for
+   * "single_phase" circuits, see PRACTICAL_MINIMUM_CROSS_SECTION_MM2. */
+  circuitCategory?: CircuitCategory;
+}
+
+export interface CrossSectionRow {
+  crossSectionMm2: number;
+  ratedCurrentA: number | null;
+  correctedCurrentA: number | null;
+  currentSufficient: boolean | null;
+  voltageDropPercent: number | null;
+  voltageDropSufficient: boolean | null;
+  /** Longest single-run length (m) at which this cross-section still meets
+   * the requested max voltage drop, at the given load/cosPhi/phase - the
+   * inverse of the voltage-drop formula used for voltageDropPercent above.
+   * null when the operating current is zero (division by zero). */
+  maxLengthForVoltageDropM: number | null;
+  recommended: boolean;
+}
+
+export interface ProtectionCoordinationResult {
+  ratedCurrentA: number;
+  /** Simplified In <= Iz check only - not full overload (I2 <= 1.45 x Iz)
+   * or short-circuit coordination. */
+  coordinated: boolean;
+}
+
+export interface CableSizingResult {
+  /** false: no verified capacity table for this layingMethod/insulation
+   * combination - every field below is null/empty, never a guess. */
+  dataVerified: boolean;
+  operatingCurrentA: number;
+  /** max(operatingCurrentA, existingProtectionRatedCurrentA). The cable
+   * capacity (Iz) must cover the protective device's rated current (In),
+   * not just the actual load current - a breaker only trips at In, so a
+   * cable sized for Ib alone could overheat under sustained load below the
+   * trip threshold. This is the value cross-section selection actually
+   * uses; operatingCurrentA is kept for display/voltage-drop only. */
+  designCurrentA: number;
+  crossSectionByCapacityMm2: number | null;
+  crossSectionByVoltageDropMm2: number | null;
+  recommendedCrossSectionMm2: number | null;
+  voltageDropAtRecommendedPercent: number | null;
+  combinedDerationFactor: number;
+  harmonicReductionApplied: boolean;
+  /** true if the recommendation was raised to satisfy
+   * PRACTICAL_MINIMUM_CROSS_SECTION_MM2 (a convention, not a thermal/
+   * voltage-drop requirement of this specific circuit). */
+  practicalMinimumApplied: boolean;
+  rows: CrossSectionRow[];
+  protectionCoordination: ProtectionCoordinationResult | null;
+}
+
+function operatingCurrentA(input: CableSizingInput): number {
+  if (input.mode === "current") {
+    return input.currentA ?? 0;
+  }
+  const powerW = (input.powerKw ?? 0) * 1000;
+  return input.phase === 1
+    ? powerW / (input.voltage * input.cosPhi)
+    : powerW / (Math.sqrt(3) * input.voltage * input.cosPhi);
+}
+
+export function calculateCableSizing(input: CableSizingInput): CableSizingResult {
+  const ib = operatingCurrentA(input);
+
+  const designCurrentA =
+    input.existingProtectionRatedCurrentA != null
+      ? Math.max(ib, input.existingProtectionRatedCurrentA)
+      : ib;
+
+  if (!isCableSizingDataVerified(input.layingMethod, input.insulation)) {
+    return {
+      dataVerified: false,
+      operatingCurrentA: ib,
+      designCurrentA,
+      crossSectionByCapacityMm2: null,
+      crossSectionByVoltageDropMm2: null,
+      recommendedCrossSectionMm2: null,
+      voltageDropAtRecommendedPercent: null,
+      combinedDerationFactor: 1,
+      harmonicReductionApplied: false,
+      practicalMinimumApplied: false,
+      rows: [],
+      protectionCoordination: null,
+    };
+  }
+
+  const coreCount: CoreCount = input.phase === 1 ? 2 : 3;
+  const aluminumFactor =
+    input.conductorMaterial === "aluminum" ? ALUMINUM_CAPACITY_FACTOR : 1;
+  const kappa = KAPPA[input.conductorMaterial];
+  const group = LAYING_METHOD_GROUP[input.layingMethod];
+  const temperatureTable =
+    group === "air" ? TEMPERATURE_FACTOR_AIR : TEMPERATURE_FACTOR_GROUND;
+  const groupingTable = group === "air" ? GROUPING_FACTOR_AIR : GROUPING_FACTOR_GROUND;
+  const temperatureFactor = temperatureTable[input.ambientTemperatureC] ?? 1;
+  const groupingFactor = groupingTable[input.groupingCircuits] ?? 1;
+
+  const harmonicReductionApplied =
+    input.phase === 3 && input.harmonicNeutralLoad === "15to33Percent";
+  const harmonicFactor = harmonicReductionApplied ? HARMONIC_REDUCTION_FACTOR : 1;
+
+  const combinedDerationFactor = temperatureFactor * groupingFactor * harmonicFactor;
+  const voltageDropCoefficient = input.phase === 1 ? 2 : Math.sqrt(3);
+
+  const ratedCurrents = CURRENT_CAPACITY_A[input.layingMethod]![coreCount];
+  const correctedCurrents = ratedCurrents.map((value) =>
+    value == null ? null : value * aluminumFactor * combinedDerationFactor
+  );
+  const voltageDrops = CROSS_SECTIONS_MM2.map(
+    (crossSection) =>
+      ((voltageDropCoefficient * input.lengthM * ib * input.cosPhi) /
+        (kappa * crossSection) /
+        input.voltage) *
+      100
+  );
+
+  const indexByCapacity = correctedCurrents.findIndex(
+    (value) => value != null && value >= designCurrentA
+  );
+  const indexByVoltageDrop = voltageDrops.findIndex(
+    (value) => value <= input.maxVoltageDropPercent
+  );
+  let indexRecommended = -1;
+  if (indexByCapacity >= 0 && indexByVoltageDrop >= 0) {
+    indexRecommended = Math.max(indexByCapacity, indexByVoltageDrop);
+  }
+
+  // Practical-minimum convention (see PRACTICAL_MINIMUM_CROSS_SECTION_MM2):
+  // only ever raises the recommendation, never lowers what the thermal/
+  // voltage-drop calculation already required.
+  const practicalMinimumMm2 = input.circuitCategory
+    ? PRACTICAL_MINIMUM_CROSS_SECTION_MM2[input.circuitCategory]
+    : undefined;
+  let practicalMinimumApplied = false;
+  if (
+    indexRecommended >= 0 &&
+    practicalMinimumMm2 != null &&
+    CROSS_SECTIONS_MM2[indexRecommended] < practicalMinimumMm2
+  ) {
+    const minimumIndex = CROSS_SECTIONS_MM2.indexOf(
+      practicalMinimumMm2 as (typeof CROSS_SECTIONS_MM2)[number]
+    );
+    if (minimumIndex >= 0) {
+      indexRecommended = minimumIndex;
+      practicalMinimumApplied = true;
+    }
+  }
+
+  const voltageDropCurrentBasis = ib * input.cosPhi;
+  const rows: CrossSectionRow[] = CROSS_SECTIONS_MM2.map((crossSection, i) => {
+    const rated = ratedCurrents[i];
+    const corrected = correctedCurrents[i];
+    const currentSufficient = corrected == null ? null : corrected >= designCurrentA;
+    const voltageDropSufficient = voltageDrops[i] <= input.maxVoltageDropPercent;
+    const maxLengthForVoltageDropM =
+      rated == null || voltageDropCurrentBasis <= 0
+        ? null
+        : (input.maxVoltageDropPercent * kappa * crossSection * input.voltage) /
+          (100 * voltageDropCoefficient * voltageDropCurrentBasis);
+    return {
+      crossSectionMm2: crossSection,
+      ratedCurrentA: rated,
+      correctedCurrentA: corrected,
+      currentSufficient,
+      voltageDropPercent: rated == null ? null : voltageDrops[i],
+      voltageDropSufficient: rated == null ? null : voltageDropSufficient,
+      maxLengthForVoltageDropM,
+      recommended: i === indexRecommended,
+    };
+  });
+
+  let protectionCoordination: ProtectionCoordinationResult | null = null;
+  if (input.existingProtectionRatedCurrentA != null && indexRecommended >= 0) {
+    const recommendedRow = rows[indexRecommended];
+    protectionCoordination = {
+      ratedCurrentA: input.existingProtectionRatedCurrentA,
+      coordinated:
+        recommendedRow.correctedCurrentA != null &&
+        input.existingProtectionRatedCurrentA <= recommendedRow.correctedCurrentA,
+    };
+  }
+
+  return {
+    dataVerified: true,
+    operatingCurrentA: ib,
+    designCurrentA,
+    crossSectionByCapacityMm2:
+      indexByCapacity >= 0 ? CROSS_SECTIONS_MM2[indexByCapacity] : null,
+    crossSectionByVoltageDropMm2:
+      indexByVoltageDrop >= 0 ? CROSS_SECTIONS_MM2[indexByVoltageDrop] : null,
+    recommendedCrossSectionMm2:
+      indexRecommended >= 0 ? CROSS_SECTIONS_MM2[indexRecommended] : null,
+    voltageDropAtRecommendedPercent:
+      indexRecommended >= 0 ? voltageDrops[indexRecommended] : null,
+    combinedDerationFactor,
+    harmonicReductionApplied,
+    practicalMinimumApplied,
+    rows,
+    protectionCoordination,
+  };
+}
+
+export interface CableSizingAlert {
+  kind: "critical" | "warn" | "info" | "ok";
+  text: string;
+}
+
+export function buildCableSizingAlerts(
+  input: CableSizingInput,
+  result: CableSizingResult
+): CableSizingAlert[] {
+  const alerts: CableSizingAlert[] = [];
+
+  if (!result.dataVerified) {
+    alerts.push({
+      kind: "critical",
+      text: `Für Verlegeart ${input.layingMethod} mit Isolierstoff ${input.insulation.toUpperCase()} liegen keine geprüften Strombelastbarkeits-Tabellenwerte vor - keine Dimensionierung möglich.`,
+    });
+    return alerts;
+  }
+
+  const byCapacity = result.crossSectionByCapacityMm2;
+  const byDrop = result.crossSectionByVoltageDropMm2;
+  const recommended = result.recommendedCrossSectionMm2;
+
+  if (byCapacity == null) {
+    const dueToBreaker =
+      input.existingProtectionRatedCurrentA != null &&
+      input.existingProtectionRatedCurrentA > result.operatingCurrentA;
+    alerts.push({
+      kind: "critical",
+      text: dueToBreaker
+        ? `Vorhandene Sicherung (${input.existingProtectionRatedCurrentA} A) übersteigt die Belastbarkeit aller Standardquerschnitte bei Verlegeart ${input.layingMethod} - nicht der Betriebsstrom (${result.operatingCurrentA.toFixed(1)} A). Größere Verlegeart oder kleinere Sicherung prüfen.`
+        : `Betriebsstrom ${result.operatingCurrentA.toFixed(1)} A übersteigt die Belastbarkeit aller Standardquerschnitte bei Verlegeart ${input.layingMethod}.`,
+    });
+  } else if (byDrop != null && byDrop > byCapacity) {
+    alerts.push({
+      kind: "warn",
+      text: `Spannungsfall ist maßgeblich, nicht die Belastbarkeit: ${byCapacity} mm² würde thermisch reichen, ${byDrop} mm² ist wegen ΔU ≤ ${input.maxVoltageDropPercent}% nötig.`,
+    });
+  }
+  if (result.harmonicReductionApplied) {
+    alerts.push({
+      kind: "info",
+      text: "Reduktionsfaktor 0,86 wegen Oberschwingungsanteil 15-33 % im Neutralleiter angewendet (IEC 60364-5-52).",
+    });
+  }
+  if (input.phase === 3 && input.harmonicNeutralLoad === "over33Percent") {
+    alerts.push({
+      kind: "warn",
+      text: "3. Harmonische > 33 %: Neutralleiter muss wie ein Außenleiter dimensioniert werden - hier nicht automatisch berücksichtigt.",
+    });
+  }
+  if (result.practicalMinimumApplied) {
+    alerts.push({
+      kind: "info",
+      text: `Auf ${result.recommendedCrossSectionMm2} mm² angehoben (Praxis-Mindestquerschnitt für 1-phasige Stromkreise, keine reine Berechnungsanforderung).`,
+    });
+  }
+  if (result.combinedDerationFactor < 1) {
+    alerts.push({
+      kind: "info",
+      text: `Korrekturfaktor angewendet: ${result.combinedDerationFactor.toFixed(2)} (Temperatur × Häufung${result.harmonicReductionApplied ? " × Oberschwingungen" : ""}).`,
+    });
+  }
+  if (result.protectionCoordination) {
+    alerts.push(
+      result.protectionCoordination.coordinated
+        ? {
+            kind: "info",
+            text: `Vorhandene Sicherung (${result.protectionCoordination.ratedCurrentA} A) ist bei der Dimensionierung berücksichtigt (In ≤ Iz, vereinfachte Prüfung, ersetzt keine vollständige Überlast-/Kurzschlussprüfung).`,
+          }
+        : {
+            kind: "warn",
+            text: `Vorhandene Sicherung (${result.protectionCoordination.ratedCurrentA} A) übersteigt die Belastbarkeit des empfohlenen Querschnitts - Koordination prüfen.`,
+          }
+    );
+  }
+  if (alerts.length === 0 && recommended != null) {
+    alerts.push({
+      kind: "ok",
+      text: `${recommended} mm² (${input.conductorMaterial === "aluminum" ? "Alu" : "Cu"}) deckt bei ${input.lengthM} m Länge sowohl Belastbarkeit (${result.designCurrentA.toFixed(1)} A${input.existingProtectionRatedCurrentA != null ? ", inkl. vorhandener Sicherung" : ""}) als auch Spannungsfall (≤ ${input.maxVoltageDropPercent}%) ab.`,
+    });
+  }
+  const recommendedRow = result.rows.find((row) => row.recommended);
+  if (recommendedRow?.maxLengthForVoltageDropM != null) {
+    alerts.push({
+      kind: "info",
+      text: `Maximale Länge bei ${recommendedRow.crossSectionMm2} mm² und ΔU ≤ ${input.maxVoltageDropPercent}%: ${recommendedRow.maxLengthForVoltageDropM.toFixed(0)} m.`,
+    });
+  }
+  return alerts;
+}
--- a/src/cable-sizing/domain/cable-sizing-contracts.ts
+++ b/src/cable-sizing/domain/cable-sizing-contracts.ts
@ -0,0 +1,49 @@
+// Request/response validation for the cable-sizing HTTP API. Pure - depends
+// only on zod and the sibling calculation module, never on db/server.
+import { z } from "zod";
+import {
+  CONDUCTOR_MATERIALS,
+  HARMONIC_NEUTRAL_LOAD_OPTIONS,
+  INSULATION_MATERIALS,
+  LAYING_METHODS,
+  type CableSizingInput,
+} from "./cable-sizing-calculation.js";
+
+export const cableSizingRequestSchema = z
+  .object({
+    phase: z.union([z.literal(1), z.literal(3)]),
+    mode: z.enum(["power", "current"]),
+    powerKw: z.number().min(0).optional(),
+    currentA: z.number().min(0).optional(),
+    cosPhi: z.number().min(0.6).max(1),
+    voltage: z.number().positive(),
+    lengthM: z.number().min(0),
+    layingMethod: z.enum(LAYING_METHODS),
+    conductorMaterial: z.enum(CONDUCTOR_MATERIALS),
+    insulation: z.enum(INSULATION_MATERIALS),
+    ambientTemperatureC: z.number(),
+    groupingCircuits: z.number().int().min(1),
+    maxVoltageDropPercent: z.number().min(1).max(5),
+    harmonicNeutralLoad: z.enum(HARMONIC_NEUTRAL_LOAD_OPTIONS),
+    existingProtectionRatedCurrentA: z.number().positive().optional(),
+    circuitCategory: z.enum(["lighting", "single_phase", "three_phase"]).optional(),
+    // Optional context, persisted with the audit-log entry only - never
+    // used for the calculation itself.
+    context: z
+      .object({
+        projectId: z.string().min(1).optional(),
+        circuitId: z.string().min(1).optional(),
+        equipmentIdentifier: z.string().min(1).optional(),
+      })
+      .optional(),
+  })
+  .refine((v) => (v.mode === "power" ? v.powerKw != null : v.currentA != null), {
+    message: "powerKw is required for mode 'power', currentA for mode 'current'",
+  });
+
+export type CableSizingRequest = z.infer<typeof cableSizingRequestSchema>;
+
+// Structural check that the request schema stays a superset of the domain
+// input shape (minus context, which is API-only metadata).
+type _AssertAssignable<T extends CableSizingInput> = T;
+type _Check = _AssertAssignable<Omit<CableSizingRequest, "context">>;
--- a/src/db/migrations/0007_watery_kingpin.sql
+++ b/src/db/migrations/0007_watery_kingpin.sql
@ -0,0 +1,14 @@
+CREATE TABLE `cable_sizing_calculations` (
+	`id` text PRIMARY KEY NOT NULL,
+	`project_id` text,
+	`circuit_id` text,
+	`equipment_identifier` text,
+	`input` text NOT NULL,
+	`result` text NOT NULL,
+	`applied_to_circuit` integer DEFAULT 0 NOT NULL,
+	`created_at` integer DEFAULT (unixepoch()) NOT NULL,
+	FOREIGN KEY (`project_id`) REFERENCES `projects`(`id`) ON UPDATE no action ON DELETE set null,
+	FOREIGN KEY (`circuit_id`) REFERENCES `circuits`(`id`) ON UPDATE no action ON DELETE set null
+);
+--> statement-breakpoint
+CREATE INDEX `cable_sizing_calculations_circuit_id_idx` ON `cable_sizing_calculations` (`circuit_id`);
--- a/src/db/migrations/meta/0007_snapshot.json
+++ b/src/db/migrations/meta/0007_snapshot.json
--- a/src/db/migrations/meta/_journal.json
+++ b/src/db/migrations/meta/_journal.json
@ -50,6 +50,13 @@
      "when": 1786043080323,
      "tag": "0006_damp_skrulls",
      "breakpoints": true
+    },
+    {
+      "idx": 7,
+      "version": "6",
+      "when": 1786114831610,
+      "tag": "0007_watery_kingpin",
+      "breakpoints": true
    }
  ]
 }
--- a/src/db/repositories/cable-sizing-calculation.repository.ts
+++ b/src/db/repositories/cable-sizing-calculation.repository.ts
@ -0,0 +1,37 @@
+import { desc, eq } from "drizzle-orm";
+import type { AppDatabase } from "../database-context.js";
+import {
+  cableSizingCalculations,
+  type CableSizingCalculation,
+  type NewCableSizingCalculation,
+} from "../schema/cable-sizing-calculations.js";
+
+// Plain repository, no revision/command semantics - mirrors
+// application-repositories.ts's flat repository pattern, not the
+// project-command-transaction boundary. A calculation is an audit record,
+// not a project mutation.
+export class CableSizingCalculationRepository {
+  constructor(private readonly db: AppDatabase) {}
+
+  create(input: NewCableSizingCalculation): CableSizingCalculation {
+    return this.db.insert(cableSizingCalculations).values(input).returning().get();
+  }
+
+  listByCircuit(circuitId: string): CableSizingCalculation[] {
+    return this.db
+      .select()
+      .from(cableSizingCalculations)
+      .where(eq(cableSizingCalculations.circuitId, circuitId))
+      .orderBy(desc(cableSizingCalculations.createdAt))
+      .all();
+  }
+
+  markApplied(id: string): CableSizingCalculation | undefined {
+    return this.db
+      .update(cableSizingCalculations)
+      .set({ appliedToCircuit: 1 })
+      .where(eq(cableSizingCalculations.id, id))
+      .returning()
+      .get();
+  }
+}
--- a/src/db/schema/cable-sizing-calculations.ts
+++ b/src/db/schema/cable-sizing-calculations.ts
@ -0,0 +1,30 @@
+import { sql } from "drizzle-orm";
+import { index, integer, sqliteTable, text } from "drizzle-orm/sqlite-core";
+import { circuits } from "./circuits.js";
+import { projects } from "./projects.js";
+
+// Audit log for the cable-sizing module (see docs/cable-sizing-module.md).
+// Deliberately separate from the revision/command system: a calculation is
+// not itself a project mutation, only applying its result via the existing
+// circuit.update command is. circuitId/projectId are nullable and
+// onDelete "set null" so a deleted circuit or project never blocks or
+// cascades into losing the audit trail.
+export const cableSizingCalculations = sqliteTable(
+  "cable_sizing_calculations",
+  {
+    id: text("id").primaryKey(),
+    projectId: text("project_id").references(() => projects.id, { onDelete: "set null" }),
+    circuitId: text("circuit_id").references(() => circuits.id, { onDelete: "set null" }),
+    equipmentIdentifier: text("equipment_identifier"),
+    input: text("input", { mode: "json" }).notNull(),
+    result: text("result", { mode: "json" }).notNull(),
+    appliedToCircuit: integer("applied_to_circuit").notNull().default(0),
+    createdAt: integer("created_at", { mode: "timestamp" })
+      .notNull()
+      .default(sql`(unixepoch())`),
+  },
+  (table) => [index("cable_sizing_calculations_circuit_id_idx").on(table.circuitId)]
+);
+
+export type CableSizingCalculation = typeof cableSizingCalculations.$inferSelect;
+export type NewCableSizingCalculation = typeof cableSizingCalculations.$inferInsert;
--- a/src/frontend/components/cable-sizing-api.ts
+++ b/src/frontend/components/cable-sizing-api.ts
@ -0,0 +1,71 @@
+// Thin, self-contained fetch client for the cable-sizing module. Deliberately
+// not merged into ../utils/api.ts to keep this module's frontend footprint a
+// single new file rather than growing the existing shared API file - see
+// docs/cable-sizing-module.md.
+import type {
+  CableSizingAlert,
+  CableSizingInput,
+  CableSizingResult,
+  InsulationMaterial,
+  LayingMethod,
+} from "../../cable-sizing/domain/cable-sizing-calculation";
+
+async function request<T>(url: string, init?: RequestInit): Promise<T> {
+  const response = await fetch(url, {
+    ...init,
+    headers: { "Content-Type": "application/json", ...init?.headers },
+    cache: "no-store",
+  });
+  if (!response.ok) {
+    let details = await response.text();
+    try {
+      const parsed = JSON.parse(details) as { error?: unknown };
+      if (typeof parsed.error === "string") details = parsed.error;
+    } catch {
+      // keep raw text
+    }
+    throw new Error(details || `Anfrage fehlgeschlagen (Status ${response.status})`);
+  }
+  return response.json() as Promise<T>;
+}
+
+export interface CalculateCableSizingResponse {
+  calculationId: string;
+  result: CableSizingResult;
+  alerts: CableSizingAlert[];
+}
+
+export function calculateCableSizing(
+  input: CableSizingInput,
+  context?: { projectId?: string; circuitId?: string; equipmentIdentifier?: string }
+): Promise<CalculateCableSizingResponse> {
+  return request("/api/cable-sizing/calculate", {
+    method: "POST",
+    body: JSON.stringify({ ...input, context }),
+  });
+}
+
+export function markCableSizingCalculationApplied(calculationId: string): Promise<unknown> {
+  return request(`/api/cable-sizing/calculations/${calculationId}/applied`, {
+    method: "POST",
+  });
+}
+
+export interface LayingMethodOption {
+  method: LayingMethod;
+  label: string;
+  dataVerified: boolean;
+}
+
+export function listLayingMethods(): Promise<LayingMethodOption[]> {
+  return request("/api/cable-sizing/laying-methods");
+}
+
+export interface InsulationMaterialOption {
+  insulation: InsulationMaterial;
+  label: string;
+}
+
+export function listInsulationMaterials(): Promise<InsulationMaterialOption[]> {
+  return request("/api/cable-sizing/insulation-materials");
+}
--- a/src/frontend/components/cable-sizing-modal.tsx
+++ b/src/frontend/components/cable-sizing-modal.tsx
@ -0,0 +1,523 @@
+"use client";
+
+import { type FormEvent, useMemo, useState } from "react";
+import {
+  CONDUCTOR_MATERIALS,
+  CROSS_SECTIONS_MM2,
+  GROUPING_LABELS,
+  GROUPING_OPTIONS,
+  HARMONIC_NEUTRAL_LOAD_LABELS,
+  HARMONIC_NEUTRAL_LOAD_OPTIONS,
+  INSULATION_MATERIALS,
+  INSULATION_MATERIAL_LABELS,
+  LAYING_METHODS,
+  LAYING_METHOD_GROUP,
+  LAYING_METHOD_LABELS,
+  PRACTICAL_MINIMUM_CROSS_SECTION_MM2,
+  TEMPERATURE_FACTOR_AIR,
+  TEMPERATURE_FACTOR_GROUND,
+  type CableSizingAlert,
+  type CableSizingInput,
+  type CableSizingResult,
+  type CircuitCategory,
+  type ConductorMaterial,
+  type HarmonicNeutralLoad,
+  type InsulationMaterial,
+  type LayingMethod,
+} from "../../cable-sizing/domain/cable-sizing-calculation";
+import { calculateCableSizing } from "./cable-sizing-api";
+import type { CircuitTreeCircuitDto } from "../types";
+import { FormModal } from "./form-modal";
+
+interface CableSizingModalProps {
+  circuit: CircuitTreeCircuitDto;
+  circuitCategory?: CircuitCategory;
+  isSaving: boolean;
+  projectId: string;
+  onClose: () => void;
+  onApply: (patch: {
+    cableCrossSection: string;
+    cableType?: string;
+    cableLength?: number;
+  }) => Promise<void>;
+}
+
+function temperatureOptions(method: LayingMethod) {
+  const table =
+    LAYING_METHOD_GROUP[method] === "air" ? TEMPERATURE_FACTOR_AIR : TEMPERATURE_FACTOR_GROUND;
+  return Object.keys(table).map(Number).sort((a, b) => a - b);
+}
+
+// Extracts the first decimal number from a free-text cross-section entry
+// like "2.5 mm²" or "2,5". Returns null if nothing parseable is found -
+// used only for the manual-entry sanity check below, never persisted.
+function parseCrossSectionMm2(text: string): number | null {
+  const match = text.match(/([0-9]+(?:[.,][0-9]+)?)/);
+  if (!match) return null;
+  const value = Number(match[1].replace(",", "."));
+  return Number.isFinite(value) ? value : null;
+}
+
+export function CableSizingModal({
+  circuit,
+  circuitCategory,
+  isSaving,
+  projectId,
+  onClose,
+  onApply,
+}: CableSizingModalProps) {
+  const [layingMethod, setLayingMethod] = useState<LayingMethod>("C");
+  const [insulation, setInsulation] = useState<InsulationMaterial>("pvc");
+  const [conductorMaterial, setConductorMaterial] = useState<ConductorMaterial>("copper");
+  const [ambientTemperatureC, setAmbientTemperatureC] = useState(30);
+  const [groupingCircuits, setGroupingCircuits] = useState(1);
+  const [cosPhi, setCosPhi] = useState(1);
+  const [maxVoltageDropPercent, setMaxVoltageDropPercent] = useState(3);
+  const [harmonicNeutralLoad, setHarmonicNeutralLoad] = useState<HarmonicNeutralLoad>("none");
+  const [lengthM, setLengthM] = useState(circuit.cableLength ?? 0);
+  const [phase, setPhase] = useState<1 | 3>(circuit.voltage === 400 ? 3 : 1);
+
+  // Manual override: applying this never requires a calculation. Prefilled
+  // from the circuit's current values so opening the modal on an already
+  // specified cable doesn't lose that data.
+  const [manualCableType, setManualCableType] = useState(circuit.cableType ?? "");
+  const [manualCrossSection, setManualCrossSection] = useState(circuit.cableCrossSection ?? "");
+
+  const [calculating, setCalculating] = useState(false);
+  const [calculationId, setCalculationId] = useState<string | null>(null);
+  const [result, setResult] = useState<CableSizingResult | null>(null);
+  const [alerts, setAlerts] = useState<CableSizingAlert[]>([]);
+  const [error, setError] = useState<string | null>(null);
+
+  const voltage = circuit.voltage ?? (phase === 1 ? 230 : 400);
+
+  const input: CableSizingInput = useMemo(
+    () => ({
+      phase,
+      mode: "power",
+      powerKw: circuit.circuitTotalPower,
+      cosPhi,
+      voltage,
+      lengthM,
+      layingMethod,
+      conductorMaterial,
+      insulation,
+      ambientTemperatureC,
+      groupingCircuits,
+      maxVoltageDropPercent,
+      harmonicNeutralLoad,
+      existingProtectionRatedCurrentA: circuit.protectionDevice?.ratedCurrentA,
+      circuitCategory,
+    }),
+    [
+      phase,
+      circuit.circuitTotalPower,
+      cosPhi,
+      voltage,
+      lengthM,
+      layingMethod,
+      conductorMaterial,
+      insulation,
+      ambientTemperatureC,
+      groupingCircuits,
+      maxVoltageDropPercent,
+      harmonicNeutralLoad,
+      circuit.protectionDevice?.ratedCurrentA,
+      circuitCategory,
+    ]
+  );
+
+  const manualCrossSectionWarning = useMemo(() => {
+    const trimmed = manualCrossSection.trim();
+    if (!trimmed) return null;
+    const parsed = parseCrossSectionMm2(trimmed);
+    if (parsed == null) return null;
+    if (!(CROSS_SECTIONS_MM2 as readonly number[]).includes(parsed)) {
+      return `${parsed} mm² ist kein Standard-Querschnitt (${CROSS_SECTIONS_MM2.join(", ")}).`;
+    }
+    if (
+      result?.dataVerified &&
+      result.recommendedCrossSectionMm2 != null &&
+      parsed < result.recommendedCrossSectionMm2
+    ) {
+      return `${parsed} mm² ist kleiner als die zuletzt berechnete Empfehlung (${result.recommendedCrossSectionMm2} mm²) - passt nicht zu Last${circuit.protectionDevice ? "/Sicherung" : ""} und Verlegeart.`;
+    }
+    const practicalMinimum = circuitCategory
+      ? PRACTICAL_MINIMUM_CROSS_SECTION_MM2[circuitCategory]
+      : undefined;
+    if (practicalMinimum != null && parsed < practicalMinimum) {
+      return `${parsed} mm² liegt unter dem Praxis-Mindestquerschnitt für 1-phasige Stromkreise (${practicalMinimum} mm²).`;
+    }
+    return null;
+  }, [manualCrossSection, result, circuit.protectionDevice, circuitCategory]);
+
+  function handleLayingMethodChange(method: LayingMethod) {
+    setLayingMethod(method);
+    const options = temperatureOptions(method);
+    const reference = LAYING_METHOD_GROUP[method] === "air" ? 30 : 20;
+    setAmbientTemperatureC(options.includes(reference) ? reference : options[0]);
+    setResult(null);
+    setCalculationId(null);
+  }
+
+  async function handleCalculate() {
+    setCalculating(true);
+    setError(null);
+    try {
+      const response = await calculateCableSizing(input, {
+        projectId,
+        circuitId: circuit.id,
+        equipmentIdentifier: circuit.equipmentIdentifier,
+      });
+      setResult(response.result);
+      setAlerts(response.alerts);
+      setCalculationId(response.calculationId);
+      // Calculating fills the manual field as a suggestion, but that field
+      // stays the single source of truth for what gets applied - the user
+      // can still edit it by hand before submitting.
+      if (response.result.dataVerified && response.result.recommendedCrossSectionMm2) {
+        setManualCrossSection(`${response.result.recommendedCrossSectionMm2} mm²`);
+      }
+    } catch (err) {
+      setError(err instanceof Error ? err.message : "Berechnung fehlgeschlagen");
+    } finally {
+      setCalculating(false);
+    }
+  }
+
+  async function handleSubmit(event: FormEvent<HTMLFormElement>) {
+    event.preventDefault();
+    if (!manualCrossSection.trim()) return;
+    await onApply({
+      cableCrossSection: manualCrossSection.trim(),
+      cableType: manualCableType.trim() || undefined,
+      cableLength: lengthM,
+    });
+  }
+
+  return (
+    <FormModal
+      description="Empfehlung auf Basis von Verlegeart, Isolierstoff, Temperatur und Häufung nach DIN VDE 0298-4, oder Kabeltyp/Querschnitt direkt manuell eintragen. Ersetzt keine vollständige Norm-Prüfung; nicht verifizierte Verlegearten liefern bewusst kein Ergebnis."
+      isSaving={isSaving}
+      onClose={onClose}
+      onSubmit={handleSubmit}
+      submitDisabled={!manualCrossSection.trim()}
+      submitLabel="Übernehmen"
+      title={`Kabel dimensionieren - ${circuit.equipmentIdentifier}${
+        circuit.displayName ? ` (${circuit.displayName})` : ""
+      }`}
+    >
+      <div className="row g-3">
+        <div className="col-12 col-md-4">
+          <label className="form-label" htmlFor="cable-sizing-length">
+            Leitungslänge (m)
+          </label>
+          <input
+            className="form-control"
+            id="cable-sizing-length"
+            min={0}
+            onChange={(event) => setLengthM(Number(event.target.value))}
+            type="number"
+            value={lengthM}
+          />
+        </div>
+        <div className="col-12 col-md-4">
+          <label className="form-label" htmlFor="cable-sizing-phase">
+            Phasen
+          </label>
+          <select
+            className="form-select"
+            id="cable-sizing-phase"
+            onChange={(event) => setPhase(Number(event.target.value) as 1 | 3)}
+            value={phase}
+          >
+            <option value={1}>1~ ({circuit.voltage ?? 230} V)</option>
+            <option value={3}>3~ ({circuit.voltage ?? 400} V)</option>
+          </select>
+        </div>
+        <div className="col-12 col-md-4">
+          <label className="form-label">Leistung (aus Stromkreis)</label>
+          <input
+            className="form-control"
+            disabled
+            value={`${circuit.circuitTotalPower.toFixed(2)} kW`}
+          />
+        </div>
+        <div className="col-12 col-md-6">
+          <label className="form-label" htmlFor="cable-sizing-method">
+            Verlegeart (DIN VDE 0298-4)
+          </label>
+          <select
+            autoFocus
+            className="form-select"
+            id="cable-sizing-method"
+            onChange={(event) => handleLayingMethodChange(event.target.value as LayingMethod)}
+            value={layingMethod}
+          >
+            {LAYING_METHODS.map((method) => (
+              <option key={method} value={method}>
+                {LAYING_METHOD_LABELS[method]}
+              </option>
+            ))}
+          </select>
+        </div>
+        <div className="col-12 col-md-6">
+          <label className="form-label" htmlFor="cable-sizing-insulation">
+            Isolierstoff
+          </label>
+          <select
+            className="form-select"
+            id="cable-sizing-insulation"
+            onChange={(event) => {
+              setInsulation(event.target.value as InsulationMaterial);
+              setResult(null);
+            }}
+            value={insulation}
+          >
+            {INSULATION_MATERIALS.map((material) => (
+              <option key={material} value={material}>
+                {INSULATION_MATERIAL_LABELS[material]}
+              </option>
+            ))}
+          </select>
+        </div>
+        <div className="col-12 col-md-6">
+          <label className="form-label" htmlFor="cable-sizing-material">
+            Leitermaterial
+          </label>
+          <select
+            className="form-select"
+            id="cable-sizing-material"
+            onChange={(event) => setConductorMaterial(event.target.value as ConductorMaterial)}
+            value={conductorMaterial}
+          >
+            {CONDUCTOR_MATERIALS.map((material) => (
+              <option key={material} value={material}>
+                {material === "copper" ? "Kupfer" : "Aluminium"}
+              </option>
+            ))}
+          </select>
+        </div>
+        <div className="col-12 col-md-6">
+          <label className="form-label" htmlFor="cable-sizing-temperature">
+            {LAYING_METHOD_GROUP[layingMethod] === "air"
+              ? "Umgebungstemperatur"
+              : "Bodentemperatur"}
+          </label>
+          <select
+            className="form-select"
+            id="cable-sizing-temperature"
+            onChange={(event) => setAmbientTemperatureC(Number(event.target.value))}
+            value={ambientTemperatureC}
+          >
+            {temperatureOptions(layingMethod).map((temp) => (
+              <option key={temp} value={temp}>
+                {temp}°C
+              </option>
+            ))}
+          </select>
+        </div>
+        <div className="col-12 col-md-6">
+          <label className="form-label" htmlFor="cable-sizing-grouping">
+            Häufung (Stromkreise nebeneinander)
+          </label>
+          <select
+            className="form-select"
+            id="cable-sizing-grouping"
+            onChange={(event) => setGroupingCircuits(Number(event.target.value))}
+            value={groupingCircuits}
+          >
+            {GROUPING_OPTIONS.map((grouping) => (
+              <option key={grouping} value={grouping}>
+                {GROUPING_LABELS[grouping]}
+              </option>
+            ))}
+          </select>
+        </div>
+        {phase === 3 && (
+          <div className="col-12 col-md-6">
+            <label className="form-label" htmlFor="cable-sizing-harmonics">
+              Oberschwingungsanteil Neutralleiter
+            </label>
+            <select
+              className="form-select"
+              id="cable-sizing-harmonics"
+              onChange={(event) =>
+                setHarmonicNeutralLoad(event.target.value as HarmonicNeutralLoad)
+              }
+              value={harmonicNeutralLoad}
+            >
+              {HARMONIC_NEUTRAL_LOAD_OPTIONS.map((option) => (
+                <option key={option} value={option}>
+                  {HARMONIC_NEUTRAL_LOAD_LABELS[option]}
+                </option>
+              ))}
+            </select>
+          </div>
+        )}
+        <div className="col-12 col-md-6">
+          <label className="form-label" htmlFor="cable-sizing-cosphi">
+            cos φ
+          </label>
+          <input
+            className="form-range"
+            id="cable-sizing-cosphi"
+            max={1}
+            min={0.6}
+            onChange={(event) => setCosPhi(Number(event.target.value))}
+            step={0.05}
+            type="range"
+            value={cosPhi}
+          />
+          <span>{cosPhi.toFixed(2)}</span>
+        </div>
+        <div className="col-12 col-md-6">
+          <label className="form-label" htmlFor="cable-sizing-voltage-drop">
+            Max. zulässiger Spannungsfall
+          </label>
+          <input
+            className="form-range"
+            id="cable-sizing-voltage-drop"
+            max={5}
+            min={1}
+            onChange={(event) => setMaxVoltageDropPercent(Number(event.target.value))}
+            step={0.5}
+            type="range"
+            value={maxVoltageDropPercent}
+          />
+          <span>{maxVoltageDropPercent.toFixed(1)} %</span>
+        </div>
+      </div>
+
+      <button
+        className="btn btn-secondary mt-3"
+        disabled={calculating}
+        onClick={handleCalculate}
+        type="button"
+      >
+        {calculating ? "Berechnet…" : "Berechnen"}
+      </button>
+      {error && <p className="text-danger mt-2">{error}</p>}
+
+      {result && !result.dataVerified && (
+        <div className="alert alert-danger mt-3">
+          Für Verlegeart {layingMethod} mit Isolierstoff {insulation.toUpperCase()} liegen keine
+          geprüften Strombelastbarkeits-Tabellenwerte vor - keine Dimensionierung möglich. Bitte
+          eine andere Verlegeart/Isolierstoff-Kombination wählen.
+        </div>
+      )}
+
+      {result && result.dataVerified && (
+        <div className="mt-3">
+          <div className="row g-2">
+            <div className="col-6 col-md-3">
+              <div className="text-muted small">Betriebsstrom Ib</div>
+              <div>{result.operatingCurrentA.toFixed(1)} A</div>
+            </div>
+            <div className="col-6 col-md-3">
+              <div className="text-muted small">
+                Bemessungsstrom (für Querschnittswahl){circuit.protectionDevice ? " *" : ""}
+              </div>
+              <div>
+                <strong>{result.designCurrentA.toFixed(1)} A</strong>
+              </div>
+            </div>
+            <div className="col-6 col-md-3">
+              <div className="text-muted small">Empfohlener Querschnitt</div>
+              <div>
+                <strong>
+                  {result.recommendedCrossSectionMm2
+                    ? `${result.recommendedCrossSectionMm2} mm²`
+                    : "-"}
+                </strong>
+              </div>
+            </div>
+            <div className="col-6 col-md-3">
+              <div className="text-muted small">ΔU bei Empfehlung</div>
+              <div>
+                {result.voltageDropAtRecommendedPercent != null
+                  ? `${result.voltageDropAtRecommendedPercent.toFixed(2)} %`
+                  : "-"}
+              </div>
+            </div>
+            <div className="col-6 col-md-3">
+              <div className="text-muted small">Korrekturfaktor</div>
+              <div>{result.combinedDerationFactor.toFixed(2)}</div>
+            </div>
+            <div className="col-6 col-md-3">
+              <div className="text-muted small">Max. Länge (ΔU-Grenze)</div>
+              <div>
+                {result.rows.find((row) => row.recommended)?.maxLengthForVoltageDropM != null
+                  ? `${result.rows
+                      .find((row) => row.recommended)!
+                      .maxLengthForVoltageDropM!.toFixed(0)} m`
+                  : "-"}
+              </div>
+            </div>
+          </div>
+          {alerts.map((alert, index) => (
+            <div
+              className={`alert mt-2 alert-${
+                alert.kind === "critical"
+                  ? "danger"
+                  : alert.kind === "warn"
+                    ? "warning"
+                    : alert.kind === "ok"
+                      ? "success"
+                      : "info"
+              }`}
+              key={index}
+            >
+              {alert.text}
+            </div>
+          ))}
+          {circuit.protectionDevice && (
+            <p className="text-muted small mt-2 mb-0">
+              * Bemessungsstrom = Maximum aus Betriebsstrom und vorhandener Sicherung
+              ({circuit.protectionDevice.ratedCurrentA} A) - die Sicherung löst erst bei ihrem
+              eigenen Bemessungsstrom aus, das Kabel muss also dafür ausgelegt sein, nicht nur
+              für die tatsächliche Last.
+            </p>
+          )}
+        </div>
+      )}
+
+      <hr className="my-3" />
+      <p className="text-muted small mb-2">
+        Querschnitt manuell eingeben oder eine Berechnung oben übernehmen - beides schreibt in
+        dasselbe Feld, du kannst es vor dem Übernehmen frei anpassen.
+      </p>
+      <div className="row g-3">
+        <div className="col-12 col-md-6">
+          <label className="form-label" htmlFor="cable-sizing-manual-type">
+            Kabeltyp
+          </label>
+          <input
+            className="form-control"
+            id="cable-sizing-manual-type"
+            onChange={(event) => setManualCableType(event.target.value)}
+            placeholder="z.B. NYM-J 3x1.5"
+            value={manualCableType}
+          />
+        </div>
+        <div className="col-12 col-md-6">
+          <label className="form-label" htmlFor="cable-sizing-manual-cross-section">
+            Querschnitt
+          </label>
+          <input
+            className="form-control"
+            id="cable-sizing-manual-cross-section"
+            onChange={(event) => setManualCrossSection(event.target.value)}
+            placeholder="z.B. 4 mm²"
+            value={manualCrossSection}
+          />
+        </div>
+        {manualCrossSectionWarning && (
+          <div className="col-12">
+            <div className="alert alert-warning mb-0">{manualCrossSectionWarning}</div>
+          </div>
+        )}
+      </div>
+    </FormModal>
+  );
+}
--- a/src/frontend/components/circuit-tree-editor.tsx
+++ b/src/frontend/components/circuit-tree-editor.tsx
@ -137,6 +137,7 @@ import type {
 import { DistributionBoardComponentModal } from "./distribution-board-component-modal";
 import { CircuitGroupModal } from "./circuit-group-modal";
 import { CircuitProtectionModal } from "./circuit-protection-modal";
+import { CableSizingModal } from "./cable-sizing-modal";
 import {
  circuitGroupCategoryLabels,
  type CircuitGroupCategory,
@ -278,6 +279,8 @@ export function CircuitTreeEditor(props: { projectId: string; circuitListId: str
    useState<CircuitGroupEditorIntent | null>(null);
  const [protectionEditorCircuit, setProtectionEditorCircuit] =
    useState<CircuitTreeCircuitDto | null>(null);
+  const [cableSizingEditorCircuit, setCableSizingEditorCircuit] =
+    useState<CircuitTreeCircuitDto | null>(null);
  const [projectDevices, setProjectDevices] = useState<ProjectDeviceDto[]>([]);
  const [isProjectDeviceDrawerOpen, setIsProjectDeviceDrawerOpen] =
    useState(false);
@ -1398,6 +1401,31 @@ export function CircuitTreeEditor(props: { projectId: string; circuitListId: str
    });
  }

+  async function handleApplyCableSizing(patch: {
+    cableCrossSection: string;
+    cableType?: string;
+    cableLength?: number;
+  }) {
+    const circuit = cableSizingEditorCircuit;
+    if (!circuit) {
+      return;
+    }
+    await runCommand({
+      label: "Kabel dimensionieren",
+      redo: async () => {
+        const result = await updateCircuitById(
+          projectId,
+          getExpectedProjectRevision(),
+          circuit.id,
+          patch
+        );
+        applyProjectCommandResult(result);
+        setCableSizingEditorCircuit(null);
+        return null;
+      },
+    });
+  }
+
  async function handleRedo() {
    if (historyBusy || isSaving || !historyState || historyState.redoDepth === 0) {
      return;
@ -3139,6 +3167,20 @@ export function CircuitTreeEditor(props: { projectId: string; circuitListId: str
          onSave={handleSaveCircuitProtection}
        />
      ) : null}
+      {cableSizingEditorCircuit ? (
+        <CableSizingModal
+          circuit={cableSizingEditorCircuit}
+          circuitCategory={
+            data.sections.find(
+              (section) => section.id === cableSizingEditorCircuit.sectionId
+            )?.category
+          }
+          isSaving={isSaving}
+          projectId={projectId}
+          onClose={() => setCableSizingEditorCircuit(null)}
+          onApply={handleApplyCableSizing}
+        />
+      ) : null}
      <div className="editor-toolbar">
        <button
          type="button"
@ -4240,10 +4282,15 @@ export function CircuitTreeEditor(props: { projectId: string; circuitListId: str
                        row.circuit &&
                        row.rowType !== "deviceRow"
                    );
+                    const isCableSizingTrigger = Boolean(
+                      (column.key === "cableSummary" || column.key === "cableCrossSection") &&
+                        row.circuit &&
+                        row.rowType !== "deviceRow"
+                    );
                    return (
                      <td
                        key={column.key}
-                        className={`${column.numeric ? "num" : ""} ${cell.editable ? "cell-editable" : ""} ${isProtectionTrigger ? "cell-protection-trigger" : ""} ${isSelected ? "cell-selected" : ""} ${hasIdentifierConflict ? "cell-invalid" : ""} ${
+                        className={`${column.numeric ? "num" : ""} ${cell.editable ? "cell-editable" : ""} ${isProtectionTrigger ? "cell-protection-trigger" : ""} ${isCableSizingTrigger ? "cell-cable-sizing-trigger" : ""} ${isSelected ? "cell-selected" : ""} ${hasIdentifierConflict ? "cell-invalid" : ""} ${
                          Boolean(row.device) && column.key === "displayName" && (row.rowType === "deviceRow" || row.rowType === "circuitCompact")
                            ? "device-drag-handle"
                            : ""
@ -4263,7 +4310,9 @@ export function CircuitTreeEditor(props: { projectId: string; circuitListId: str
                        title={
                          isProtectionTrigger
                            ? "Schutzgerät bearbeiten"
-                            : column.key === "equipmentIdentifier" &&
+                            : isCableSizingTrigger
+                              ? "Kabel dimensionieren"
+                              : column.key === "equipmentIdentifier" &&
                          row.circuit &&
                          (row.rowType === "circuitCompact" ||
                            row.rowType === "circuitSummary" ||
@ -4342,6 +4391,10 @@ export function CircuitTreeEditor(props: { projectId: string; circuitListId: str
                            setProtectionEditorCircuit(row.circuit!);
                            return;
                          }
+                          if (isCableSizingTrigger) {
+                            setCableSizingEditorCircuit(row.circuit!);
+                            return;
+                          }
                          if (cell.editable) {
                            handleRowSelectionClick(row, column.key, {
                              ctrlKey: event.ctrlKey,
--- a/src/server/composition/application-repositories.ts
+++ b/src/server/composition/application-repositories.ts
@ -13,6 +13,7 @@ import { ProjectRepository } from "../../db/repositories/project.repository.js";
 import { RoomRepository } from "../../db/repositories/room.repository.js";
 import { ExternalCsvConfigurationRepository } from "../../db/repositories/external-csv-configuration.repository.js";
 import { ExternalModelStateRepository } from "../../db/repositories/external-model-state.repository.js";
+import { CableSizingCalculationRepository } from "../../db/repositories/cable-sizing-calculation.repository.js";

 export const circuitDeviceRowRepository =
  new CircuitDeviceRowRepository(db);
@ -33,3 +34,4 @@ export const roomRepository = new RoomRepository(db);
 export const externalCsvConfigurationRepository =
  new ExternalCsvConfigurationRepository(db);
 export const externalModelStateRepository = new ExternalModelStateRepository(db);
+export const cableSizingCalculationRepository = new CableSizingCalculationRepository(db);
--- a/src/server/controllers/cable-sizing.controller.ts
+++ b/src/server/controllers/cable-sizing.controller.ts
@ -0,0 +1,99 @@
+import { randomUUID } from "node:crypto";
+import type { Request, Response } from "express";
+import { cableSizingRequestSchema } from "../../cable-sizing/domain/cable-sizing-contracts.js";
+import {
+  calculateCableSizing,
+  buildCableSizingAlerts,
+  LAYING_METHODS,
+  LAYING_METHOD_LABELS,
+  LAYING_METHOD_VERIFIED,
+  INSULATION_MATERIALS,
+  INSULATION_MATERIAL_LABELS,
+} from "../../cable-sizing/domain/cable-sizing-calculation.js";
+import { cableSizingCalculationRepository } from "../composition/application-repositories.js";
+
+// Stateless calculation, not a project command: no expectedRevision, no
+// circuit mutation here. The caller applies the result via the existing
+// circuit.update command (POST /api/projects/:projectId/commands) if the
+// user confirms it. See docs/cable-sizing-module.md.
+export async function calculateCableSizingHandler(req: Request, res: Response) {
+  const parsed = cableSizingRequestSchema.safeParse(req.body);
+  if (!parsed.success) {
+    return res.status(400).json({ error: parsed.error.flatten() });
+  }
+
+  const { context, ...input } = parsed.data;
+  const result = calculateCableSizing(input);
+  const alerts = buildCableSizingAlerts(input, result);
+
+  let entry;
+  try {
+    entry = await cableSizingCalculationRepository.create({
+      id: randomUUID(),
+      projectId: context?.projectId ?? null,
+      circuitId: context?.circuitId ?? null,
+      equipmentIdentifier: context?.equipmentIdentifier ?? null,
+      input,
+      result,
+      appliedToCircuit: 0,
+    });
+  } catch (error) {
+    // context.circuitId/projectId are caller-supplied and only used for the
+    // audit-log entry, not the calculation itself - a stale or unknown id
+    // (e.g. a circuit deleted between page load and this request) should be
+    // a normal 400, not a raw 500 from the foreign-key constraint.
+    if (
+      error &&
+      typeof error === "object" &&
+      "code" in error &&
+      (error as { code?: string }).code === "SQLITE_CONSTRAINT_FOREIGNKEY"
+    ) {
+      return res
+        .status(400)
+        .json({ error: "Unknown context.projectId or context.circuitId" });
+    }
+    throw error;
+  }
+
+  return res.status(201).json({ calculationId: entry.id, result, alerts });
+}
+
+export async function listLayingMethodsHandler(_req: Request, res: Response) {
+  return res.json(
+    LAYING_METHODS.map((method) => ({
+      method,
+      label: LAYING_METHOD_LABELS[method],
+      dataVerified: LAYING_METHOD_VERIFIED[method],
+    }))
+  );
+}
+
+export async function listInsulationMaterialsHandler(_req: Request, res: Response) {
+  return res.json(
+    INSULATION_MATERIALS.map((insulation) => ({
+      insulation,
+      label: INSULATION_MATERIAL_LABELS[insulation],
+    }))
+  );
+}
+
+export async function markCalculationAppliedHandler(req: Request, res: Response) {
+  const { calculationId } = req.params;
+  if (typeof calculationId !== "string") {
+    return res.status(400).json({ error: "Invalid calculationId" });
+  }
+  const updated = await cableSizingCalculationRepository.markApplied(calculationId);
+  if (!updated) {
+    return res.status(404).json({ error: "Calculation not found" });
+  }
+  return res.json(updated);
+}
+
+export async function listCalculationsForCircuitHandler(req: Request, res: Response) {
+  const { circuitId } = req.params;
+  if (typeof circuitId !== "string") {
+    return res.status(400).json({ error: "Invalid circuitId" });
+  }
+  const rows = await cableSizingCalculationRepository.listByCircuit(circuitId);
+  return res.json(rows);
+}
--- a/src/server/index.ts
+++ b/src/server/index.ts
@ -1,6 +1,7 @@
 import express from "express";
 import { globalDeviceRouter } from "./routes/global-device.routes.js";
 import { projectDeviceRouter } from "./routes/project-device.routes.js";
+import { cableSizingRouter } from "./routes/cable-sizing.routes.js";
 import { projectRouter } from "./routes/project.routes.js";
 import { errorMiddleware } from "./middleware/error.middleware.js";
 import { createLogger, toErrorMeta } from "../shared/logging/logger.js";
@ -49,6 +50,7 @@ app.get("/health", (_req, res) => {
 app.use("/api/projects", projectRouter);
 app.use("/api/global-devices", globalDeviceRouter);
 app.use("/api/project-devices", projectDeviceRouter);
+app.use("/api/cable-sizing", cableSizingRouter);

 app.use(errorMiddleware);

--- a/src/server/routes/cable-sizing.routes.ts
+++ b/src/server/routes/cable-sizing.routes.ts
@ -0,0 +1,19 @@
+import express from "express";
+import * as cableSizingController from "../controllers/cable-sizing.controller.js";
+
+export const cableSizingRouter = express.Router();
+
+cableSizingRouter.post("/calculate", cableSizingController.calculateCableSizingHandler);
+cableSizingRouter.get("/laying-methods", cableSizingController.listLayingMethodsHandler);
+cableSizingRouter.get(
+  "/insulation-materials",
+  cableSizingController.listInsulationMaterialsHandler
+);
+cableSizingRouter.post(
+  "/calculations/:calculationId/applied",
+  cableSizingController.markCalculationAppliedHandler
+);
+cableSizingRouter.get(
+  "/circuits/:circuitId/calculations",
+  cableSizingController.listCalculationsForCircuitHandler
+);
--- a/tests/cable-sizing-calculation.repository.test.ts
+++ b/tests/cable-sizing-calculation.repository.test.ts
@ -0,0 +1,152 @@
+import path from "node:path";
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+import { eq } from "drizzle-orm";
+import { migrate } from "drizzle-orm/better-sqlite3/migrator";
+import { createDatabaseContext, type DatabaseContext } from "../src/db/database-context.js";
+import { CableSizingCalculationRepository } from "../src/db/repositories/cable-sizing-calculation.repository.js";
+import { circuitLists } from "../src/db/schema/circuit-lists.js";
+import { circuitSections } from "../src/db/schema/circuit-sections.js";
+import { circuits } from "../src/db/schema/circuits.js";
+import { projects } from "../src/db/schema/projects.js";
+import { DistributionBoardFixtureRepository } from "./support/distribution-board-fixture.js";
+
+function createRepository() {
+  const context = createDatabaseContext(":memory:");
+  migrate(context.db, { migrationsFolder: path.resolve("src", "db", "migrations") });
+  return new CableSizingCalculationRepository(context.db);
+}
+
+function createRepositoryWithCircuits(): {
+  repository: CableSizingCalculationRepository;
+  context: DatabaseContext;
+  circuitOneId: string;
+  circuitTwoId: string;
+} {
+  const context = createDatabaseContext(":memory:");
+  migrate(context.db, { migrationsFolder: path.resolve("src", "db", "migrations") });
+  context.db.insert(projects).values({ id: "project-1", name: "Test project" }).run();
+  const board = new DistributionBoardFixtureRepository(context.db).createWithCircuitListAndDefaultSections(
+    "project-1",
+    "UV-01"
+  );
+  const circuitList = context.db
+    .select()
+    .from(circuitLists)
+    .where(eq(circuitLists.distributionBoardId, board.id))
+    .get();
+  if (!circuitList) {
+    throw new Error("fixture did not create a circuit list");
+  }
+  const section = context.db
+    .select()
+    .from(circuitSections)
+    .where(eq(circuitSections.circuitListId, circuitList.id))
+    .get();
+  if (!section) {
+    throw new Error("fixture did not create a section");
+  }
+  const circuitOneId = "circuit-1";
+  const circuitTwoId = "circuit-2";
+  context.db
+    .insert(circuits)
+    .values([
+      {
+        id: circuitOneId,
+        circuitListId: section.circuitListId,
+        sectionId: section.id,
+        equipmentIdentifier: "-1F1.1",
+      },
+      {
+        id: circuitTwoId,
+        circuitListId: section.circuitListId,
+        sectionId: section.id,
+        equipmentIdentifier: "-1F1.2",
+      },
+    ])
+    .run();
+  return {
+    repository: new CableSizingCalculationRepository(context.db),
+    context,
+    circuitOneId,
+    circuitTwoId,
+  };
+}
+
+describe("CableSizingCalculationRepository", () => {
+  it("creates an entry with input/result JSON and defaults appliedToCircuit to false", () => {
+    const repository = createRepository();
+    const entry = repository.create({
+      id: "calc-1",
+      projectId: null,
+      circuitId: null,
+      equipmentIdentifier: "-1F1.1",
+      input: { layingMethod: "C" },
+      result: { recommendedCrossSectionMm2: 4 },
+      appliedToCircuit: 0,
+    });
+    assert.equal(entry.id, "calc-1");
+    assert.equal(entry.appliedToCircuit, 0);
+    assert.deepEqual(entry.input, { layingMethod: "C" });
+    assert.deepEqual(entry.result, { recommendedCrossSectionMm2: 4 });
+  });
+
+  it("lists entries by circuitId, most recent first", () => {
+    // createdAt defaults to SQLite's unixepoch() (second resolution), so two
+    // inserts in the same test can tie - pass explicit, distinct timestamps
+    // instead of racing the clock. circuitId has a real foreign key into
+    // circuits (foreign_keys = ON), so this needs actual circuit rows, not
+    // arbitrary strings.
+    const { repository, circuitOneId, circuitTwoId } = createRepositoryWithCircuits();
+    repository.create({
+      id: "calc-a",
+      projectId: null,
+      circuitId: circuitOneId,
+      equipmentIdentifier: null,
+      input: {},
+      result: {},
+      appliedToCircuit: 0,
+      createdAt: new Date(1000),
+    });
+    repository.create({
+      id: "calc-b",
+      projectId: null,
+      circuitId: circuitOneId,
+      equipmentIdentifier: null,
+      input: {},
+      result: {},
+      appliedToCircuit: 0,
+      createdAt: new Date(2000),
+    });
+    repository.create({
+      id: "calc-other-circuit",
+      projectId: null,
+      circuitId: circuitTwoId,
+      equipmentIdentifier: null,
+      input: {},
+      result: {},
+      appliedToCircuit: 0,
+      createdAt: new Date(3000),
+    });
+
+    const rows = repository.listByCircuit(circuitOneId);
+    assert.equal(rows.length, 2);
+    assert.equal(rows[0].id, "calc-b");
+    assert.equal(rows[1].id, "calc-a");
+  });
+
+  it("marks an entry as applied", () => {
+    const repository = createRepository();
+    repository.create({
+      id: "calc-1",
+      projectId: null,
+      circuitId: null,
+      equipmentIdentifier: null,
+      input: {},
+      result: {},
+      appliedToCircuit: 0,
+    });
+    const updated = repository.markApplied("calc-1");
+    assert.equal(updated?.appliedToCircuit, 1);
+  });
+});
--- a/tests/cable-sizing-calculation.test.ts
+++ b/tests/cable-sizing-calculation.test.ts
@ -0,0 +1,209 @@
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+import {
+  buildCableSizingAlerts,
+  calculateCableSizing,
+  isCableSizingDataVerified,
+  type CableSizingInput,
+} from "../src/cable-sizing/domain/cable-sizing-calculation.js";
+
+const BASE_INPUT: CableSizingInput = {
+  phase: 1,
+  mode: "power",
+  powerKw: 5,
+  cosPhi: 1,
+  voltage: 230,
+  lengthM: 30,
+  layingMethod: "C",
+  conductorMaterial: "copper",
+  insulation: "pvc",
+  ambientTemperatureC: 30,
+  groupingCircuits: 1,
+  maxVoltageDropPercent: 3,
+  harmonicNeutralLoad: "none",
+};
+
+describe("calculateCableSizing", () => {
+  it("matches the verified reference case (1~, 5 kW, 230 V, 30 m, method C, copper)", () => {
+    const result = calculateCableSizing(BASE_INPUT);
+    assert.equal(result.dataVerified, true);
+    assert.ok(Math.abs(result.operatingCurrentA - 21.739) < 0.01);
+    assert.equal(result.crossSectionByCapacityMm2, 2.5);
+    assert.equal(result.crossSectionByVoltageDropMm2, 4);
+    assert.equal(result.recommendedCrossSectionMm2, 4);
+  });
+
+  it("returns dataVerified: false and no numeric result for an unverified laying method", () => {
+    const result = calculateCableSizing({ ...BASE_INPUT, layingMethod: "A2" });
+    assert.equal(result.dataVerified, false);
+    assert.equal(result.recommendedCrossSectionMm2, null);
+    assert.equal(result.rows.length, 0);
+    // The operating current itself does not depend on the capacity table
+    // and is still reported so the UI can show at least that much.
+    assert.ok(Math.abs(result.operatingCurrentA - 21.739) < 0.01);
+  });
+
+  it("returns dataVerified: false for xlpe regardless of method", () => {
+    const result = calculateCableSizing({ ...BASE_INPUT, insulation: "xlpe" });
+    assert.equal(result.dataVerified, false);
+  });
+
+  it("isCableSizingDataVerified matches the six ported, verified methods only", () => {
+    for (const method of ["A1", "B2", "C", "E", "D1", "D2"] as const) {
+      assert.equal(isCableSizingDataVerified(method, "pvc"), true, method);
+    }
+    for (const method of ["A2", "B1", "F", "G"] as const) {
+      assert.equal(isCableSizingDataVerified(method, "pvc"), false, method);
+    }
+  });
+
+  it("applies the 0.86 harmonic reduction factor only for three-phase + 15to33Percent", () => {
+    const threePhase: CableSizingInput = {
+      ...BASE_INPUT,
+      phase: 3,
+      mode: "current",
+      currentA: 10,
+      powerKw: undefined,
+      voltage: 400,
+    };
+    const base = calculateCableSizing({ ...threePhase, harmonicNeutralLoad: "none" });
+    const derated = calculateCableSizing({
+      ...threePhase,
+      harmonicNeutralLoad: "15to33Percent",
+    });
+    assert.equal(base.harmonicReductionApplied, false);
+    assert.equal(derated.harmonicReductionApplied, true);
+    assert.ok(
+      Math.abs(derated.combinedDerationFactor - base.combinedDerationFactor * 0.86) < 1e-9
+    );
+
+    const singlePhaseWithHarmonics = calculateCableSizing({
+      ...BASE_INPUT,
+      harmonicNeutralLoad: "15to33Percent",
+    });
+    assert.equal(singlePhaseWithHarmonics.harmonicReductionApplied, false);
+  });
+
+  it("uses max(operatingCurrentA, existingProtectionRatedCurrentA) as the design current for cross-section selection", () => {
+    // Load alone (21.7 A) would recommend 4 mm² (see the reference case
+    // above); a 32 A breaker on the same circuit must still be covered by
+    // the cable (In <= Iz), so the recommendation should grow accordingly.
+    const withoutBreaker = calculateCableSizing(BASE_INPUT);
+    const withBreaker = calculateCableSizing({
+      ...BASE_INPUT,
+      existingProtectionRatedCurrentA: 32,
+    });
+    assert.equal(withoutBreaker.designCurrentA, withoutBreaker.operatingCurrentA);
+    assert.equal(withBreaker.designCurrentA, 32);
+    assert.ok(
+      (withBreaker.recommendedCrossSectionMm2 ?? 0) >=
+        (withoutBreaker.recommendedCrossSectionMm2 ?? 0)
+    );
+    assert.ok(withBreaker.protectionCoordination?.coordinated);
+  });
+
+  it("reports an oversized breaker as the limiting factor when no cross-section can cover it", () => {
+    const result = calculateCableSizing({
+      ...BASE_INPUT,
+      existingProtectionRatedCurrentA: 1000,
+    });
+    assert.equal(result.designCurrentA, 1000);
+    assert.equal(result.recommendedCrossSectionMm2, null);
+    // No cross-section satisfies the design current at all, so there is no
+    // "recommended but under-protected" case to flag - protectionCoordination
+    // is only meaningful once a recommendation exists.
+    assert.equal(result.protectionCoordination, null);
+
+    const alerts = buildCableSizingAlerts(
+      { ...BASE_INPUT, existingProtectionRatedCurrentA: 1000 },
+      result
+    );
+    assert.equal(alerts.length, 1);
+    assert.equal(alerts[0].kind, "critical");
+    assert.ok(alerts[0].text.includes("Vorhandene Sicherung"));
+  });
+});
+
+describe("buildCableSizingAlerts", () => {
+  it("returns a single critical alert for an unverified combination, no numeric claims", () => {
+    const input: CableSizingInput = { ...BASE_INPUT, layingMethod: "G" };
+    const result = calculateCableSizing(input);
+    const alerts = buildCableSizingAlerts(input, result);
+    assert.equal(alerts.length, 1);
+    assert.equal(alerts[0].kind, "critical");
+    assert.ok(alerts[0].text.includes("keine geprüften"));
+  });
+
+  it("returns an ok alert plus a max-length info alert for a clean, unremarkable case", () => {
+    const input: CableSizingInput = {
+      ...BASE_INPUT,
+      mode: "current",
+      currentA: 15,
+      powerKw: undefined,
+      lengthM: 3,
+    };
+    const result = calculateCableSizing(input);
+    const alerts = buildCableSizingAlerts(input, result);
+    assert.equal(alerts.length, 2);
+    assert.equal(alerts[0].kind, "ok");
+    assert.equal(alerts[1].kind, "info");
+    assert.ok(alerts[1].text.includes("Maximale Länge"));
+  });
+});
+
+describe("maxLengthForVoltageDropM", () => {
+  it("is the inverse of the voltage-drop formula: recalculating at that length gives back the limit", () => {
+    const result = calculateCableSizing(BASE_INPUT);
+    const recommendedRow = result.rows.find((row) => row.recommended);
+    assert.ok(recommendedRow?.maxLengthForVoltageDropM != null);
+
+    const atMaxLength = calculateCableSizing({
+      ...BASE_INPUT,
+      lengthM: recommendedRow!.maxLengthForVoltageDropM!,
+    });
+    const rowAtSameCrossSection = atMaxLength.rows.find(
+      (row) => row.crossSectionMm2 === recommendedRow!.crossSectionMm2
+    );
+    assert.ok(
+      Math.abs(rowAtSameCrossSection!.voltageDropPercent! - BASE_INPUT.maxVoltageDropPercent) <
+        0.01
+    );
+  });
+
+  it("is null when there is no current flowing (division by zero guard)", () => {
+    const result = calculateCableSizing({ ...BASE_INPUT, mode: "current", currentA: 0, powerKw: undefined });
+    assert.ok(result.rows.every((row) => row.maxLengthForVoltageDropM === null));
+  });
+});
+
+describe("practical minimum cross-section for single_phase circuits", () => {
+  it("raises a smaller calculated recommendation to 2.5 mm² for single_phase circuits", () => {
+    // 1 A load at 30 m would normally recommend 1.5 mm² by calculation alone.
+    const smallLoad: CableSizingInput = { ...BASE_INPUT, mode: "current", currentA: 1, powerKw: undefined };
+    const withoutCategory = calculateCableSizing(smallLoad);
+    const withCategory = calculateCableSizing({ ...smallLoad, circuitCategory: "single_phase" });
+
+    assert.equal(withoutCategory.recommendedCrossSectionMm2, 1.5);
+    assert.equal(withoutCategory.practicalMinimumApplied, false);
+    assert.equal(withCategory.recommendedCrossSectionMm2, 2.5);
+    assert.equal(withCategory.practicalMinimumApplied, true);
+  });
+
+  it("never lowers a recommendation that already needs more than the practical minimum", () => {
+    const result = calculateCableSizing({ ...BASE_INPUT, circuitCategory: "single_phase" });
+    assert.equal(result.recommendedCrossSectionMm2, 4);
+    assert.equal(result.practicalMinimumApplied, false);
+  });
+
+  it("does not apply to lighting or three_phase categories", () => {
+    const smallLoad: CableSizingInput = { ...BASE_INPUT, mode: "current", currentA: 1, powerKw: undefined };
+    assert.equal(
+      calculateCableSizing({ ...smallLoad, circuitCategory: "lighting" }).recommendedCrossSectionMm2,
+      1.5
+    );
+    assert.equal(
+      calculateCableSizing({ ...smallLoad, circuitCategory: "three_phase" }).recommendedCrossSectionMm2,
+      1.5
+    );
+  });
+});