src/app/globals.css

@@ -1233,11 +1233,22 @@ a.kpi:hover {
 }
 
 .tree-grid .cell-cable-sizing-trigger::before {
-  content: "🧮";
+  content: "";
   position: absolute;
   left: 0.2em;
-  font-size: 0.8em;
+  top: 50%;
-  opacity: 0.8;
+  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 {

src/cable-sizing/domain/cable-sizing-calculation.ts

@@ -209,6 +209,21 @@ export const GROUPING_LABELS: Record<number, string> = {
   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 {
@@ -235,6 +250,9 @@ export interface CableSizingInput {
   /** 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 {
@@ -244,6 +262,11 @@ export interface CrossSectionRow {
   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;
 }
 
@@ -272,6 +295,10 @@ export interface CableSizingResult {
   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;
 }
@@ -305,6 +332,7 @@ export function calculateCableSizing(input: CableSizingInput): CableSizingResult
       voltageDropAtRecommendedPercent: null,
       combinedDerationFactor: 1,
       harmonicReductionApplied: false,
+      practicalMinimumApplied: false,
       rows: [],
       protectionCoordination: null,
     };
@@ -351,11 +379,38 @@ export function calculateCableSizing(input: CableSizingInput): CableSizingResult
     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,
@@ -363,6 +418,7 @@ export function calculateCableSizing(input: CableSizingInput): CableSizingResult
       currentSufficient,
       voltageDropPercent: rated == null ? null : voltageDrops[i],
       voltageDropSufficient: rated == null ? null : voltageDropSufficient,
+      maxLengthForVoltageDropM,
       recommended: i === indexRecommended,
     };
   });
@@ -392,6 +448,7 @@ export function calculateCableSizing(input: CableSizingInput): CableSizingResult
       indexRecommended >= 0 ? voltageDrops[indexRecommended] : null,
     combinedDerationFactor,
     harmonicReductionApplied,
+    practicalMinimumApplied,
     rows,
     protectionCoordination,
   };
@@ -448,6 +505,12 @@ export function buildCableSizingAlerts(
       text: "3. Harmonische > 33 %: Neutralleiter muss wie ein Außenleiter dimensioniert werden - hier nicht automatisch berücksichtigt.",
     });
   }
+  if (result.practicalMinimumApplied) {
+    alerts.push({
+      kind: "info",
+      text: `Auf ${result.recommendedCrossSectionMm2} mm² angehoben (Praxis-Mindestquerschnitt für 1-phasige Stromkreise, keine reine Berechnungsanforderung).`,
+    });
+  }
   if (result.combinedDerationFactor < 1) {
     alerts.push({
       kind: "info",
@@ -473,5 +536,12 @@ export function buildCableSizingAlerts(
       text: `${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;
 }

src/cable-sizing/domain/cable-sizing-contracts.ts

@@ -26,6 +26,7 @@ export const cableSizingRequestSchema = z
     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

src/frontend/components/cable-sizing-modal.tsx

@@ -3,6 +3,7 @@
 import { type FormEvent, useMemo, useState } from "react";
 import {
   CONDUCTOR_MATERIALS,
+  CROSS_SECTIONS_MM2,
   GROUPING_LABELS,
   GROUPING_OPTIONS,
   HARMONIC_NEUTRAL_LOAD_LABELS,
@@ -12,11 +13,13 @@ import {
   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,
@@ -28,6 +31,7 @@ import { FormModal } from "./form-modal";
 
 interface CableSizingModalProps {
   circuit: CircuitTreeCircuitDto;
+  circuitCategory?: CircuitCategory;
   isSaving: boolean;
   projectId: string;
   onClose: () => void;
@@ -44,8 +48,19 @@ function temperatureOptions(method: LayingMethod) {
   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,
@@ -92,6 +107,7 @@ export function CableSizingModal({
       maxVoltageDropPercent,
       harmonicNeutralLoad,
       existingProtectionRatedCurrentA: circuit.protectionDevice?.ratedCurrentA,
+      circuitCategory,
     }),
     [
       phase,
@@ -107,9 +123,34 @@ export function CableSizingModal({
       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);
@@ -162,7 +203,9 @@ export function CableSizingModal({
       onSubmit={handleSubmit}
       submitDisabled={!manualCrossSection.trim()}
       submitLabel="Übernehmen"
-      title={`Kabel dimensionieren - ${circuit.equipmentIdentifier}`}
+      title={`Kabel dimensionieren - ${circuit.equipmentIdentifier}${
+        circuit.displayName ? ` (${circuit.displayName})` : ""
+      }`}
     >
       <div className="row g-3">
         <div className="col-12 col-md-4">
@@ -401,6 +444,16 @@ export function CableSizingModal({
               <div className="text-muted small">Korrekturfaktor</div>
               <div>{result.combinedDerationFactor.toFixed(2)}</div>
             </div>
+            <div className="col-6 col-md-3">
+              <div className="text-muted small">Max. Länge (ΔU-Grenze)</div>
+              <div>
+                {result.rows.find((row) => row.recommended)?.maxLengthForVoltageDropM != null
+                  ? `${result.rows
+                      .find((row) => row.recommended)!
+                      .maxLengthForVoltageDropM!.toFixed(0)} m`
+                  : "-"}
+              </div>
+            </div>
           </div>
           {alerts.map((alert, index) => (
             <div
@@ -459,6 +512,11 @@ export function CableSizingModal({
             value={manualCrossSection}
           />
         </div>
+        {manualCrossSectionWarning && (
+          <div className="col-12">
+            <div className="alert alert-warning mb-0">{manualCrossSectionWarning}</div>
+          </div>
+        )}
       </div>
     </FormModal>
   );

src/frontend/components/circuit-tree-editor.tsx

@@ -3170,6 +3170,11 @@ export function CircuitTreeEditor(props: { projectId: string; circuitListId: str
       {cableSizingEditorCircuit ? (
         <CableSizingModal
           circuit={cableSizingEditorCircuit}
+          circuitCategory={
+            data.sections.find(
+              (section) => section.id === cableSizingEditorCircuit.sectionId
+            )?.category
+          }
           isSaving={isSaving}
           projectId={projectId}
           onClose={() => setCableSizingEditorCircuit(null)}

tests/cable-sizing-calculation.test.ts

@@ -134,7 +134,7 @@ describe("buildCableSizingAlerts", () => {
     assert.ok(alerts[0].text.includes("keine geprüften"));
   });
 
-  it("returns a single ok alert for a clean, unremarkable case", () => {
+  it("returns an ok alert plus a max-length info alert for a clean, unremarkable case", () => {
     const input: CableSizingInput = {
       ...BASE_INPUT,
       mode: "current",
@@ -144,7 +144,66 @@ describe("buildCableSizingAlerts", () => {
     };
     const result = calculateCableSizing(input);
     const alerts = buildCableSizingAlerts(input, result);
-    assert.equal(alerts.length, 1);
+    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
+    );
   });
 });