A 50 kW/415 V feeder where the smaller cable that looks fine on paper actually fails the ampacity check — and the next size up clears both checks comfortably.
| Load | 50 kW, 415 V three-phase, power factor 0.85 |
| Conductor / insulation | Copper, PVC (70°C) |
| Installation method | Method C (clipped direct) |
| Route length | 50 m |
| Maximum permitted voltage drop | 5% |
IEC 60364-5-52 Table B.52.4 (Method C, PVC 70°C, copper, three loaded conductors) gives the tabulated current-carrying capacity.
| Size | Table ampacity | Required (Ib) | Result |
|---|---|---|---|
| 16 mm² | 76 A | 81.8 A | FAILS — 76 A < 81.8 A |
16 mm² looks like a reasonable first guess for an 82 A load, but it's 5.8 A short of the table rating.
| Size | Table ampacity | Required (Ib) | Result |
|---|---|---|---|
| 25 mm² | 96 A | 81.8 A | PASSES — 96 A ≥ 81.8 A |
2.64% is comfortably under the 5% limit, so voltage drop does not override the ampacity-driven selection here.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| 16 mm² ampacity | ≥ 81.8 A | 76 A | ✗ FAIL |
| 25 mm² ampacity | ≥ 81.8 A | 96 A | ✓ PASS |
| 25 mm² voltage drop | ≤ 5% | 2.64% | ✓ PASS |
Key insight: Ampacity and voltage drop are independent checks and either one can govern, depending on run length. Short, heavily loaded runs like this one are usually ampacity-limited; long, lightly loaded runs are usually voltage-drop-limited. Always check both rather than assuming one.
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Open Cable Sizing & Voltage Drop calculator →Voltage drop scales linearly with length while ampacity doesn't change with length at all, so there's always some length beyond which voltage drop overtakes ampacity as the limiting factor. For this 25 mm² cable at 2.64% for 50 m, voltage drop would reach the 5% limit at roughly 95 m — beyond that, a longer run at the same load would need a larger cable purely to control voltage drop, even though 25 mm² would still pass on ampacity alone.
This is exactly the table that was cross-checked cell-by-cell against the official IEC 60364-5-52:2009 standard PDF — two genuine transcription errors were found and corrected in that review (see the standards audit report), which is why every ampacity figure quoted in this example traces directly back to the standard's own published tables rather than a secondary/approximated source.