How to Size a Cable to IEC 60364 — With a Live Calculator
Three checks, one cable size. Learn the IEC 60364 method for thermal current capacity, voltage drop, and short-circuit withstand — then run it on your own system.
Cable sizing is one of the most repetitive calculations in electrical engineering — and one of the easiest to get wrong. Choose too small a cable and you risk overheating, excessive voltage drop, or failure to withstand a fault. Choose too large and you waste money on copper.
IEC 60364-5-52 gives us a systematic three-check method. The largest cable cross-section required by any of the three checks governs the final selection.
The Three Checks
| Check | Standard Reference | What it verifies | Governs when… |
|---|---|---|---|
| 1. Thermal (current capacity) | IEC 60364-5-52 Table B.52 | Cable doesn’t overheat at rated current after derating | High load current or poor installation conditions |
| 2. Voltage drop | IEC 60364-5-52 Annex G | Voltage at load terminals stays within ±4% (or ±3% for lighting) | Long cable runs with moderate current |
| 3. Short-circuit withstand | IEC 60364-4-43 Adiabatic equation | Cable survives fault current until breaker clears | High fault levels with slow protection clearing times |
Check 1 — Thermal Current Capacity
The design current Ib must not exceed the derated cable current rating Iz:
It = tabulated rating · Ca = ambient temp factor · Cg = grouping factor · Ci = installation method factor
The installation method has the biggest effect. A cable buried in the ground (Method D) can carry significantly more current than the same cable clipped to a wall in a 45°C plant room (Method C with high ambient derating).
| Method | Description | Typical derating vs Method C |
|---|---|---|
| A1/A2 | Conduit in insulated wall | −15 to −25% |
| B1/B2 | Conduit on wall / trunking | −10 to −15% |
| C | Clipped direct to surface | Reference (0%) |
| D1/D2 | Buried in ground | +10 to +20% |
| E/F | Free air / perforated tray | +15 to +30% |
Check 2 — Voltage Drop
L = cable length (km) · r = resistance (Ω/km) · x = reactance (Ω/km) · cosφ = power factor
IEC 60364-5-52 recommends a maximum voltage drop of 4% for power circuits and 3% for lighting from the origin of the installation to the point of use. Long LV runs — MCC to a remote motor, or sub-distribution board at 200m — are almost always governed by voltage drop, not thermal rating.
Check 3 — Short-Circuit Withstand
IEC 60364-4-43 uses the adiabatic equation — assuming all fault heat stays in the conductor:
S = minimum cross-section (mm²) · Isc = fault current (A) · t = clearing time (s) · k = 143 for Cu/XLPE, 115 for Cu/PVC
This check only applies when you know the prospective fault current and the protection clearing time. For a 16 kA fault cleared in 0.1 s on a copper XLPE cable: S ≥ 16000 × √0.1 / 143 = 35.4 mm². You need at least 50 mm² to withstand the fault thermally.
Live Calculator — Size Your Cable Now
Enter your load and installation data. All three checks run simultaneously — the governing criterion determines the final recommendation.
Full GridSense study includes multi-cable grouping, SC withstand verification, cable schedule export, and IEC-compliant PDF report with all derating factors shown.
Worked Example — 55 kW Motor, 80m Run
A 55 kW, 415V, 3-phase motor in a plant room at 40°C, cables clipped direct (Method C), copper XLPE, 80m run, 4% voltage drop limit.
Design current: Ib = 55000 / (√3 × 415 × 0.85) = 90 A
Thermal check: At 40°C the temperature correction factor for XLPE is 0.91. A 25 mm² Cu XLPE cable has It = 110A. Derated: 110 × 0.91 = 100A > 90A. ✓
Voltage drop at 80m on 25 mm² (r = 0.727 Ω/km): ΔV = √3 × 90 × 0.08 × 0.727 × 0.85 = 7.8V = 1.9%. ✓ within 4%.
Result: 25 mm² copper XLPE, governed by thermal rating. The calculator above will confirm this — try entering the same values.
Summary
The IEC 60364 cable sizing method is a three-check process: thermal capacity after derating, voltage drop over the full run length, and short-circuit withstand if fault data is available. The worst-case check governs the final selection. The calculator above runs all three simultaneously against the same engine used in certified engineering studies.
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