What this calculator does
Cable sizing has two independent constraints: the conductor must carry the current without overheating, and it must not waste too much voltage over the run. This calculator solves both together and returns the size that governs, which is the number most site engineers actually need.
How to use it
- Enter the load current, the supply voltage and the one-way route length.
- Select the circuit type — the three-phase case allows roughly 13 % more current for the same drop.
- Set the acceptable voltage-drop percentage.
- Read the recommended size together with its ampacity and the size that ampacity alone would require.
Governing formula
R_max = (%limit/100) · V · 1000 / (k · I · L) Where:
-
V= Supply voltage [V] -
I= Load current [A] -
L= One-way run length [m]
Derived Equations:
R_max = (p/100)·V·1000 / (k·I·L) size = first table row with R ≤ R_max and A ≥ I ΔV% = k·I·R·L / 1000 / V × 100 Worked example — 32 A, 45 m, 230 V single-phase at a 3 % limit
The figures below were produced by running this page's own engine with the example inputs (Design load current (A), Supply voltage (V), One-way conductor length (m), Circuit type, Acceptable voltage drop (%)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Recommended conductor size | 10 mm² |
|---|---|
| Resulting voltage drop | 2.29 % |
| Ampacity of that size | 55 A |
| Minimum size by ampacity alone | 4 mm² |
| Maximum permitted resistance | 2.4 Ω/km |
| Assessment | Minimum 10 mm² (55 A ampacity) holds drop to 2.3%. |
- With k = 2 the largest acceptable loop resistance falls well below the 6 mm² that 32 A ampacity alone would permit.
- Compare the recommended size against the "ampacity alone" size — when they differ, the drop governs and the cable is oversized for current but correct for the run.
Assumptions & limitations
Engineering calculation. Selects the smallest standard copper size that satisfies BOTH the ampacity table and your voltage-drop limit. Derating factors, grouping and ambient correction are not applied.
- Ampacity values are for copper in free air at 30 °C; apply the installation-method and grouping derating factors from the code before ordering.
- No thermal protection coordination, short-circuit capacity or earth-fault loop checks.
- Aluminium conductors are not in the table — convert to an equivalent copper resistance before use or size separately.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- IEC 60364-5-52 — current-carrying capacity and voltage drop.
- NEC 310.16 — allowable ampacities of insulated conductors.
- BS 7671 Appendix 4 — current-carrying capacity.