How the cable size is chosen
A wire is large enough when it passes two checks. It must carry the load current without overheating, and the voltage lost along it must stay within the limit. The calculator finds the smallest standard size for each check and recommends the larger of the two.
Current check: rated current of the size ≥ I
Voltage drop check: k · L · I · ρ ÷ A ≤ allowed % · U
- k
- 2 for single-phase and DC, 1.732 for three-phase
- L
- one-way length
- I
- load current in A
- ρ
- resistivity at 75 °C: copper 0.0210, aluminum 0.0345 Ω·mm²/m
- A
- conductor cross-section in mm²
- U
- supply voltage
On short runs the current rating decides. On long runs, and at low voltages such as 12 V or 24 V, voltage drop usually decides.
Worked example
A 30 A, 240 V single-phase load is 150 ft from the panel, copper wire, 3% allowed. For current, 10 AWG is enough (30 A under NEC 240.4(D)). For voltage drop, 10 AWG gives 4.6%, 8 AWG 2.9%. The calculator recommends 8 AWG.
Current ratings used
| AWG, copper | NEC, 75 °C | mm², copper | IEC method C, single-phase |
|---|---|---|---|
| 14 AWG | 15 A | 1.5 mm² | 19.5 A |
| 12 AWG | 20 A | 2.5 mm² | 27 A |
| 10 AWG | 30 A | 4 mm² | 36 A |
| 8 AWG | 50 A | 6 mm² | 46 A |
| 6 AWG | 65 A | 10 mm² | 63 A |
| 4 AWG | 85 A | 16 mm² | 85 A |
| 2 AWG | 115 A | 25 mm² | 112 A |
| 1/0 AWG | 150 A | 35 mm² | 138 A |
NEC values from Table 310.16, 75 °C column, with the 240.4(D) limits for 14 to 10 AWG. IEC values for PVC-insulated copper clipped direct at 30 °C, two loaded conductors.
When the rating must be reduced
The ratings above assume normal conditions. They drop when cables are bundled or run in a shared conduit, when the ambient temperature is above 30 °C, when the cable runs through thermal insulation, and in the US when terminals are only rated for 60 °C. Apply the correction factors of your code (NEC 310.15, BS 7671 Appendix 4, AS/NZS 3008) before the final choice, and have the work done by a licensed electrician.