Long cable runs lose voltage. Keep the drop within about 3% of supply voltage (≈ 6.9 V on a 230 V circuit). Use the values below with this formula:
Voltage drop (V) = (mV/A/m × Current A × Length m) ÷ 1000
| Conductor size (sq mm) | Approx. voltage drop (mV/A/m) |
|---|---|
| 1.0 | 44 |
| 1.5 | 29 |
| 2.5 | 18 |
| 4.0 | 11 |
| 6.0 | 7.3 |
| 10 | 4.4 |
| 16 | 2.8 |
Worked example
A 2.5 sq mm wire carrying 20 A over a 30 m run: 18 × 20 × 30 ÷ 1000 = 10.8 V — that is 4.7% of 230 V, over the 3% limit. Step up to 4 sq mm: 11 × 20 × 30 ÷ 1000 = 6.6 V (2.9%) — within limit.
For an instant check across sizes, loads and run lengths, use the APAR Wire-Size Calculator, which applies both ampacity and the 3% voltage-drop rule for you.