Cable size for 40 A over 20 m, single-phase

For 40 A over 20 m on a 230 V single-phase supply, use at least 10 mm² (8 AWG) copper cable, protected by a 50 A breaker. Voltage drop: 1.6%.

Assumes copper cable clipped to a wall (or flexible battery cable for DC), 30 °C, PVC insulation. Change anything below.

The same current over other distances

Distance	Cable size	Voltage drop
10 m	10 mm² (8 AWG)	0.8%
20 m	10 mm² (8 AWG)	1.6%
30 m	10 mm² (8 AWG)	2.3%
50 m	10 mm² (8 AWG)	3.9%
100 m	16 mm² (6 AWG)	4.9%

10 mm²
8 AWG: 50 A
Breaker: 1.6%
Voltage drop: 22 m
3-core cable

Current at the end of the cableDistance (one way)m

VoltageV

How the cable runs

Use at least10 mm²8 AWG

Breaker50 A

Voltage drop1.6% · 3.6 V

Current40 A

Cable carries up to63 A

Heat decides this size: it carries the breaker current without overheating, and the voltage drop is fine.

Cores

3 cores: line, neutral and earth

Which cable

Twin & Earth 6242Y (UK) or NYM-J (Europe)

Fixed wiring clipped to walls indoors.

H07RN-F (rubber, flexible)

Machines, workshops and anything that moves or vibrates.

Several circuits bundled together carry less each: allow for grouping (IEC 60364-5-52 Table B.52.17).
Check the earth fault loop impedance and the RCD protection required by your local rules.

Guidance based on IEC 60364-5-52 for copper cables. Always check against your local rules (BS 7671, NEC, NF C 15-100, RTIEBT…) and the cable maker’s data.

Planning a whole solar system?Describe it and we size every cable, fuse and part, with a wiring diagram.