Cable size for 20 A over 50 m, three-phase

For 20 A over 50 m on a 400 V three-phase supply, use at least 4 mm² (12 AWG) copper cable, protected by a 25 A breaker. Voltage drop: 2.4%.

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	4 mm² (12 AWG)	0.5%
20 m	4 mm² (12 AWG)	1%
30 m	4 mm² (12 AWG)	1.5%
50 m	4 mm² (12 AWG)	2.4%
100 m	4 mm² (12 AWG)	4.9%

4 mm²
12 AWG: 25 A
Breaker: 2.4%
Voltage drop: 56 m
5-core cable

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

Voltage between phasesV

How the cable runs

Use at least4 mm²12 AWG

Breaker25 A

Voltage drop2.4% · 9.74 V

Current20 A

Cable carries up to32 A

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

Cores

5 cores: L1, L2, L3, neutral and earth (4 cores if no neutral is needed)

Which cable

NYM-J / XVB / 5-core LSZH

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.