Cable size for 20 A over 5 m, DC

For 20 A over 5 m on a 12 V DC supply, use at least 16 mm² (6 AWG) copper cable, protected by a 25 A fuse. Voltage drop: 2.3%.

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
1 m	2.5 mm² (14 AWG)	3%
2 m	6 mm² (10 AWG)	2.5%
3 m	10 mm² (8 AWG)	2.2%
5 m	16 mm² (6 AWG)	2.3%
10 m	25 mm² (4 AWG)	3%

16 mm²
6 AWG: 25 A
Fuse: 2.3%
Voltage drop: 6 m
2-core cable

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

Voltage

How the cable runs

Use at least16 mm²6 AWG

Fuse25 A

Voltage drop2.3% · 0.28 V

Current20 A

Cable carries up to114 A

The distance decides this size: 1.5 mm² would stay cool enough, but over 5 m it loses too much voltage.

Cores

2 cores (+ and −), or two single cables, red and black

Which cable

Tinned flexible battery cable (ISO 6722 / SAE J1127)

Vans and boats: resists vibration and corrosion.

Welding cable (H01N2-D)

Very flexible, for heavy battery and inverter leads.

Several circuits bundled together carry less each: allow for grouping (IEC 60364-5-52 Table B.52.17).

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.