Cable size for 30 A over 3 m, DC

For 30 A over 3 m on a 12 V DC supply, use at least 16 mm² (6 AWG) copper cable, protected by a 40 A fuse. Voltage drop: 2.1%.

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	4 mm² (12 AWG)	2.8%
2 m	10 mm² (8 AWG)	2.2%
3 m	16 mm² (6 AWG)	2.1%
5 m	25 mm² (4 AWG)	2.3%
10 m	50 mm² (1 AWG)	2.3%

16 mm²
6 AWG: 40 A
Fuse: 2.1%
Voltage drop: 4 m
2-core cable

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

Voltage

How the cable runs

Use at least16 mm²6 AWG

Fuse40 A

Voltage drop2.1% · 0.25 V

Current30 A

Cable carries up to114 A

The distance decides this size: 4 mm² would stay cool enough, but over 3 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.