How the cable size is chosen
Current flows from the battery to the load and back, so the cable length counts twice. The calculator works out the resistance of the whole loop, the voltage lost across it at your current and the thinnest standard size that keeps that loss within your limit. Then it checks that the size can carry the current with the margin a fuse needs.
R = ρ × L_total / A ρ = 0.01724 Ω·mm²/m (copper, 20 °C) ΔV = I × R drop % = ΔV / V × 100 A_min = ρ × L_total × I / (V × drop % / 100)
At low voltage the drop matters much more than at mains voltage. Losing 0.5 V is less than 0.3 % of 230 V, but more than 4 % of 12 V: enough to make a compressor fridge cut out or an inverter shut down on low voltage.
3 % or 10 %: which voltage drop to allow
Marine practice (ABYC E-11) uses two limits: 3 % for circuits where voltage matters, such as panelboard main feeders, electronic equipment, navigation lights and bilge blowers, and 10 % for loads that tolerate it, such as cabin lighting. Vans and off-grid systems follow similar logic: 3 % for anything with a motor, a charger or electronics, up to 10 % for lights.
Maximum cable length at 12 V
| Size | 5 A | 10 A | 20 A | 30 A | 50 A |
|---|---|---|---|---|---|
| 16 AWG | 9 ft | 4.5 ft | 2.2 ft | 1.5 ft | 0.9 ft |
| 14 AWG | 14 ft | 7.1 ft | 3.6 ft | 2.4 ft | 1.4 ft |
| 12 AWG | 23 ft | 11 ft | 5.7 ft | 3.8 ft | 2.3 ft |
| 10 AWG | 36 ft | 18 ft | 9 ft | 6 ft | 3.6 ft |
| 8 AWG | 57 ft | 29 ft | 14 ft | 9.6 ft | 5.7 ft |
| 6 AWG | 91 ft | 46 ft | 23 ft | 15 ft | 9.1 ft |
| 4 AWG | 145 ft | 72 ft | 36 ft | 24 ft | 14 ft |
| 2 AWG | 230 ft | 115 ft | 58 ft | 38 ft | 23 ft |
Longest one-way run, in feet, for a 3 % drop at 12 V on a two-wire circuit with copper at 20 °C. At 24 V the lengths double.
Frequently asked questions
What size wire do I need for 12 V at 10 amps?
It depends on the length. With a 3 % drop limit, 10 A at 12 V needs 14 AWG for a 6 ft run (one way) and 10 AWG for 15 ft. In metric, that is about 2.5 mm² for 2 m and 6 mm² for 5 m.
Do I enter the one-way or the round-trip length?
The one-way length, from the battery or fuse box to the load. The calculator doubles it for the return wire, unless you choose a chassis return.
Should I design for a 3 % or a 10 % voltage drop?
Use 3 % for anything with electronics, a motor or a charger, and for the main cables from the battery. 10 % is acceptable for lighting and other loads that work fine at a slightly lower voltage.
Why does a 24 V system need thinner cables?
For the same power, doubling the voltage halves the current, and the percentage drop falls to a quarter. A 120 W load over 5 m needs 6 mm² at 12 V but only 1.5 mm² at 24 V.
What fuse should protect the cable?
The fuse protects the cable, so its rating must not exceed what the cable can carry. Pick at least 1.25 times the load current so it doesn’t blow in normal use, and fit it as close to the battery as possible.
How do mm² and AWG sizes compare?
AWG numbers go down as the wire gets thicker. Common pairs are 1.5 mm² ≈ 16 AWG, 2.5 mm² ≈ 14 AWG, 4 mm² ≈ 12 AWG, 6 mm² ≈ 10 AWG, 10 mm² ≈ 8 AWG, 16 mm² ≈ 6 AWG and 25 mm² ≈ 4 AWG. The sizes are close but not identical, so check the drop again when you swap one for the other.