12V cable size chart for campervans (mm², length and fuse)
Choosing cable for a 12 V van takes two checks: 1. Voltage drop: is the cable thick enough for its length? 2. Current rating: is the cable rated above the fuse that protects it?
Chart: maximum one-way length (metres) for a 3% drop at 12 V
3% of 12 V = 0.36 V, the usual limit for fridges, pumps, chargers and anything sensitive. "—" means the cable's current rating (105 °C insulation) is too low for the fuse that current needs.
| Current | 1.5 mm² | 2.5 mm² | 4 mm² | 6 mm² | 10 mm² | 16 mm² | 25 mm² | 35 mm² | 50 mm² |
|---|---|---|---|---|---|---|---|---|---|
| 5 A | 3.1 | 5.1 | 8.2 | 12.3 | 20.6 | 32.9 | 51.4 | 72.0 | 102.9 |
| 10 A | 1.5 | 2.6 | 4.1 | 6.2 | 10.3 | 16.5 | 25.7 | 36.0 | 51.4 |
| 15 A | 1.0 | 1.7 | 2.7 | 4.1 | 6.9 | 11.0 | 17.1 | 24.0 | 34.3 |
| 20 A | 0.8 | 1.3 | 2.1 | 3.1 | 5.1 | 8.2 | 12.9 | 18.0 | 25.7 |
| 30 A | — | — | 1.4 | 2.1 | 3.4 | 5.5 | 8.6 | 12.0 | 17.1 |
| 40 A | — | — | — | 1.5 | 2.6 | 4.1 | 6.4 | 9.0 | 12.9 |
| 60 A | — | — | — | — | 1.7 | 2.7 | 4.3 | 6.0 | 8.6 |
| 80 A | — | — | — | — | — | 2.1 | 3.2 | 4.5 | 6.4 |
| 100 A | — | — | — | — | — | 1.6 | 2.6 | 3.6 | 5.1 |
Length is one-way (battery to appliance). The formula already counts the return cable.
The formula (so you can check any chart, including this one)
Area (mm²) = 0.0175 × 2 × length (m) × current (A) ÷ (12 × allowed drop)
- 0.0175 Ω·mm²/m: copper resistivity (a slightly conservative value).
- × 2: positive and negative both drop voltage.
- Allowed drop: 0.03 for critical circuits, 0.10 for non-critical lighting.
Example: a fridge drawing 3.75 A, 4 m from the battery, 3% drop: 0.0175 × 2 × 4 × 3.75 ÷ 0.36 = 1.46 mm², so 1.5 mm² works on voltage drop. Many builders use 2.5 mm² to allow for start-up current and future changes.
Cable current ratings and fuses
| mm² | Rating, 70 °C insulation | Rating, 105 °C insulation |
|---|---|---|
| 1.5 | 18 A | 25 A |
| 2.5 | 25 A | 35 A |
| 4 | 35 A | 45 A |
| 6 | 45 A | 60 A |
| 10 | 65 A | 90 A |
| 16 | 90 A | 130 A |
| 25 | 120 A | 170 A |
| 35 | 160 A | 210 A |
| 50 | 210 A | 270 A |
Source: ISO 10133:2017 Table A.1 (single conductor, 30 °C ambient). Bundled cables or hot locations need derating.
Fuse rule of thumb: the next standard size at or above 1.25 × the normal current, and never above the cable's rating. Fit it as close to the battery as possible: within about 175 mm (7 in) is widely used guidance (ABYC E-11).
▶ The free calculator gives you the cable size, fuse and actual voltage drop for any run.
Common mistakes
- Using solid-core household cable. Use flexible multi-strand automotive or marine cable, ideally tinned.
- A thinner negative than positive. They must match.
- Ignoring the manual. Inverters and DC-DC chargers specify cable sizes. Follow them if they're bigger.
General planning information, not professional electrical advice.