Campervan Cable Size Chart: What mm² Do You Actually Need?
Cable sizing is where most self-builds quietly go wrong. Get it right and your lights stay bright, your fridge stays cold, and nothing gets warm that should not be. This guide gives you the chart, the maths behind it, and the rules of thumb installers actually use.
Try it live
Check your cable sizes live — calculator opens pre-configured for this setup
The rule that matters: current goes there AND back
Every circuit has two conductors. A 60A load 3 metres from the battery is not a 3-metre problem — it is a 6-metre problem, because the current travels out on the positive cable and returns on the negative. Cable charts that only price the positive leg undersize your wiring by roughly half.
The formula: required cross-sectional area A = (2 × I × L × ρ) ÷ ΔV, where I is current, L is one-way length, ρ is 0.0175 Ω·mm²/m for copper, and ΔV is the acceptable drop — 3% of 12V, or 0.36V.
12V cable size chart (3% drop, copper, round trip)
*At these currents ampacity (heat), not voltage drop, sets the size — keep runs under 1.5m regardless.
| Load | 1m run | 3m run | 5m run | Fuse (1.25×) |
|---|---|---|---|---|
| 5A — lights, fans | 1mm² | 2.5mm² | 4mm² | 7.5A |
| 10A — pump, USB, fridge | 1.5mm² | 4mm² | 6mm² | 15A |
| 20A — heater, sockets | 2.5mm² | 6mm² | 10mm² | 25A |
| 40A — DC-DC input | 6mm² | 16mm² | 25mm² | 50A |
| 60A — main feed | 10mm² | 25mm² | 35mm² | 80A |
| 100A — small inverter | 16mm² | 50mm² | 95mm² | 125A |
| 170A — 2000W inverter | 35mm² | 70mm²* | 95mm²* | 200A |
| 250A — 3000W inverter | 50mm²* | 95mm²* | — | 350A |
Two numbers, not one
Cable must satisfy two checks. Voltage drop keeps your devices running properly — a fridge that sees 11.2V instead of 12.5V cycles badly and dies early. Ampacity keeps the cable cool enough not to melt its insulation. A cable that passes the drop check on a short run can still be too small for the current.
Rough continuous ampacity for tri-rated copper in a van: 2.5mm² ≈ 20A, 4mm² ≈ 30A, 6mm² ≈ 42A, 10mm² ≈ 55A, 16mm² ≈ 75A, 25mm² ≈ 100A, 35mm² ≈ 135A, 50mm² ≈ 170A, 70mm² ≈ 210A, 95mm² ≈ 250A.
Mistakes that show up months later
- Sizing from the one-way distance — always double it for the return leg
- Using the load amps but forgetting the fuse protects the cable, not the device
- CCA (copper-clad aluminium) "welding cable" in fine-strand sizes — it carries ~60% of the current of real copper
- A fuse fitted far from the battery — anything before the fuse is an unfused short risk; 300mm is the accepted limit
- Reusing the same fuse size for every circuit — each branch is sized individually
Common questions
What size cable for a 12V fridge in a campervan?
A compressor fridge draws around 4A running with higher surge. For a typical 4m run use 2.5–4mm², fused at 10A — and check the drop, not just ampacity.
Is 10mm² enough for a campervan main feed?
At 60A over a 3m one-way run, 10mm² drops roughly 6% — too much. 25mm² brings it back under 3%. Check your actual current and distance in the calculator.
Can I mix cable sizes in one van build?
Yes — and you should. The main battery feed is heavy gauge, each branch circuit is sized to its own load and fuse.
Check your cable sizes live