Wire gauge & fuse calculator
At 12 V a 3 % voltage-drop budget is only 0.36 V, so on van-length runs the drop — not the current — usually decides your cable. This sizes for both and tells you which one governed.
Battery to appliance. The return leg is added for you.
Recommended cable
3.31 mm² (12 AWG)
Fuse
15 A
Voltage drop
0.32 V (2.7 %)
Round-trip length
6 m
Voltage drop set this size.
Planning figures, not certification. Standards are summarised for builders; your inspector, your van's plate and your own measurements always win.
How this is calculated
- Cross-section from the drop budget: A = ρ × L × I ÷ V_drop, with ρ = 0.0178 Ω·mm²/m for warm stranded copper and L the round-trip length (both conductors).
- Ampacity from ABYC E-11 for 105 °C insulation outside engine spaces, up to three bundled conductors. The thicker of the two requirements is the recommendation.
- Fuse at 1.2 × the continuous load, rounded up to a size that is actually sold, and never above the cable's ampacity — the fuse protects the wire, not the appliance.
Sources
- ABYC E-11 — AC & DC electrical systems on boats (the ampacity table the van world borrows)
- Explorist.life — wire sizing and fuse sizing guides
- FarOutRide — van electrical system guide
Questions builders ask
What size cable do I need for a 12 V camper circuit?
Size it for voltage drop first. A 20 A load on a 4 m one-way run is 8 m of copper there and back, which needs about 8 mm² (AWG 8) to stay inside a 3 % drop — while ampacity alone would pass 0.82 mm² (AWG 18). Work out both and fit the thicker one.
What size fuse do I need for a 12 V camper circuit?
Fuse at 1.2 × the continuous load, rounded up to a size that is actually sold, and never above the cable's ampacity. A 20 A continuous load takes a 25 A fuse. The fuse protects the wire, not the appliance — so a bigger fuse always means a bigger cable too.
Does cable length or current decide wire size in a van?
On van-length runs, length. Both rules scale with current, but only the voltage-drop rule also scales with length, and at 12 V a 3 % budget is just 0.36 V. Circuits beyond roughly 3 m one-way are usually drop-governed; only short, high-current runs like battery to inverter are decided by ampacity.
Keep reading
Plan the whole van, not just this number
VanPlot is a 3D layout planner for camper conversions. Place real components in your real van and watch weight, axle loads, power and cost update live — this calculator is one panel of it.
Who makes this
I'm Vladimir. I'm planning my own conversion, and none of the tools I tried could tell me whether a layout would fit or what it would do to the rear axle, so I made this one. I work on it alone. If something's wrong or missing, write to me — the address goes to my inbox.
Vladimir Šterjoski