Cable and fuse calculator
Cable size and fuse for your camper, at 12, 24 or 48 V
cable, with a fuse
Voltage drop decides: a thinner cable would leave the appliance with less voltage than allowed.
Current decides: the cable is short, but it has to carry the current without overheating.
The cable has been upsized so that a standard fuse fits below what the cable can carry.
- Size by voltage drop
- Actual drop
- Cable carries
No size up to 120 mm² works. Move the appliance closer to the battery, raise the voltage or split the circuit.
Know the watts but not the amps? Divide by the voltage: 600 W at 12 V is 50 A.
VanVolto designs the whole camper system: loads, battery, panels, DC-DC charger and inverter, derates the cable for heat and bundling, and reads your Victron devices over Bluetooth. Meet VanVolto, coming soon to Google Play. Not sure which battery yet? Work it out.
How it works
Current flows out and back through the cable, so the length counts twice. The smallest size that loses no more voltage than allowed is S = 2 × L × I / (γ × ΔU), with L the one-way length in metres, I the current, γ the conductivity of copper (56 m/(Ω·mm²)) and ΔU the allowed drop in volts. With 20 A, 3 m and 3 % of 12 V (0.36 V) you get 5.95 mm², so the standard cable is 6 mm².
The cable also has to carry the current without overheating. The calculator uses the current rating of PVC-insulated copper in conduit or trunking at 30 °C, from the harmonised standard HD 60364-5-52 (table B.52.2, method B1). It is a cautious choice: in the engine bay, in the sun or bundled with other cables, a cable carries less and needs a bigger size.
The fuse protects the cable: it goes as close to the battery as possible, rated at least 1.25 times the working current and no higher than what the cable can carry. If no standard fuse fits between those two values, the cable is upsized.
Results are for guidance. A permanent installation in a vehicle may need approval, with paperwork from a workshop.