Wires and fuses: what size?
At 12 V, currents are strong. A wire that's too small heats up, wastes your energy and can start a fire. Enter the voltage, current and length: we give you the wire gauge and the fuse that protects it.
Tool: Work out your wire and fuseWork out your wire and fuse
Length is the distance between the two devices, one way only. The calculation counts there and back.
in metres
3% in general, 1 to 2% between battery and inverter
1/0
wire gauge (AWG)
54 mm²
cross-section
250 A
fuse
0.18 V
lost in the wire
A copper wire of at least 1/0 AWG (53.5 mm²).
A 250 A fuse, on the power source side (battery, panels or generator), as close as possible.
With this wire, you lose 0.18 V (1.5%) from end to end.
In watts, we count the current drawn from the battery, inverter losses included: about 185 A.
Copper, resistance at 20 °C. Gauge chosen to meet both the acceptable loss and the current the wire carries without heating: a single cable in free air, 90 °C insulation, like a battery cable. If your wires are bundled in a conduit, go bigger. Your country's electrical code always has the last word.
The fuse rule
- The fuse protects the wire, not the device: it must blow before the wire heats up.
- It must be at least 25% bigger than the normal current, so it doesn't blow for nothing.
- But never bigger than what the wire carries. If there's no room between the two, use a bigger wire.
Words to know
- AWG
- The American wire gauge. The smaller the number, the bigger the wire. After 1 come 1/0, 2/0, 3/0, 4/0 (“one aught”, “two aught”…).
- mm²
- The wire's cross-section in square millimetres: the measure used in Europe. 4 AWG is about 21 mm².
- Voltage drop
- Voltage lost in a wire that's too long or too thin. It turns into heat, and your devices get less.
- Ampacity
- The current a wire carries without overheating. It depends on its size and its insulation.
- Stranded wire
- Made of many small strands: flexible, it resists vibration. It's what you want in a trailer, van or boat.
Classic traps
- Calculating with the one-way length only: current goes there and back. The tool counts both.
- Buying aluminium or “CCA” (copper-clad aluminium) wire sold as copper: it conducts much worse and heats up.
- Putting in a bigger fuse because that one “kept blowing”: the wire will burn instead.
- A badly crimped lug: a poor contact heats as much as a wire that's too small. Use the right crimper, or have it done.
Going further
Next step
Where to put the fuses
The whole system on one diagram: every fuse and switch, numbered and explained.