Reference · 4 min read

DC cable and fuse sizing

Every DC cable has to pass two tests: it must not get dangerously hot, and it must not waste too much voltage over its length. Size for whichever test needs the thicker cable, then fuse to protect that cable.

Step 1: work out the current

For a DC appliance, current is its watts divided by system voltage. For an inverter, divide by its efficiency as well, because the DC side has to supply the inverter's losses too.

Inverter DC current (A) = continuous W ÷ (battery V × 0.9)

A 2,000 W inverter on 12 V draws about 185 A at full load. The inverter calculator works this out from your own loads.

Step 2: check voltage drop

Current flows out and back, so the length in the formula is twice the one-way run. Copper's resistivity is about 0.0172 Ω·mm² per metre.

Minimum area (mm²) = 2 × one-way length (m) × current (A) × 0.0172 ÷ allowed drop (V)

Marine wiring practice commonly allows a 3% drop for anything critical and up to 10% for lighting. On a 12 V system, 3% is only 0.36 V, which is why 12 V cable gets thick so fast.

Run	Current	Allowed drop (3%)	Minimum area	Next size up
2,000 W inverter, 12 V, 1.5 m each way	185 A	0.36 V	26.5 mm²	35 mm²
Same inverter on 24 V	93 A	0.72 V	6.6 mm²	10 mm²
30 W roof fan, 12 V, 6 m each way	2.5 A	0.36 V	1.4 mm²	1.5 mm²

Doubling the voltage halves the current and doubles the allowed drop, so the cable can have a quarter of the area. That is the cable saving behind the advice to step up to 24 V or 48 V.

Step 3: check ampacity

Voltage drop says nothing about heat. Short, heavy runs are usually decided by ampacity; long, light runs by voltage drop.

Ampacity depends on the insulation's temperature rating and on how the cable is installed: in free air, bundled with others, in conduit or in a hot engine bay. No single table fits every cable, so take the figure from your cable manufacturer's datasheet for your installation.

Then use the larger of the two results. The inverter run above passes voltage drop at 35 mm², but whether 35 mm² is rated for 185 A plus margin in your installation is a datasheet question. If it is not, go up a size.

Step 4: the fuse protects the cable

A fuse is not there to protect the appliance. It is there to stop the cable overheating when something shorts. Two limits set its rating:

• At or below the cable's ampacity, so the fuse blows before the cable cooks.
• At least about 125% of the normal load, so it does not blow in normal use. The inverter calculator applies the same 25% margin.

If those two limits cross, the cable is too small. Fit a bigger cable, not a bigger fuse.

Where a smaller cable branches off a busbar fed by a larger one, the branch needs its own fuse, sized to the smaller cable.

Where the fuse goes, and which type

Fit the main fuse as close to the battery positive terminal as practical. Marine standards set that distance in centimetres, not metres, because the cable before the fuse is unprotected.

Job	Common fuse types
Main fuse on a lithium bank	Class T, or a terminal-mounted MRBF fuse rated for the fault current
Main fuse on a lead acid bank	ANL or MEGA
Inverter or charger feed from a busbar	MEGA or ANL, sized to that cable
Lights, pump, fan and socket circuits	Blade fuses in a fuse block

A lithium bank can deliver thousands of amps into a short. Class T fuses are built to interrupt around 20,000 A; whatever you fit on a lithium bank, check its interrupt rating.

Before you power up

• Lugs crimped with the right die for the cable size, then tested by pulling hard.
• Tinned copper cable and lugs anywhere damp.
• Heat-shrink over every lug, and terminals tightened to the manufacturer's torque.
• Cable supported along its run, with grommets wherever it passes through metal.
• A wiring diagram with every cable size and fuse rating, kept somewhere outside the build.

This guide gives planning estimates. Electrical installation carries real risk of fire and injury. Have your design checked against the regulations that apply where you are, by someone qualified.