Two things make marine DC cabling different from household wiring, and both push the same direction: heavier cable than you expect.
Low voltage means high current. The same power at 12V draws roughly four times the current it would at 48V. Current is what causes voltage drop and heat.
The circuit is a loop. Current travels out to the load and back through the return conductor. The conductor length in the calculation is therefore twice the cable route length. A "3 metre run" is 6 metres of conductor.
Voltage drop usually decides the size, not current rating
A cable can be rated to carry a current without melting and still be far too small for the job, because by the time the current arrives the voltage has sagged. On a 12V system there is very little headroom: a 0.4V drop is over 3% of the supply.
This is why inverter and windlass circuits on 12V boats use conductors that look absurd to anyone used to 240V wiring.
What target drop to use
Convention distinguishes between circuits where voltage matters and circuits where it does not:
- Around 3% for circuits where performance depends on voltage — inverters, electronics, navigation equipment, charging circuits.
- Up to about 10% for non-critical circuits where a small loss is tolerable.
Charging circuits deserve the tighter figure. Voltage drop between an alternator or charger and the battery directly reduces charging effectiveness, and can cause a charger to behave as though the battery is fuller than it is.
What the calculator does not cover
The calculation here is voltage drop for copper at about 20°C. It is not a current rating, and it is not a complete cable specification. Still to consider:
- Current rating and temperature. A hot cable carries less than a cold one. Engine-room ambient temperature matters.
- Bundling. Cables run together cannot shed heat as well as a single cable in free air.
- Insulation and construction. Marine cable is tinned and finely stranded for a reason — vibration and salt destroy solid or untinned conductors.
- Terminations. A perfect cable with a poor crimp is a hot joint waiting to happen.
- Over-current protection. Every conductor needs protection sized for the cable, close to the source of power.
- AS/NZS 3004.2. The standard sets requirements this calculator does not attempt to reproduce.
High-current DC work is where marine electrical faults become fires. Have it designed or checked by a licensed marine electrician.