Cable & fuse sizing
Marine Cable Size Calculator
Pick the right DC cable cross-section and fuse for your boat. Enter current, one-way length and system voltage — Marine Power Designer returns the cable size, voltage drop and recommended fuse against ABYC or ISO marine wiring standards.
Voltage drop vs ampacity
Two limits decide marine cable size: ampacity (how much current the cable can carry without overheating at marine ambient temperatures) and voltage drop (how much voltage you lose over the run). On 12V systems, voltage drop almost always picks the cable — not ampacity.
ABYC vs ISO defaults
ABYC E-11 (USA) and ISO 10133 (international) agree on the principle but differ on ampacity tables and conductor temperature ratings. Pick the standard you certify against; the calculator applies the matching table.
What to enter
- Continuous current in amps (or inverter watts → calculator derives the DC current)
- One-way cable length in metres or feet (the tool doubles it for the round trip)
- System voltage: 12V, 24V or 48V
- Critical / non-critical circuit (sets the 3% vs 10% drop limit)
- Standard: ABYC or ISO
Frequently asked questions
What voltage drop is acceptable on a boat?
ABYC E-11 allows 3% drop for critical circuits (electronics, panels) and 10% for non-critical. ISO 10133 is similar. Marine Power Designer defaults to 3% for critical loads.
Why are marine cables bigger than house cables?
Marine ambient temperatures are higher, runs are longer, conductors are stranded fine-wire for vibration, and voltage drop matters more on 12V. ABYC ampacity tables are stricter than residential NEC.
What fuse size should I use?
Fuse rating must protect the cable, not the load. Pick the fuse just above the continuous current and below the cable's ampacity at its installed temperature. The calculator suggests a standard fuse size automatically.
Does cable length really matter that much?
Yes — voltage drop is linear with length and current. A 100A run at 12V over 5m needs roughly twice the cross-section of the same run at 2.5m. Going to 24V or 48V is often cheaper than upsizing cable.
Ready to size your system?
Marine Power Designer runs in your browser. No sign-up needed to try the guided wizard.
Open the calculator