Find the right wire gauge for any boat circuit: 12, 24 or 48 volt DC, solar panel strings, or 120/240 volt AC shore power. Enter the voltage, current and length of the run. The calculator checks both voltage drop and how much current the wire can safely carry, gives you the minimum wire size and the largest fuse or breaker that protects it, and links the matching wire, lugs and fuses.
Compare every wire size for this circuit
Sized for Ancor tinned marine wire, which has 105°C insulation. Wire with a lower temperature rating carries less current, so these results only apply to wire with the same rating.
How the marine wire size calculator works
Boat wiring has to pass two separate checks, and the wire size you need is the larger of the two.
1. Voltage drop
Every foot of wire has resistance, so some voltage is lost along the way. Too much drop makes motors run hot, electronics reset, and inverters shut down early. The calculator uses the standard marine formula: circular mils = 10.75 × amps × round-trip feet ÷ allowed volts of drop. It then rounds up to the next wire size. Critical circuits are held to 3% drop and non-critical circuits to 10%.
2. Ampacity (how much current the wire can carry)
Current heats wire. The safe current depends on the insulation's temperature rating, whether the wire runs through a hot engine room, and how many current-carrying wires are bundled together. Bundling and engine room heat both lower the rating. The calculator uses the marine ampacity tables for 105°C wire like Ancor.
3. Fuse or breaker size
The fuse or breaker protects the wire, so its rating can't be higher than the wire's ampacity. The calculator shows that maximum and suggests a standard size that fits your load. Put the fuse as close to the power source as you can, within 7 inches of the battery for most circuits.
Bigger runs and paralleled cables
When even 4/0 isn't enough, the calculator sizes two identical cables run side by side, protected by one fuse sized to their combined capacity. Both cables must be the same length, gauge and type, and should be labeled as a pair.
Marine wire size FAQs
How do I figure out what size wire my boat circuit needs?
You need three numbers: the system voltage, the maximum current the circuit draws, and the one-way length from the power source to the device. Enter them above and choose whether the circuit is critical. The calculator checks voltage drop and ampacity and gives you the larger wire size of the two.
Why is the wire length doubled?
Current travels out on the positive wire and back on the negative, so the voltage drop happens over the full round trip. Enter the one-way distance and the calculator doubles it for you.
What's the difference between 3% and 10% voltage drop?
A 3% limit is used for circuits where low voltage causes problems or safety issues: inverters, chargers, panel feeds, electronics, navigation lights, and bilge pumps and blowers. A 10% limit is acceptable for non-critical loads like general cabin lighting.
How do I size wire for solar panels?
Choose DC and enter the string's operating voltage. For current, use 1.25 times the panel's short-circuit current (Isc) to allow for bright conditions. Make sure the wire's voltage rating is higher than the string's maximum open-circuit voltage, especially on cold mornings.
Does running wire through the engine room change the size?
Yes. The higher temperature in an engine space lowers how much current a wire can safely carry. Answer "Yes" to the engine room question and the calculator uses the lower engine room ratings.
What size fuse do I need for my wire?
The fuse or breaker rating can't be higher than the wire's ampacity, because its job is to protect the wire. The calculator shows the maximum and suggests a standard size above your load. If your equipment maker specifies a fuse, follow their instructions as long as it doesn't exceed the wire's capacity.
Can I use automotive or household wire on a boat?
Use stranded, tinned copper marine wire. Solid household wire cracks from vibration, and bare copper corrodes quickly in salt air. Tinned marine wire like Ancor is built for the marine environment and has 105°C insulation.
Can I run AC and DC wire with this calculator?
Yes. Choose AC for 120 or 240 volt shore power, generator and inverter output circuits. For AC, a three-conductor cable (hot, neutral, ground) counts as one bundle. The calculator suggests Ancor triplex cable in the matching gauge.