What size wire and fuse does this circuit need?

How this works

A wire has to pass two separate tests. ABYC, the American Boat & Yacht Council, puts it plainly: conductor sizing can’t be based on voltage drop alone, and when the two tests give different sizes, use the larger one.

1. Voltage drop

Every wire has resistance, so some voltage is lost along it. Too much drop makes inverters shut off early and fridges struggle. ABYC E-11 allows a 3% drop on critical circuits (main feeds, electronics) and 10% on others such as cabin lighting. At 12 V, that is 0.36 V or 1.2 V.

• Required size (circular mils) = 10.75 × amps × round-trip length in feet ÷ allowed drop in volts.
• 10.75 is the resistance constant for copper used by ABYC and in US Coast Guard rules (46 CFR 183.340).
• The circuit is the round trip. Current flows out on the positive wire and back on the negative, so a load 10 feet from the battery is a 20-foot circuit. Enter the one-way distance; the calculator doubles it. If the negative goes to a bus bar or chassis point that is closer, measure the real total and enter half of it.
• The calculator picks the smallest wire with at least that many circular mils.

2. Ampacity: what the wire can carry

Ampacity is the most current a wire can carry before its insulation gets too hot. It depends on the insulation’s temperature rating and where the wire runs. We use the allowable amperage table in US Coast Guard rule 33 CFR 183.425, which is the same table ABYC E-11 uses. In an engine space, multiply by the engine-space factor (0.58 for 60 °C, 0.75 for 75 °C, 0.78 for 80 °C, 0.82 for 90 °C, 0.85 for 105 °C, 0.89 for 125 °C). Bundling lowers ampacity too: the same rule multiplies it by 0.70 when 3 current-carrying wires run together, 0.60 for 4 to 6 and 0.50 for 7 to 24. The calculator doesn’t apply this, so for bundled runs multiply the “carries” figure yourself, or go up a size.

3. The fuse protects the wire

• The fuse must be at least the load current, or it will blow in normal use. We start at 125% of the load and round up to a common fuse size, to avoid nuisance blowing.
• The fuse must not be more than the wire’s ampacity, so it blows before the wire overheats. If the fuse would be too big for the voltage-drop wire, the calculator moves up a wire size. (US Coast Guard rules allow up to 150% of ampacity on circuits under 50 V; we keep to 100%, which is stricter.)
• Place the fuse at the battery end of the positive wire. Boat rules (33 CFR 183.455 and ABYC E-11) call for it within 7 inches of the power source, or within 40 or 72 inches only when the cable is in a protective sheath.

The starting example is 25 A, 10 feet one way, 12 V, critical (3%), 60 °C wire outside the engine space. (60 °C is the starting choice because it gives the safest answer when you don’t know your wire’s rating.) Voltage drop needs 8 AWG (about 14,900 circular mils required). The fuse is 35 A (25 × 1.25 = 31.25, rounded up), and 8 AWG at 60 °C can carry 55 A, so the fuse protects it.

Wire sizes used in the calculator

Resistance is for solid copper at 20 °C, for reference. The voltage-drop formula uses ABYC’s constant of 10.75, which gives slightly higher resistance and so slightly bigger wire. 18 AWG is left out because a separate wire must be at least 16 AWG under 33 CFR 183.425.

Safety

This calculator is for planning only and does not replace ABYC E-11, NFPA 1192 for RVs, your local electrical code or a qualified installer. Use stranded copper wire rated for vehicles or boats, crimp connections with the right tool, protect wires where they pass through metal, and add a battery disconnect. Lithium batteries can deliver very high fault currents; use a fuse with a high enough interrupt rating, such as a Class T fuse, at the battery. Mains (120 V or 230 V) wiring is different and is not covered here.

Sources

• ABYC, “What is voltage drop?”: 3% and 10% limits, round-trip length, and using the larger of the voltage-drop and ampacity sizes.
• 33 CFR Part 183, Subpart I (eCFR): Table 5 allowable amperage of conductors, engine-space factors, minimum wire size, fuse rating and location.
• 46 CFR 183.340 (govinfo): the circular-mil voltage-drop formula with K = 10.75.
• HyperPhysics, Georgia State University: common wire gauges: circular mils and resistance per AWG.
• Blue Sea Systems, wire selection chart and Blue Sea Systems, ABYC E-11 excerpts: how marine wire charts apply the same rules.