DC Distribution, Conductors, Overcurrent Protection, and Wiring Methods
Core concepts
DC distribution starts at the battery bank: main disconnect, high-current fuse or breaker, busbars, branch panel, and individual overcurrent protection. ABYC requires overcurrent protection on the ungrounded (positive) conductor within 7 in of the power source, with limited exceptions such as sheathed conductors or raceways, where the distance may extend to about 40 in. Fuses and breakers protect conductors, not appliances; the device rating must not exceed the conductor ampacity after derating. Switches and breakers belong in the positive leg only. The hull must never carry return current: use a two-wire system with a dedicated negative return. Marine cable should be stranded, tinned copper with 105 °C insulation, supported, and protected from chafe. Color conventions: DC positive red; DC negative black or yellow; AC neutral white; AC safety ground green. Older boats may use black for DC negative, a classic trap when AC is also present.
Traps
Do not place a fuse in the negative leg, use the hull as a ground return, or protect a 6 AWG feeder with a 100 A breaker if its derated ampacity is lower. Watch for double-lugging at battery posts and for breakers used as switches if not rated for it. A common error is sizing a fuse to the load's running current rather than to the conductor.
Worked example
A 20 A load is 15 ft from the panel, 30 ft round trip. At 12 V, 3% is 0.36 V; maximum loop resistance is 0.36 ÷ 20 = 0.018 Ω, or 0.6 mΩ/ft. 10 AWG (about 1.0 mΩ/ft) gives 0.6 V (5%) and is too small; 8 AWG (about 0.63 mΩ/ft) gives about 0.38 V (3.2%); 6 AWG gives about 0.25 V (2.1%). Choose 6 AWG and a 25–30 A breaker, confirming ampacity in the bundle.