ABYC E-11 Marine DC Wire Sizing
Why a dockside NEC wire size undersizes on a boat: ABYC E-11 sizes on the ROUND-TRIP length (out and back), CM = 10.75 x current x (2 x length) / V_drop, not the NEC one-way habit -- and the marine allowable drop is stricter where it matters, 3% for panelboard feeders and nav/critical loads (10% non-critical). A 20 A nav feeder, 25 ft one-way, 12 V, 3% has only 0.36 V of headroom -> 29,861 circular mils -> #4 wire (#6 falls short); relax to 10% non-critical and it drops to 8,958 CM -> #10, three sizes smaller. The ABYC ampacity table sets a separate floor the drop size must clear. A design aid; the standard, wire temperature rating, and installation govern.
Formula and source
V_drop = drop_pct/100 x system_voltage; CM = 10.75 x current x (2 x length) / V_drop; AWG = smallest standard size with circular-mil area >= CM.
The ABYC E-11 (AC & DC Electrical Systems on Boats) DC conductor sizing by voltage drop, on the round-trip length, by name; the standard, the wire's temperature rating, and the installation govern.
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