Distance is one way; the calculator doubles it for the round trip (positive + negative).
Voltage drop = 2 × length (ft) × amps × ohms per 1,000 ft ÷ 1,000. Resistances are for stranded copper at 20°C (NEC Chapter 9, Table 8), e.g. 10 AWG = 1.21 Ω, 4 AWG = 0.308 Ω, 2/0 = 0.0967 Ω per 1,000 ft. 3% is the usual target for important loads; 10% may be acceptable for some lighting. The calculator also checks a conservative ampacity limit so the wire does not overheat. Use pure copper (not copper-clad aluminum) and wire rated for your temperature and installation.
Safety: These are general estimates, not a design. Follow your equipment manufacturer's instructions and local electrical code, and consult a qualified electrician for anything connected to shore power, a house, or high-current batteries.
For a short run (about 5 ft one way) 1 AWG copper meets a 3% drop and ampacity; longer runs need 1/0–2/0. Use the calculator for your exact length.
For the same power, 12 V carries 10 times the current of 120 V, and voltage drop rises with current. Moving to 24 V or 48 V cuts wire size dramatically.
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