Enter the load and one-way distance; get the smallest real conductor that passes both the drop budget and the ampacity check on a 12 V system.
12 V is the harshest sizing environment in common use — a 0.36 V budget at 3% — and the biggest audience: vans, boats, RVs, automotive and small solar. Rules of thumb that work at 120 V fail by a factor of ten here.
Generic wire-size tools solve ρ·L/A and report a theoretical gauge — famously including sizes like "13 AWG" that don't exist at the supply house, with no check that the wire can carry the current thermally. This calculator returns only real, purchasable conductors, enforces the NEC ampacity floor for the load, applies the drop budget, and shows the verdict — wire you can actually buy and legally install. Every result comes with the fan chart, upgrade economics, and a PDF report.
Siblings: DC wire size hub · 12V voltage drop (check an existing wire) · metric sizes.
| Vd(max) | budget volts = limit% × source voltage |
| Rmax | largest acceptable resistance, Ω/kft |
| answer | smallest gauge with R ≤ Rmax and Table 310.16 ampacity ≥ I |
Two gates, not one: a gauge must pass the drop budget AND carry the current thermally. Physics-only calculators stop at the first gate.
Work it in three lines: budget = 3% × 12 V = 0.36 V; R(max) = 0.36 × 1000 ÷ (2 × 50 × 20) = 0.180 Ω/kft; smallest gauge under that with ampacity ≥ 50 A is 1 AWG (0.154 Ω/kft, 130 A). Actual drop: 0.31 V = 2.57%.
At a strict 3% budget, a 50 A motor at 20 ft one-way needs 1 AWG copper — 4 AWG lands at 5.1%, acceptable only as a non-critical 5%-class choice. At 30 ft, 3% demands 2/0. Undersized cable is the #1 cause of 'weak' trolling motors.
Only for non-critical resistive loads. Compressor fridges cut out near 11.5 V, so their circuits deserve the 3% treatment.