Enter your load, run length and maximum % drop — this solver walks up the AWG ladder and returns the smallest conductor that passes both the ampacity floor and your voltage drop budget.
Two independent rules govern conductor sizing. Ampacity (NEC Table 310.16) is the safety limit — the maximum continuous current a conductor can carry without overheating its insulation. Voltage drop is the performance limit — the NEC recommends no more than 3% on branch circuits and 5% total. On short runs ampacity usually governs; past roughly 100 feet, voltage drop takes over and forces conductors one or two sizes larger than ampacity alone would require.
This calculator tests each size in order against both rules, so the answer is the smallest conductor that is both safe and within your drop budget.
| 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.
By system voltage: 120 V · 208 V · 240 V · 277 V · 480 V · 12 V · 24 V · 48 V — and the reference hubs: wire tables, by gauge, by amperage, by distance.
Work it in three lines: budget = 3% × 240 V = 7.20 V; R(max) = 7.20 × 1000 ÷ (2 × 50 × 120) = 0.600 Ω/kft; smallest gauge under that with ampacity ≥ 50 A is 6 AWG (0.491 Ω/kft, 65 A). Actual drop: 5.89 V = 2.46%.
Whichever demands the larger conductor. Ampacity is a hard safety requirement; the 3%/5% drop figures are NEC recommendations. On long runs, voltage drop almost always forces the larger size.
Use 3% for branch circuits (panel to outlet) and budget the full path to 5%. Many engineers spec 2% for feeders to leave 3% for branches.