Voltage Drop Calculator

Long run? Check it before you pull wire.

The formula

Single-phase drop = 2 × K × I × L ÷ circular mils, with K ≈ 12.9 for copper and 21.2 for aluminum (Ω·cmil/ft); three-phase runs multiply by 0.866. The NEC recommends keeping branch-circuit drop within 3% (5% combined with the feeder) — a guideline, not a mandate, but the number inspectors and engineers design to.

Where drop bites

Distance hurts quietly: a 20 A load on 12 AWG copper at 120 V passes at 40 ft but reaches ≈3.3% by 50 ft one-way. Detached garages, well pumps, gate openers and EV chargers at the end of long runs are the classic cases — size the conductor for the distance, not just the breaker.

Related: wire size calculator (with built-in drop check) · ampacity chart