Single- or three-phase · copper or aluminum · NEC 2023
Loading the calculator…
Worked example — for #10 copper, 20 A, 150 ft one way, 240 V single-phase, 3.0 % recommendation, the engine returns:
3.11% drop (7.5 V, 232.5 V at the load) — EXCEEDS the limit
How it works. Voltage drop is a performance problem the NEC comments on rather than mandates. A long run at high current loses voltage in the conductor itself, so equipment at the far end sees less than the panel does. The usual working target is 3% on a branch circuit and 5% total including the feeder — a recommendation, not a requirement.
What this covers: Chapter 9 circular-mil (K-constant) method. Ignores reactance, power factor and raceway material — fine for short, high-power-factor runs; less accurate for large conductors, steel raceway, or motor loads.
Educational reference, not code. Based on NEC (NFPA 70) 2023 (Ch. 9 · Table 8). Your state or local AHJ may enforce a different edition or amend it — theirs is the one that counts. No NEC text is reproduced here.