Single-phase voltage drop is calculated using VD = 2 × K × I × D / CM, where the factor of 2 accounts for both the hot and neutral conductors carrying the full load current. This formula applies to 120V and 240V single-phase circuits commonly found in residential wiring, including branch circuits to outlets, branch circuits to appliances, and feeders to sub-panels. The NEC recommends a maximum 3% voltage drop for branch circuits and a combined total of 5% for feeders plus branch circuits. Voltage drop is especially important on longer runs — a 12 AWG circuit feeding an outlet 150 feet away may exceed the 3% threshold at full 20A load.
For a 120V single-phase 20A branch circuit at 150 ft using 12 AWG copper: VD = 2 × 12.9 × 20 × 150 / 6,530 ≈ 11.9V (9.9%). This significantly exceeds the NEC 3% guideline — upgrade to 8 AWG to bring VD under 3%.
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