Voltage drop on 120V branch circuits is most critical for circuits serving sensitive electronics, motors, and appliances requiring stable voltage. At 120V, even a small drop — say 5 volts — represents over 4% of the supply voltage and can cause motor overheating, flickering lighting, or nuisance tripping of equipment. The NEC recommends a maximum 3% voltage drop (3.6V on a 120V circuit) for branch circuits, and no more than 5% combined from the service to the load. Common 120V circuits at risk: outlets for power tools in detached garages, workshop circuits, or any 15A or 20A circuit with runs exceeding 75 feet.
For a 120V 15A circuit at 100 ft using 14 AWG copper: VD = 2 × 12.9 × 15 × 100 / 4,110 ≈ 9.4V (7.8%). Far exceeds 3% guideline — use 10 AWG to achieve VD ≈ 3.0%.
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