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Voltage drop is the reduction in voltage along a conductor due to resistance. NEC recommends maximum 3% drop on branch circuits and 5% total feeder + branch.
Approx formula (single-phase): VD = (2 × K × I × L) / CM.
Worked example: V circuit, 20A load, 100 ft one-way, #12 copper (6530 CM), K=12.9: VD ≈ (2 × 12.9 × 20 × 100) / 6530 ≈ 7.9V → ~6.6% (too high for 3% recommendation).
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