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Voltage drop
Approximate drop for copper or aluminum conductors, single- or three-phase.
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VD = (2 or √3) × K × I × L ÷ CM, where K is about 12.9 Ω·cmil/ft for copper and 21.2 for aluminum, L is the one-way length in feet and CM is the conductor’s area in circular mils.
The NEC’s informational notes suggest keeping branch-circuit drop to 3% and feeder plus branch to 5%. This is an estimate: it ignores reactance and temperature, so check critical runs with a full calculation.
Sources: NFPA 70 (NEC) 210.19(A) Informational Note and Chapter 9, Table 8 conductor areas.