VOLTAGE DROP
VOLTAGE DROP
VD = 2·K·I·L ÷ CM
V
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RESULT
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NEC recommends ≤3% drop on branch circuits, ≤5% total. K = 12.9 for copper.
About this calculator
This voltage drop calculator computes the voltage lost in a copper wire run, which the NEC recommends keeping at or below 3% on branch circuits and 5% combined feeder + branch. Use one-way distance — the calculator doubles it for round-trip. K = 12.9 for copper at 75°C. For aluminum wire, multiply the drop by 1.6.
Common questions
What's an acceptable voltage drop?
NEC recommends max 3% on branch circuits and max 5% combined feeder + branch. Code is a recommendation, not a hard requirement, but exceeding it causes lights to dim, motors to run hot, and electronics to misbehave.
When do I need to upsize wire for distance?
A 15A circuit on 14 AWG copper exceeds 3% drop past about 50 ft (one-way). Past 50 ft, jump to 12 AWG. Past 100 ft, jump to 10 AWG. Long subpanel runs commonly need 8 or 6 AWG even at low amperage.
Does aluminum wire have more voltage drop?
Yes — aluminum has about 1.6× the resistance of same-gauge copper. SE-rated aluminum wire is sized larger to compensate. For a given amperage, aluminum service entrance cable is typically 2 sizes larger (e.g., 4/0 AL replaces 2/0 CU).