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Voltage Drop Calculator

Calculate voltage drop over a wire run from gauge, length, current, and voltage: single- or three-phase, copper or aluminum. WattSimple flags the NEC 3% recommendation and tells you the smallest wire that meets it.

Phase

Voltage drop

3.11%

Volts dropped
7.46 V
Voltage at load
232.5 V
Over the recommended 3%. Upsize to 8 AWG copper to drop to 1.95%.

The NEC's 3% (branch) / 5% (total) figures are recommendations (210.19 & 215.2 informational notes), not hard rules. Wire & breaker chart. Confirm with a licensed electrician + AHJ.

Measure the real amps and volts

The calculator predicts the drop from wire size and length. A multimeter shows the real one: read the voltage at the far outlet with the load running and compare it to the reading at the panel. A clamp meter reads the current on one conductor at the same time. Opening a panel or a live box is work for someone qualified.

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Frequently asked questions

This calculator uses the circular-mils method: VD = (factor × K × length × amps) ÷ circular mils, where K ≈ 12.9 for copper (21.2 for aluminum) and the factor is 2 for single-phase or 1.732 for three-phase. Length is the one-way run distance. The result is shown as both volts and a percentage of system voltage.

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Sources & standards

This tool's methodology and data follow US standards and authoritative public datasets: