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Voltage Drop Calculator

Copper or aluminum, single- or three-phase, any conductor. Get the percent drop, a pass/fail against the NEC 3% guideline, and the smallest wire that fixes a long run.

Voltage drop
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Enter load, length, and voltage
Method: K-method (VD = 2·K·I·L ÷ CM single-phase, 1.732 three-phase; K = 12.9 copper / 21.2 aluminum), NEC Chapter 9 circular mils. The 3% branch / 5% total figure is an NEC recommendation, not a hard rule.
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Why voltage drop matters

Every foot of wire has resistance, and resistance eats voltage. Push a big load down a long run and the far end sags — motors run hot and short their life, LED drivers flicker, heaters underperform, and controls get flaky. The fix is almost always more copper: a bigger conductor has more circular mils, less resistance, and less drop. The NEC doesn't mandate a limit, but it recommends holding a branch circuit or feeder under 3%, and the whole path under 5%. On long homeruns, EV chargers, well pumps, and outbuildings, voltage drop — not ampacity — is usually what decides your wire size.

Questions

What is an acceptable voltage drop?
The NEC recommends no more than 3% on a branch circuit or feeder and no more than 5% for the two combined. It's an informational recommendation for efficiency, not a hard code requirement — but most inspectors and good electricians hold to it, especially on long runs and motor loads.
How do you calculate voltage drop?
Single-phase: 2 × K × I × L ÷ CM, where K is 12.9 for copper and 21.2 for aluminum, I is the load in amps, L is the one-way length in feet, and CM is the circular mils of the conductor. Three-phase swaps the 2 for 1.732. Divide by the system voltage for the percent.
How do you fix too much voltage drop?
Size the wire up (more circular mils, less resistance), shorten the run, raise the voltage, or split the load. Upsizing is the usual move — the calculator shows the smallest AWG that brings the run back under 3%.
Is this accurate and free?
It's free and runs in your browser. It uses the standard K-method (DC resistance) with NEC Chapter 9 circular mils — plenty for everyday runs. For long, large, or low-power-factor jobs, verify against NEC Chapter 9 Table 9 (AC resistance and reactance).

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For reference and estimating only. Based on NEC (NFPA 70) Chapter 9 values and the standard K-method. Not a substitute for the National Electrical Code, a licensed electrician's judgment, or your authority having jurisdiction (AHJ). Verify every result against your locally adopted code before installing.