How this calculation works
The calculation is resistance times current times length. What changes between single-phase and three-phase is the multiplier on the length.
Single-phase uses 2 because the current goes out and comes back — the round trip is twice the one-way distance. Three-phase uses the square root of 3 because the phase currents are 120 degrees apart and never all peak together. L is one-way distance in both cases, and R is the conductor resistance in ohms per 1,000 feet.
The percentage is what people argue about. The Code does not enforce a voltage drop limit for general branch circuits and feeders. The 3% and 5% figures live in informational notes, which are advisory. But a lot of AHJs treat them as expected practice, plenty of specs make them contractual, and some specific installations — sensitive electronic equipment, fire pumps, EV supply equipment — do carry enforceable limits.
Treat 3% on a branch circuit and 2% on a feeder as the working target, with 5% combined as the ceiling. It is what almost every engineer and inspector expects to see.
Where voltage drop actually bites
Motors are the worst case. Torque falls off with the square of the voltage, so a 10% drop costs about 19% of your starting torque. A motor that starts fine on a short run will sit and hum at the end of a long one, trip on overload, and cook itself over a few months.
Low-voltage systems are the other one. On a 24 volt control circuit, losing 2 volts is over 8%. The same 2 volts on a 480 volt feeder is nothing. That is why the percentage matters more than the raw number.
One thing this calculator does not do: it uses the DC resistance values from Chapter 9, Table 8. For large conductors in steel raceway at high current, reactance starts to matter and Table 9 impedance is the more accurate approach. Below 4/0 or so the difference is small enough to ignore.
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Frequently asked questions
Is voltage drop actually enforceable?
Usually not on its own. The 3% and 5% figures appear in informational notes, which are advisory rather than enforceable. Some specific installations do carry hard limits, and many project specifications make the percentages contractual. Ampacity is the enforceable sizing rule; voltage drop is a performance requirement that your AHJ or your spec may or may not hold you to.
Do I use one-way or round-trip distance?
Enter the one-way distance. The calculator handles the round trip for you — that is what the factor of 2 in the single-phase formula does. Measuring the total conductor length and entering that would double-count and give you twice the real drop.
Why does aluminum need a bigger conductor?
Aluminum has roughly 60% the conductivity of copper, so for the same size it has about 1.6 times the resistance and drops about 1.6 times the voltage. That is why aluminum feeders typically run one to two sizes larger than the copper equivalent for the same job.
Does upsizing for voltage drop change the ground wire?
Yes. If you increase the ungrounded conductors for voltage drop, 250.122(B) requires the equipment grounding conductor to be increased proportionally by circular mil area. This is one of the most commonly missed items on an inspection.
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nfpa.org for complete and authoritative code text. Always verify requirements with the code edition adopted by your jurisdiction and your Authority Having Jurisdiction (AHJ).