Conductor sizing
Estimate voltage drop, percent drop, and end voltage for any run — copper or aluminum, single- or three-phase — and size the conductor to NEC's 3% branch and 5% combined guidance.
Voltage drop
—Enter a run to estimate voltage drop.
| Voltage drop | — |
|---|---|
| Percent drop | — |
| Voltage at load | — |
| Resistance used | — |
The estimate uses the single-conductor formula Vdrop = K × length × current × (R ÷ 1000). K is 2 for single-phase and √3 (about 1.732) for three-phase. R is the conductor's DC resistance in ohms per 1000 ft, taken from NEC Chapter 9, Table 8 for uncoated copper and aluminum. Percent drop is the drop divided by the source voltage.
The NEC suggests (informational note, not a hard rule) a branch circuit stay at or below 3% and the feeder plus branch stay at or below 5%. The calculator flags both. When a run exceeds 3%, it suggests the thinnest conductor that brings it back under 3% so you don't over-buy copper.
Based on NEC Ch.9 Table 8 (DC resistance, ~75°C, uncoated conductors).
Voltage drop = K × length (ft) × current (A) × (R ÷ 1000), where K is 2 for single-phase or √3 for three-phase, and R is the conductor's DC resistance in ohms per 1000 ft. Percent drop is the voltage drop divided by the source voltage, times 100.
The NEC's informational notes recommend keeping a branch circuit at or below 3% and the feeder plus branch combined at or below 5%. These are recommendations (FPN), not enforceable rules, but most designers and inspectors treat 3% as the practical target.
For the same size, aluminum has higher resistance than copper, so it drops more voltage. To carry the same load over the same distance with a similar drop, aluminum usually needs to be one or two sizes larger than copper.
It estimates voltage drop only, using NEC Chapter 9 Table 8 DC resistance. It does not size for ampacity, temperature derating, conduit fill, power factor, or continuous-load factors, and it is not a substitute for a licensed electrician or your local code (AHJ).
Estimate only, based on NEC Chapter 9 Table 8 DC resistance. This is not a substitute for a licensed electrician or your local code (AHJ). It does not fully model ampacity, temperature derating, power factor, or continuous-load factors. Always verify against the current NEC and local code before wiring. Based on NEC Ch.9 Table 8.