Voltage Drop Calculator

It works out how much voltage is lost along a wire carrying a given current. Enter the supply type, voltage (120/240 V split-phase or 208/480 V three-phase), current and run length in feet (m), then the conductor material and size, and it shows the drop in volts, the drop as a percentage of the supply, whether that clears your limit, and the voltage still available at the load.

Voltage drop = k × I × L × R, where I is the current, L is the one-way run length and R is the conductor resistance per foot (metre) — resistivity ÷ cross-sectional area, so a lower AWG number (bigger mm²) means less drop. The factor k is 2 for DC and single-phase circuits — to account for the return conductor — and √3 for three-phase. Dividing the drop by the supply voltage gives the percentage.

Enter the one-way route length — the distance from the supply to the load, in feet (m). The calculator already accounts for the return path (the ×2 for single-phase and DC), so you should not double the length yourself.

A common rule of thumb — and the NEC recommendation — is up to 3% on a branch circuit and 5% total to the furthest point (feeder plus branch). For example a 20 A, 240 V load run 100 ft on 10 AWG (6 mm²) copper drops about 5 V, roughly 2% — inside the 3% target. The exact limit depends on your local wiring rules, so set the “max allowed drop” to whatever applies to you and the tool flags a pass or fail against it.

Resistance falls as the conductor gets thicker, so a bigger cross-section — a lower AWG number — drops less voltage; roughly, going up two AWG sizes (doubling the area) halves the drop. Copper carries current with about a third less resistance than aluminum of the same size, so an aluminum run needs to be a gauge or two larger to match it.

It is a solid sizing guide. The figures use conductor resistance at a typical operating temperature and ignore reactance, which only becomes significant on large three-phase runs (say 480 V feeders). It does not replace a full circuit design — always confirm the ampacity (NEC current-carrying capacity), protection and installation method against your wiring rules.