Every cable has resistance, so it loses a little voltage along its length — and the longer the run and the higher the current, the more it loses. Size a circuit only on ampacity and a long run can arrive with too little voltage to work properly. Voltage drop is the second half of wire sizing.
The formula
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 (resistivity ÷ cross-sectional area — so a lower AWG number, a bigger mm², means less drop). The factor k is 2 for DC and single-phase circuits (it counts the return conductor) and √3 for three-phase. Divide the drop by the supply voltage for the percentage.
Enter the one-way route length, not the loop — the ×2 already accounts for the return path.
What drop is acceptable?
The 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). Your local wiring rules set the real limit — design to whatever applies.
Example: a 20 A, 240 V load run 100 ft on 10 AWG (6 mm²) copper drops about 5 V — roughly 2%, comfortably inside the 3% target.
Size and material matter most
Resistance falls as the conductor gets thicker, so going up two AWG sizes (doubling the area) roughly halves the drop. And 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.
| To cut voltage drop… | Effect |
|---|---|
| Up two AWG sizes (2× area) | ~half the drop |
| Copper instead of aluminum | ~⅓ less drop, same size |
| Halve the run length | ~half the drop |
Voltage Drop Calculator
Enter the supply type, current, run length and conductor size — it returns the drop in volts and %, a pass/fail against your limit, and the voltage left at the load.
When a long run means upsizing
On a long run the ampacity might be fine while the drop fails, so you size up for distance. A circuit that needs 12 AWG for its current may need 10 or 8 AWG once it runs 150 ft — not because the wire would overheat, but because the load would starve. The calculator flags exactly when that switch happens.
This isn’t the whole design
The figures use conductor resistance at a typical operating temperature and ignore reactance, which only matters on large three-phase runs (say 480 V feeders). Always confirm ampacity, protection and installation method against your wiring rules — voltage drop is one check among several.