Voltage Drop Calculator
Voltage lost in a copper run, and whether the gauge is adequate.
Uncoated copper at 75 °C, per NEC Chapter 9 Table 8. Aluminium conductors drop roughly 60% more for the same size.
How it works
Current through a conductor loses voltage to its resistance, and that loss is heat. Single-phase circuits count the run twice — out and back — so the drop uses twice the one-way length. Three-phase uses a factor of √3.
The common guideline is 3% on a branch circuit and 5% overall including the feeder. It is not usually a code requirement, but it is where motors start running hot, LED drivers get unhappy and long runs stop working properly at the far end.
Common questions
What if I fail the 3% guideline?
Go up a wire size, shorten the run, or raise the system voltage. Doubling the voltage quarters the loss for the same delivered power, which is the entire argument for high-voltage transmission.