What is voltage drop?

Voltage drop is the voltage lost between the source and the load because the wire itself resists current. Push 120 volts into a long run and the far end might see 114; the missing volts became heat in the copper.

Ohm's law runs the show: drop equals current times the wire's resistance. More amps, longer wire, or thinner wire all mean more loss.

The symptoms are familiar: lights that dim when the compressor starts, tools that feel weak at the end of a cord, chargers that run slow in the garage.

The NEC recommends holding drop to 3 percent on a branch circuit. The cure is always the same: shorter runs or fatter wire.

See the loss on any run with the voltage drop calculator before the wire goes in.