What is the 3% voltage drop rule?

The 3 percent voltage drop rule is the NEC's recommendation that a branch circuit lose no more than 3 percent of its voltage at design load, with feeder plus branch held to 5 percent combined. It lives in informational notes to 210.19 and 215.2.

Informational means unenforceable in most jurisdictions; the exceptions are specific articles like fire pumps where a hard limit applies, and any job whose specs adopt the number contractually.

On 120 volts, 3 percent is only 3.6 volts, gone in about 100 feet of loaded 12 gauge.

Treat it as the design line: motors, EV chargers, and long detached-building runs misbehave beyond it even though no inspector writes the ticket.

The NEC voltage drop calculator tests any run against the 3 and 5 percent lines instantly.