Voltage drop grows with run length — the same wire loses more volts the farther it goes
Same 12 AWG wire, same 15 A load — only the distance changes. Past ~75–100 ft, voltage drop (not ampacity) usually decides the wire size.
What this diagram shows
Three horizontal wire runs from the same panel on 12 AWG copper at 15 amps and 120 volts. At 30 feet the load receives about 118.2 volts, a 1.5 percent drop, within the 3 percent recommendation. At 100 feet the load receives about 114.1 volts, a 4.9 percent drop, exceeding the recommendation. At 200 feet the load receives about 108.1 volts, a 9.9 percent drop. The 3 percent figure is an NEC Informational Note recommendation, not enforceable code — long runs are why detached buildings and well pumps often need larger wire than ampacity alone requires.
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Voltage Drop Calculator
How much voltage do you lose over a wire run? Free NEC K-factor calculator for volts and % drop by wire size, length, and load — with the 3% check.
