What Size Wire for 20 Amps at 100 Feet?

For a 20 amp, 120 V circuit that runs 100 feet one way, use 8 AWG copper to stay within a 3% voltage drop. 12 AWG is the minimum for a 20 amp breaker but it loses about 6.6% at that distance.

The answer

8 AWG copper

20 A, 100 ft one way, 120 V

6 AWG aluminum (75 °C terminals)

Smallest wire for 20 A at 120 V with a 3% voltage drop, by one-way length
Case Copper · 75 °CCopper · NM / 60 °CAluminum · 75 °CDrop at copper size
25 ft 120 V 12 AWG 12 AWG 10 AWG1.6%
50 ft 120 V 10 AWG 10 AWG 8 AWG2.1%
100 ft 120 V 8 AWG 8 AWG 6 AWG2.6%
150 ft 120 V 6 AWG 6 AWG 4 AWG2.5%
200 ft 120 V 4 AWG 4 AWG 3 AWG2.1%

Calculate for your exact distance

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Why 12 AWG is not enough at 100 feet

12 AWG copper is the smallest wire allowed on a 20 amp breaker, and it is the right size up to about 45 ft at full load. Beyond that, the voltage lost in the wire grows with every foot. At 100 ft, 12 AWG loses about 6.6% of 120 V, which means lights dim and motors struggle at the far end. Stepping up to 10 AWG helps but only gets to roughly 70 ft. 8 AWG keeps a full 20 A load within about 3% at 100 ft.

Why 120 V needs thicker wire than 240 V

The volts lost in a wire depend on the current, the wire resistance and the length, not on the supply voltage. A 120 V circuit therefore loses the same volts as a 240 V one carrying the same current, but those volts are twice as large a share of 120 V. That is why a 20 amp 120 V run needs heavier wire than a 20 amp load on 240 V over the same distance.

Feeding a shed or detached garage

When the building is 100 ft or more from the panel, a single long 20 amp branch circuit is rarely the cheapest answer. A 240 V feeder to a small subpanel carries more power on less copper, and splits into short, easy branch circuits at the other end. See the subpanel wire size guide. Check your local code for detached-building requirements before you dig.

How these sizes are worked out
  • Each size is the smallest copper or aluminum wire that meets two limits: the table ampacity for the breaker (NEC Table 310.16, with the NEC 240.4(D) small-conductor limits) and a 3% voltage drop. The larger of the two is shown.
  • Copper is shown for 75 °C terminals and for NM (Romex) cable or 60 °C terminals, and aluminum for 75 °C terminals.
  • Voltage drop uses conductor resistance only, with the wire at 75 °C. Reactance is ignored, which is accurate for short runs.
  • No ambient temperature or bundling derating is applied. Hot locations and conduits with many current-carrying conductors need larger wire.
  • This is general information, not a substitute for a licensed electrician or your local code. Read the disclaimer, or use the wire size calculator for your exact run.

This is general information based on NEC tables, not a permit or an installation plan. Local amendments, the equipment nameplate and an inspector decide what is acceptable. Read the disclaimer.

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