What Size Wire for 30 Amps at 100 Feet?

For a 30 amp, 240 V circuit that runs 100 feet one way, use 8 AWG copper or 6 AWG aluminum. 10 AWG is the right size on a short run, but at 100 ft it loses a little over 3% of the voltage.

The answer

8 AWG copper

30 A, 100 ft one way, 240 V

6 AWG aluminum (75 °C terminals)

Smallest wire for 30 A with a 3% voltage drop, by one-way length and voltage
Case Copper · 75 °CCopper · NM / 60 °CAluminum · 75 °CDrop at copper size
25 ft 240 V 10 AWG 10 AWG 8 AWG0.8%
50 ft 240 V 10 AWG 10 AWG 8 AWG1.5%
100 ft 240 V 8 AWG 8 AWG 6 AWG1.9%
150 ft 240 V 8 AWG 8 AWG 4 AWG2.9%
200 ft 240 V 6 AWG 6 AWG 4 AWG2.5%
50 ft 120 V RV or trailer 8 AWG 8 AWG 6 AWG1.9%
100 ft 120 V RV or trailer 6 AWG 6 AWG 4 AWG2.5%

Calculate for your exact distance

On this page

Why 10 AWG falls short at 100 feet

10 AWG copper is the correct size for a 30 amp breaker. NEC 240.4(D) caps it at 30 A, and the 60 °C column rates it exactly 30 A. But at the full 30 A over 100 ft, 10 AWG loses about 3.1% of 240 V. That is only just over the 3% target. 8 AWG brings the drop down to about 2% and is the safer choice for a long-running load or a longer run.

Dryers, water heaters and 30 amp RV outlets

Electric dryers and tank water heaters are 240 V loads and use the 240 V rows. A 30 amp RV shore-power outlet (TT-30) is a 120 V circuit, so it is shown separately in the table. The same wire loses twice the percentage at 120 V, because the lost volts are compared with half the supply voltage. The 120 V rows in the table show how much heavier the wire has to be.

Always follow the appliance nameplate: the manufacturer's required circuit size overrides any chart.

Aluminum at 30 amps

10 AWG aluminum is limited to a 25 amp breaker under NEC 240.4(D), so aluminum for a 30 amp circuit starts at 8 AWG, and a 100 ft run at 240 V needs 6 AWG. Use terminals and connectors listed for aluminum.

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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