What Size Wire for 50 Amps at 100 Feet?

For a 50 amp, 240 V circuit that runs 100 feet one way, use 6 AWG copper or 4 AWG aluminum to stay within a 3% voltage drop. 8 AWG is enough on a short run, but at 100 ft it loses a little over 3%.

The answer

6 AWG copper

50 A, 100 ft one way, 240 V

4 AWG aluminum (75 °C terminals)

Smallest wire for 50 A at 240 V with a 3% voltage drop, by one-way length
Case Copper · 75 °CCopper · NM / 60 °CAluminum · 75 °CDrop at copper size
25 ft 240 V 8 AWG 6 AWG 6 AWG0.8%
50 ft 240 V 8 AWG 6 AWG 6 AWG1.6%
100 ft 240 V 6 AWG 6 AWG 4 AWG2.0%
150 ft 240 V 4 AWG 4 AWG 3 AWG1.9%
200 ft 240 V 4 AWG 4 AWG 2 AWG2.6%

Calculate for your exact distance

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Why 8 AWG stops working at 100 feet

On its own, a 50 amp breaker only needs 8 AWG copper when the wire and terminals are rated 75 °C. That is an ampacity answer. It says nothing about distance. Every foot of wire adds resistance, and at 50 A the volts lost in the wire add up quickly: 8 AWG loses about 3.2% of 240 V over 100 ft, just past the usual 3% target. 6 AWG has roughly 63% of the resistance and comes in at about 2%.

That is the whole reason the answer changes from 8 AWG to 6 AWG somewhere between 50 and 100 ft. The table above shows where each size takes over.

How the answer changes with distance

Up to about 90 ft, 8 AWG copper (75 °C terminals) stays within 3%. From there to roughly 145 ft, 6 AWG works. Beyond that, 4 AWG copper is the next step, and it carries a 200 ft run with room to spare. Aluminum needs about two sizes more than copper for the same job.

If your run is not in the table, use the wire size calculator with your exact length, voltage and load.

NM cable is limited to the 60 °C column

NM (Romex) cable has to be sized from the 60 °C column of NEC Table 310.16 (NEC 334.80). In that column 8 AWG is rated only 40 A, so a 50 amp circuit in NM cable needs 6 AWG even on a short run. For long runs the distance rule takes over and both answers meet at 6 AWG, then 4 AWG.

A 3% drop is a target, not a code limit

The NEC mentions about 3% for a branch circuit and about 5% for the whole feeder-plus-branch run in an informational note. It is a recommendation. Motors and electronics prefer less than 3%. If you are close to a size boundary on a 100 ft run, the larger wire is cheap insurance.

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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