What Size Wire for a Subpanel Feeder?

A 60 amp subpanel feeder needs 6 AWG copper (4 AWG aluminum) out to about 120 ft, then 4 AWG copper. A 100 amp feeder needs 3 AWG copper (1 AWG aluminum) out to about 145 ft, then 2 AWG.

The answer

60 A, 100 ft one way, 240 V

6 AWG copper

THHN in conduit with 75 °C terminals

4 AWG copper

for NM (Romex) cable or 60 °C terminals

4 AWG aluminum (75 °C terminals)

Smallest feeder wire at 240 V with a 3% voltage drop, by breaker size and one-way length
Case Copper · 75 °CAluminum · 75 °CDrop at copper size
60 A feeder 50 ft run 6 AWG4 AWG1.2%
60 A feeder 100 ft run 6 AWG4 AWG2.5%
60 A feeder 150 ft run 4 AWG2 AWG2.3%
60 A feeder 200 ft run 3 AWG1 AWG2.4%
100 A feeder 50 ft run 3 AWG1 AWG1.0%
100 A feeder 100 ft run 3 AWG1 AWG2.0%
100 A feeder 150 ft run 2 AWG1/0 AWG2.4%
100 A feeder 200 ft run 1 AWG2/0 AWG2.6%

Calculate for your exact distance

On this page

The feeder breaker sets the minimum wire size

A subpanel feeder is protected by a breaker in the main panel, and the wire must be rated for that breaker. Unlike a dwelling service, a feeder that serves only part of the load does not get the 83% reduction of NEC 310.12: a 100 amp feeder breaker needs wire rated 100 A at 75 °C. A load calculation may show you need a smaller breaker than you first assumed, which also means smaller wire.

Distance changes the answer

At 50 ft, ampacity decides the size. Past about 120 ft, voltage drop takes over, and by 150 ft a 60 amp feeder needs 4 AWG copper instead of 6, and a 100 amp feeder needs 2 AWG instead of 3. Aluminum feeders are common for long, heavy runs because they cost much less per foot, but they are two sizes larger and need connectors rated for aluminum.

What a feeder to another building contains

A feeder to a subpanel normally carries four conductors: two hots, a neutral and an equipment ground. A detached garage or shed has extra grounding requirements, such as a grounding electrode. The rules vary by location, so check them with your inspector before you pull wire.

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.
  • Sizes assume terminals rated 75 °C.
  • 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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