Feed a 125 amp sub panel with 1 AWG copper or 2/0 AWG aluminum wire at 75 °C and a separate 6 AWG copper (4 AWG aluminum) ground.
125 amp sub panel feeder size
125 amp sub panel feeder at 75 °C and ground wire, copper and aluminum, and the smallest conduit for four THHN wires
Copper
Aluminum
Feeder, 75 °C
1 AWG
2/0 AWG
NM-B, 60 °C
not made
—
Ground wire
6 AWG
4 AWG
EMT conduit
1-1/4″
1-1/2″
PVC 40 conduit
1-1/4″
1-1/2″
Conduit sizes are for four THHN wires: two hots and a neutral of the feeder size plus the ground wire.
125 amp sub panel wire size by distance
Sizes that keep the voltage drop within 3% at 240 V with the panel fully loaded. When the feeder is made larger for distance, the ground wire grows with it, in copper and in aluminum.
125 amp sub panel feeder and ground wire by one-way distance, copper and aluminum at 75 °C, within 3% voltage drop at 240 V and full load
Distance
Copper feeder
Copper ground
Aluminum feeder
Aluminum ground
50 ft
1 AWG
6 AWG
2/0 AWG
4 AWG
100 ft
1 AWG
6 AWG
2/0 AWG
4 AWG
150 ft
1 AWG
6 AWG
2/0 AWG
4 AWG
200 ft
1/0 AWG
4 AWG
3/0 AWG
2 AWG
300 ft
3/0 AWG
3 AWG
250 kcmil
1 AWG
Copper feeder
Aluminum feeder
125 amp sub panel feeder size by distance, within 3% voltage drop at 240 V.
A panel is rarely loaded to its full rating for long. Sizing for the actual calculated load instead of the breaker rating gives smaller wire on long runs; the calculator below does that.
What are you sizing for?
A
ft
From the panel to the load, not there and back
Voltage
Conductor
Termination rating
Result
1 AWG copper
Ampacity sets the size; the voltage drop is within 3%.
Breaker
125 A
Voltage drop
3.9 V (1.6%)
Volts at the load
236.1 V
Ground wire
6 AWG copper
How it is calculated
Breaker: 125 A rounded up to a standard size = 125 A.
Smallest copper conductor rated for 125 A at 75 °C: 1 AWG.
Drop = 2 × K × 125 A × 100 ft ÷ 83,690 cmil = 3.85 V, target 3% of 240 V.