The calculator is preset to a 100 amp feeder at 240 volts in conduit. Enter the distance to the subpanel; a detached garage or barn at the far end of the yard often needs the next size up for voltage drop.
How to use this calculator
- Keep the current at 100 amps, the rating of the feeder breaker in the main panel.
- Choose copper or aluminum. Aluminum is by far the more common feeder at this size because it costs much less.
- Enter the one-way distance from the main panel to the subpanel along the trench or the conduit route.
- Open More options and set the allowed drop to 2 percent if you want to keep the feeder tight and leave room for the circuits beyond it.
Worked examples
A subpanel in an attached garage
Load current: 100 A · Circuit voltage: 240 V · One-way distance: 40 ft · Wire type: THHN/THWN wires in conduit (75 °C)
- Wire size
- 3 AWG copper
- Current the wire must carry
- 100.0 A
- Rated for
- 100 A
- Voltage drop
- 0.82%
- Longest run within 3%
- 147 ft
A detached workshop across the yard
Load current: 100 A · Circuit voltage: 240 V · One-way distance: 150 ft · Conductor: Aluminum
- Wire size
- 1/0 AWG aluminum
- Current the wire must carry
- 100.0 A
- Rated for
- 120 A
- Voltage drop
- 2.51%
- Longest run within 3%
- 179 ft
A basement subpanel for a finished room
Load current: 100 A · Circuit voltage: 240 V · One-way distance: 25 ft · Conductor: Aluminum
- Wire size
- 1 AWG aluminum
- Current the wire must carry
- 100.0 A
- Rated for
- 100 A
- Voltage drop
- 0.53%
- Longest run within 3%
- 142 ft
Feeder size for a 100 amp subpanel by one-way distance
| One-way distance | Copper, THHN/THWN wires in conduit (75 °C) | Aluminum |
|---|---|---|
| 25 ft | 3 AWG | 1 AWG |
| 50 ft | 3 AWG | 1 AWG |
| 100 ft | 3 AWG | 1 AWG |
| 150 ft | 2 AWG | 1/0 AWG |
| 200 ft | 1 AWG | 2/0 AWG |
Each cell: the smallest conductor rated for a hundred amps in the higher temperature column that keeps the drop within the branch-circuit limit.
Why a subpanel feeder is bigger than a service
The code lets the service conductors of a single-family house carry only 83 percent of their rating, because a whole home never draws its full service at once. A subpanel feeds part of the house or another building, so it gets no such allowance and is sized from the regular ampacity table: 3 AWG copper carries 100 amps and 1 AWG aluminum 100 amps. Compare the 100 amp service page, where the same rating takes smaller wire.
Why NM-B cable does not appear here
NM-B cable is rated from the lowest temperature column, where copper does not reach one hundred amps in the sizes this cable is made in. A 100 amp feeder is therefore run as single conductors in conduit, as SER cable inside the house, or as aluminum cable rated for burial to a detached building. Each needs two hots, a neutral and a ground, with the neutral and ground bars kept apart in the subpanel.
Distance and a detached building
A feeder is usually held to about 2 percent of drop, tighter than a branch circuit. Over a long trench that pushes the size up a step or two, and the table shows where. A detached building also needs its own grounding electrode and a disconnect. The breaker in the main panel stays at 100 amps when you upsize the wire.
Common questions
What size wire for a 100 amp subpanel?
3 AWG copper or 1 AWG aluminum, rated 100 amps and 100 amps in the higher temperature column, on a short run. A long run may need the next size up.
Can I use 4 AWG copper for a 100 amp subpanel?
No. 4 AWG copper is rated 85 amps. It appears for a 100 amp house service only because the service is sized for 83 percent of its rating, and a subpanel is not.
What size aluminum wire for a 100 amp subpanel 150 feet away?
Over that distance voltage drop usually sets the size rather than the current: enter it above to see the answer, typically one or two sizes larger than the short-run 1 AWG.
Does a 100 amp subpanel need 4 wires?
Yes: two hots, a neutral and an equipment ground. The neutral bar in the subpanel must not be bonded to the ground bar.