A 100 amp subpanel feeder typically needs #3 AWG copper or #1 AWG aluminum wire; run length, terminal ratings, and local code can change that.
A subpanel feeder is a permanent decision — wire buried in walls or underground gets expensive to redo once the drywall’s up. Choosing what size wire for a 100 amp subpanel starts with a standard spec: #3 AWG copper or #1 AWG aluminum in the 75°C column that today’s panels are rated for.
That pair of sizes covers most residential feeders, but it assumes 75°C-rated terminations, which is the norm on modern panels and breakers. If the equipment sticker says 60°C, or the run stretches well past 100 feet, the feeder moves up a size. The final number also depends on the installation method and how your local code is applied.
Minimum Feeder Wire Sizes For A 100 Amp Subpanel
Three sizes matter here: the hot conductors, the neutral, and the equipment ground. A complete feeder is four wires — two hots, one neutral, one ground — and the ground runs smaller than the hots because it only carries fault current.
This table is the sizing shortcut for a typical 120/240 volt residential feeder. Copper conducts better than aluminum, so the aluminum version runs larger for the same 100 amp load.
| Conductor | Standard Size | Notes |
|---|---|---|
| Hot wires (copper) | #3 AWG | Rated 100 amps at 75°C; the usual pick for most runs. |
| Hot wires (aluminum) | #1 AWG | Also 100 amps at 75°C; needs AL-rated lugs and anti-oxidant compound. |
| Neutral | Full size | Same gauge as the hots in standard 120/240V practice. |
| Equipment ground | #8 copper / #6 aluminum | Per NEC Table 250.122 for a 100 amp feeder. |
| Alternate sizes | #4 copper / #2 aluminum | Cited in some references; verify ratings and local approval before relying on them. |
| Common cable types | THHN/THWN-2, XHHW, SER, UF-B, URD | THHN-type in conduit; SER indoors; UF-B and URD for direct burial. |
Notice the ground is smaller than the hot conductors. That’s by design: feeder wires carry continuous load, while the equipment ground only needs enough capacity to clear a fault. These ampacity ratings line up with the figures in Elliot Electric’s residential service sizing table, which is a useful cross-check before you order wire.
Why Neutral And Ground Stay Separate At The Subpanel
At a subpanel, the neutral and the equipment ground never bond together again. The bonding screw that ties them in the main panel is simply left out or removed.
In the main panel, neutral and ground share a bus because that’s the system’s single earth bond. Bonding a second time at the subpanel lets normal neutral current travel on ground wires and metal enclosures — the kind of mistake that can energize a panel box. Code answers this by requiring an insulated neutral and a separate ground bar in the subpanel.
If you’re shopping for the enclosure, our tested 100A subpanel roundup compares panels with enough bus space to keep the two bars separate.
When Does A 100 Amp Subpanel Feeder Need Bigger Wire?
Two conditions push a 100 amp feeder up a size: a long run and low termination ratings. Beyond roughly 100 feet, voltage drop becomes the deciding factor even when the ampacity chart says the smaller wire is legal.
Voltage drop steals usable voltage the longer the wire runs. A 150-foot underground feeder often steps up one full size to stay within the common 3% recommendation for feeders. Aluminum feeders add another requirement: lugs marked AL and the anti-oxidant compound the manufacturer specifies, because improperly terminated aluminum is a common failure point.
The 83% rule also causes confusion. That allowance trims conductor size for some 100 amp services, but it doesn’t automatically transfer to subpanel feeders, and inspectors treat the two differently.
For a typical run within roughly 100 feet of the main panel, order #3 AWG copper or #1 AWG aluminum, a full-size neutral, and a #8 copper or #6 aluminum ground. Confirm the 75°C rating on the panel and breaker lugs, keep the neutral and ground bars separate, and run your final plan past the local inspector — that last check is free and catches most DIY mistakes.
FAQs
Can I feed a 100 amp subpanel with aluminum wire?
Yes, aluminum is common and code-accepted for subpanel feeders when used correctly. The standard size is #1 AWG aluminum instead of #3 copper, and every lug it connects to must be rated AL. Anti-oxidant compound and correct torque matter more with aluminum than copper, which is why many inspectors look closely at those connections.
Why is the ground wire smaller than the hot wires?
The equipment ground only has to carry fault current long enough for the breaker to trip, while the hot conductors carry the full 100 amp load continuously. NEC Table 250.122 therefore allows a smaller conductor — #8 copper or #6 aluminum for a 100 amp feeder — and that smaller size still clears faults safely.
Does the 83% rule apply to subpanel feeders?
Usually not. The 83% allowance is tied to specific service conditions and is not a general license to downsize a feeder. For a subpanel, the standard approach is sizing the conductors for the full 100 amp load at the terminal temperature rating, then checking voltage drop for the distance. Local amendments can change this.
References & Sources
- Elliot Electric Supply. “Residential Service, 3 Wire, Single Phase — Sizing Table.” Reference table for residential service and feeder conductor sizing.
