What Size Wire for a 100 Amp Subpanel?
Short answer: a 100 amp subpanel feeder needs 3 AWG copper or 1 AWG aluminum (75°C column of Table 310.16), plus an equipment grounding conductor of 8 AWG copper or 6 AWG aluminum. The 83% rule for services does not apply to a subpanel. Long runs to a detached garage or shop often need a larger size for voltage drop.
100 A subpanel feeder options
| Feeder | Hots and neutral | Ground | Notes |
|---|---|---|---|
| THHN/THWN-2 copper in conduit | 3 AWG | 8 AWG Cu | 1" EMT or PVC fits four conductors |
| XHHW-2 aluminum in conduit | 1 AWG | 6 AWG Al | Lower cost, larger conduit |
| Aluminum SER cable (inside a building) | 1-1-1-3 | included | 2-2-2-4 SER is rated 90 A |
| Aluminum URD / direct burial (to detached building) | 1 AWG | 6 AWG Al or separate Cu | Check depth per Table 300.5 |
The 75°C column is allowed because feeder breakers and panel lugs at this size are almost always marked 75°C. If any termination is rated only 60°C, 110.14(C)(1)(a) sends you to the 60°C column of the ampacity chart and a larger conductor.
Why not the 83% rule?
Table 310.12 applies only to conductors that carry the entire load of a dwelling. A subpanel feeds part of the house or another building, so size from Table 310.16. The 4 AWG copper / 2 AWG aluminum sizes on the 100 amp service page are not valid here.
What about 2-2-2-4 aluminum SER on a 100 A breaker?
2 AWG aluminum is rated 90 A at 75°C. NEC 240.4(B) allows the next standard breaker (100 A) if the calculated load on the subpanel is 90 A or less and the circuit isn't a multi-outlet branch circuit. It's a common installation, but it requires a load calculation to justify it.
100 A feeder by distance (240 V)
| Conductor | 50 ft | 100 ft | 150 ft | 200 ft |
|---|---|---|---|---|
| Copper | 3 AWG (0.8%) | 3 AWG (1.6%) | 3 AWG (2.5%) | 2 AWG (2.6%) |
| Aluminum | 1 AWG (0.8%) | 1 AWG (1.7%) | 1 AWG (2.5%) | 1/0 AWG (2.7%) |
Smallest conductor that meets both the 75°C ampacity for a 100 A circuit and a 3% voltage drop at 80 A (80% of the rating), 240 V single-phase, one-way distance. Percent drop in brackets. If the circuit uses NM-B cable or 60°C terminations, start from the 60°C column instead.
At an 80 A load, 3 AWG copper reaches 3% drop at about 184 feet and 1 AWG aluminum at about 178 feet. Past that, go up one size.
Worked example: 150 feet to a detached shop
A 100 A feeder runs 150 feet in PVC to a shop panel with a main breaker. The expected load is 80 A.
- Ampacity: 3 AWG copper or 1 AWG aluminum, 75°C column.
- Voltage drop, copper: 2 × 12.9 × 80 × 150 ÷ 52,620 = 5.88 V, or 2.5%.
- Voltage drop, aluminum: 2 × 21.2 × 80 × 150 ÷ 83,690 = 6.08 V, or 2.5%.
- Conduit, copper option: three 3 AWG THHN (0.0973 in² each) plus an 8 AWG ground (0.0366 in²) total 0.3285 in². That fits 1" EMT (40% area 0.346 in²) and 1" PVC Schedule 40 (0.333 in²), with little to spare. Most crews pull it in 1-¼" for an easier pull over a long run. For aluminum XHHW-2, take the areas from Chapter 9 Table 5.
- At the shop: a grounding electrode system, a building disconnect (the main breaker can serve), and neutral and ground separated. Check the burial depth for the wiring method in Table 300.5.
Both conductors pass. Use the voltage drop calculator for your exact length, and the conduit fill chart for other combinations.
Four-wire feeder and grounding rules
- Four wires: two hots, a neutral and an equipment grounding conductor. Three-wire feeders to detached buildings are no longer permitted for new installations (250.32(B)(1)).
- Separate neutral and ground at the subpanel. The neutral is bonded to ground only at the service (250.24(A)(5) prohibits the connection downstream). Remove the bonding screw or strap; neutrals go on the isolated neutral bar and grounds on a ground bar bonded to the enclosure (408.40).
- Detached building: needs its own grounding electrode system (ground rods, etc.) per 250.32(A), plus a disconnecting means at the building (225.31).
- Main breaker optional: a main breaker in the subpanel can serve as the building disconnect for a detached building.
Common mistakes inspectors flag
- 4 AWG copper or 2 AWG aluminum on a 100 A breaker because "that's what the service uses."
- Bonding screw left in the subpanel neutral bar.
- Grounds and neutrals on the same bar in the subpanel.
- No ground rods or building disconnect at a detached garage or shop.
- Direct-burial cable too shallow, or THHN in underground conduit without a wet-location "W" rating.
Frequently asked questions
Can I use 4 gauge copper for a 100 amp subpanel?
No. 4 AWG copper is rated 85 A at 75°C. You need 3 AWG copper (100 A). 4 AWG copper is only permitted at 100 A for a dwelling service or main feeder under Table 310.12.
What size breaker feeds a 100 amp subpanel?
A 100 A two-pole breaker in the main panel, assuming the feeder conductors are rated at least 100 A. You may feed a 100 A-rated subpanel with a smaller breaker (for example 60 A) and size the feeder for that breaker; see 60 amp wire size.
Can I put a 100 amp subpanel on a 100 amp main panel?
Yes. Panel ratings aren't added together. What matters is that the calculated load of the whole house, including the subpanel, stays within the service rating.
Does a subpanel in the same house need a ground rod?
No. A subpanel in the same building is grounded through the equipment grounding conductor run with the feeder. Ground rods are required at a separate building or structure (250.32(A)), not at an attached garage or basement subpanel.
What size conduit for a 100 amp subpanel?
Three 3 AWG THHN plus an 8 AWG ground fit in 1" EMT (0.3285 in² of the 0.346 in² allowed). Many electricians use 1-¼" for an easier pull.
Last updated: October 6, 2026. Based on NEC 2023. Check results against the code edition and local amendments your jurisdiction enforces and with the AHJ; electrical work should be done by qualified people.