The minimum NEC-compliant size of wire for 100 amp service is 3 AWG copper or 1 AWG aluminum, protected by a 100A double-pole breaker. This assumes standard residential conditions: 75°C terminations, 30°C ambient temperature, and no more than three current-carrying conductors in a raceway.
- Material: Copper (THHN/THWN-2) or Aluminum (XHHW-2)
- Temperature Column: 75°C (Standard for modern residential breakers and lugs per NEC 110.14(C))
- Ambient Temperature: 30°C (86°F) or lower
- Conduit Fill: Maximum 3 current-carrying conductors in a single raceway
The Baseline: Minimum Wire Size for 100 Amp Service
When sizing conductors, you must look at the 75°C column of NEC Table 310.16, even if your wire insulation is rated for 90°C (like THHN). This is because the breaker lugs and panel bus bars are almost universally rated for 75°C. Using the 90°C column for final ampacity sizing is a common DIY mistake that leads to failed inspections.
| AWG Size | Material | Insulation Type | Ampacity (75°C) | Allowed Breaker |
|---|---|---|---|---|
| 4 AWG | Copper | THHN/THWN-2 | 85A | 80A (Too small) |
| 3 AWG | Copper | THHN/THWN-2 | 100A | 100A (Minimum) |
| 2 AWG | Aluminum | XHHW-2 | 90A | 90A (Too small) |
| 1 AWG | Aluminum | XHHW-2 | 100A | 100A (Minimum) |
Why Not One Size Smaller? The NEC 240.4 Rule
A frequent question on the bench is why we cannot use 4 AWG copper or 2 AWG aluminum on a 100A breaker. The answer lies in NEC Article 240.4, which governs overcurrent protection.
4 AWG copper has an ampacity of 85A. If you connect it to a 100A breaker and a fault draws 95A, the breaker will not trip. The wire, however, will overheat, potentially melting the insulation and starting a fire inside your walls. The breaker must be sized to protect the weakest link in the circuit. Because 85A does not correspond to a standard breaker size (NEC 240.6), and the next standard size up is 90A, 4 AWG copper is strictly limited to a 90A breaker. To safely use a 100A breaker, your wire must be rated for at least 100A, making 3 AWG copper the absolute floor.
What Changes the Answer: Voltage Drop, Bundling, and Temperature
The baseline numbers above assume a perfect, short run in a cool basement. Real-world jobsites rarely cooperate. Here is how environmental factors force you to upsize your wire.
| Condition | NEC Rule | Required Action |
|---|---|---|
| Run exceeds 150 feet | NEC 210.19 (Informational Note) | Upsize to 2 AWG Cu or 1/0 AWG Al to maintain <3% voltage drop. |
| 4 to 6 conductors in one conduit | NEC 310.15(C)(1) | Apply 80% derating factor. 3 AWG Cu (100A x 0.8 = 80A) is now too small. Upsize to 1 AWG Cu. |
| Ambient temp 41°C - 45°C (e.g., hot attic) | NEC 310.15(B)(1) | Apply 0.82 correction factor. Upsize to 2 AWG Cu or 1/0 AWG Al. |
The Voltage Drop Check
Voltage drop is not strictly enforced by the NEC for most residential feeders, but exceeding 3% causes motors to run hot and lights to dim. Using the Southwire Voltage Drop Calculator methodology for a 240V circuit with an 80A continuous load:
- At 100 feet: 3 AWG copper yields a 3.92V drop (1.6%). Perfectly fine.
- At 150 feet: 3 AWG copper yields a 5.88V drop (2.45%). Still under the 3% threshold.
- At 200 feet: 3 AWG copper yields a 7.84V drop (3.26%). Fail. You must upsize to 2 AWG copper (which drops the loss to 2.58%) to maintain power quality.
When an Engineer or the AHJ Must Confirm Your Plan
- You are working on Service Entrance Conductors: The wires between the utility meter and your main disconnect are governed by utility rules and NEC Article 230. Many utilities require specific sizing or prohibit DIY work on the line side of the meter.
- You are paralleling conductors: If you want to run two smaller wires in parallel to achieve 100A, NEC 310.10(G) strictly forbids paralleling any wire smaller than 1/0 AWG. You cannot parallel two 3 AWG wires.
- Your local AHJ has amendments: Some municipalities mandate a minimum of 2 AWG copper for all 100A subpanel feeders regardless of length to future-proof the installation. Always check local codes.
Frequently Asked Questions
Can I use 2 AWG wire for a 100 amp service?
Yes, absolutely. 2 AWG copper is rated for 115A at 75°C. While it is physically thicker and slightly more difficult to bend in tight panel gutters, it provides excellent voltage drop headroom for longer runs and makes terminating into lugs much easier since the wire fills the lug barrel completely. Many electricians use 2 AWG as their standard for 100A feeds simply to avoid keeping 3 AWG on the truck.
What size ground wire do I need for a 100 amp feeder?
Per NEC Table 250.122, the minimum equipment grounding conductor (EGC) for a 100A breaker is 8 AWG copper or 6 AWG aluminum. Crucial Pro-Tip: If you upsized your ungrounded (hot) wires from 3 AWG to 2 AWG to mitigate voltage drop, NEC 250.122(B) requires you to upsize the ground wire proportionally. In that scenario, your copper ground must increase to 6 AWG.
Is it better to use copper or aluminum for a 100 amp subpanel?
It depends entirely on the run length and environment. For short indoor runs under 50 feet, copper (3 AWG THHN) is easier to pull through conduit and terminates cleanly without special preparation. For long outdoor trench runs (100+ feet), aluminum (1/0 AWG XHHW-2) is the undisputed winner. It saves significant money on material costs and is the industry standard for underground feeders. If using aluminum, you must clean the wire strands with a wire brush, apply Noalox anti-oxidant paste, and torque the lugs to the manufacturer's exact inch-pound specification to prevent high-resistance heating over time.
