The correct wire for a 100 amp sub panel is 3 AWG copper or 1 AWG aluminum THHN/THWN-2, protected by a 100A two-pole breaker. This baseline assumes standard 75°C terminal ratings, a 30°C ambient temperature, and no more than three current-carrying conductors in the raceway.
Baseline Assumptions Block
- Conductor Material: Copper (unless aluminum is explicitly selected)
- Insulation Type: THHN/THWN-2 or XHHW-2
- Terminal Temperature Rating: 75°C (Standard for modern breakers and lugs rated 100A and below)
- Ambient Temperature: 30°C (86°F)
- Conduit Fill: 3 or fewer current-carrying conductors (no derating required)
- System Voltage: 120/240V Single-Phase
The Baseline Sizing: Ampacity and the 75°C Rule
When sizing feeders, many DIYers make the mistake of looking at the 90°C column on an ampacity chart because THHN wire is rated for 90°C. However, NEC Article 110.14(C) dictates that you must size the wire based on the lowest temperature rating of any connected component. Since most 100A breakers and subpanel lugs are rated for 75°C, the 75°C column governs your final ampacity.
Below is the data-dense ampacity reference for the sizes surrounding the 100A threshold. Notice that 4 AWG copper falls short at 85A, making 3 AWG the absolute minimum for copper.
| AWG Size | Copper 60°C | Copper 75°C (Governing) | Copper 90°C (Derating Only) | Aluminum 75°C (Governing) |
|---|---|---|---|---|
| 4 AWG | 70A | 85A (Fails 100A) | 95A | 65A (Fails) |
| 3 AWG | 85A | 100A (Passes) | 115A | 75A (Fails) |
| 2 AWG | 95A | 115A | 130A | 90A (Fails) |
| 1 AWG | 110A | 130A | 145A | 100A (Passes) |
If you choose aluminum to save on material costs (1 AWG aluminum is significantly cheaper than 3 AWG copper), you must ensure your subpanel lugs are explicitly rated for aluminum. Most modern load centers are, but always verify the label inside the panel door. Never interchange copper and aluminum ampacity values; doing so will result in an undersized feeder and a severe fire hazard.
Voltage Drop: When Length Forces an Upsize
The ampacity table above only satisfies the NEC's minimum safety requirements for preventing wire overheating. It does not account for voltage drop. While the NEC treats voltage drop as a recommendation rather than a strict violation for most feeders, NFPA 70 (NEC) Article 215.2 Informational Note suggests keeping feeder voltage drop under 3% for reasonable efficiency.
Let us run the math for a 240V, 100A load using 3 AWG copper (Circular Mils = 52,620; K-factor for copper = 12.9).
- At 100 feet: Voltage Drop = (2 × 12.9 × 100A × 100ft) / 52,620 = 4.90V. This is 2.04% of 240V. (Passes the 3% guideline).
- At 150 feet: Voltage Drop = (2 × 12.9 × 100A × 150ft) / 52,620 = 7.35V. This is 3.06% of 240V. (Fails the 3% guideline).
If your subpanel is located 150 feet from the main breaker, 3 AWG copper will result in noticeable voltage sag when heavy loads (like a table saw or EV charger) kick on. You must upsize to 2 AWG copper, which drops the voltage loss at 150 feet down to 5.83V (2.43%).
| One-Way Distance | Recommended Copper AWG | Calculated Voltage Drop | Action Required |
|---|---|---|---|
| Up to 115 feet | 3 AWG | ≤ 2.8% | Install 3 AWG THHN/THWN-2 |
| 116 to 175 feet | 2 AWG | ≤ 3.0% | Upsize to 2 AWG to maintain efficiency |
| 176 to 220 feet | 1 AWG | ≤ 3.0% | Upsize to 1 AWG; check conduit fill |
Derating, Bundling, and When to Call the AHJ
The baseline 3 AWG copper answer assumes you are pulling three current-carrying conductors (two hot legs and one neutral) plus a bare equipment grounding conductor through a single conduit. If your installation deviates from this, the math changes entirely.
Bundling and Conduit Fill Derating
Under NEC 310.15(C)(1), if you have more than three current-carrying conductors in a single raceway, you must apply a derating factor. For 4 to 6 conductors, the derating factor is 80%. If you are pulling a second circuit through the same conduit to feed a sub-sub-panel, you now have 4 or more current-carrying conductors.
To handle a 100A breaker with an 80% derating factor, your wire must have a baseline ampacity of 125A (100A / 0.80). Because derating is calculated using the 90°C column, 3 AWG THHN (115A at 90°C) is no longer sufficient. You must step up to 2 AWG THHN (130A at 90°C) to legally protect the circuit.
Continuous Loads and AHJ Approval
A 100A breaker is rated for a maximum continuous load of 80A (100A × 0.80). If your subpanel will supply continuous loads (defined by the NEC as loads expected to run for 3 hours or more, such as commercial lighting, server racks, or certain EV chargers) that exceed 80A, you cannot use a 100A breaker. You must upsize to a 125A breaker and use 1 AWG copper wire.
When to Consult an Engineer or the AHJ
While this guide covers standard residential feeder sizing, you must pull a permit and have your local Authority Having Jurisdiction (AHJ) or a licensed electrical engineer review your plans if:
- You are installing a Service Entrance rather than a subpanel feeder (service entrance grounding and bonding rules are vastly different and strictly enforced).
- The conduit will be routed through an environment where the ambient temperature regularly exceeds 30°C (86°F), such as an uninsulated attic in a southern climate, which requires additional temperature correction factors.
- You are utilizing parallel conductors (running multiple smaller wires to achieve 100A), which is generally not permitted for residential feeders under 400A without specific engineering oversight.
Finally, when terminating your 3 AWG or 1 AWG wire into the subpanel lugs, do not guess the torque. Use a calibrated torque screwdriver or inch-pound torque wrench set to the exact value printed on the panel's wiring diagram (typically between 40 and 50 in-lbs for 100A lugs). Under-torqued lugs cause high-resistance connections that will arc and melt over time, while over-torqued lugs can strip the aluminum threads or snap the screw head.
