For a standard 100 amp breaker, you need 3 AWG copper wire or 1 AWG aluminum wire. This assumes 75°C terminations, a 30°C ambient temperature, and no more than three current-carrying conductors in the conduit. Always verify local codes, as residential service feeders have specific exceptions.
- Material: Copper (THHN/THWN-2) or Aluminum (XHHW-2)
- Termination Rating: 75°C (Standard for modern breakers and lugs per NEC 110.14(C))
- Ambient Temperature: 30°C (86°F)
- Installation Method: Raceway (conduit) or cable, not more than 3 current-carrying conductors
The Baseline: Ampacity and the 75°C Termination Rule
The most common mistake DIYers and junior electricians make is sizing wire based on the 90°C column of the ampacity table because THHN wire is rated for 90°C. However, the National Electrical Code (NEC) section 110.14(C) dictates that you must size the wire based on the lowest temperature rating of any connected component. Almost all standard 100A breakers, panelboard lugs, and disconnect switches are rated for 75°C. Therefore, the 75°C column is your legal limit for ampacity.
Why use #3 AWG and not one size smaller? A #4 AWG copper wire is only rated for 85 amps in the 75°C column. If you pull 100 amps through #4 wire, the wire will overheat before the 100A breaker trips, creating a fire hazard. The breaker must protect the wire; therefore, the wire's ampacity must equal or exceed the breaker's rating.
| Wire Size (AWG) | Copper (60°C) | Copper (75°C) | Copper (90°C) | Aluminum (75°C) |
|---|---|---|---|---|
| #4 | 70A | 85A | 95A | 65A |
| #3 | 85A | 100A | 115A | 75A |
| #2 | 95A | 115A | 130A | 90A |
| #1 | 110A | 130A | 145A | 100A |
Source: Adapted from Cerrowire / NEC Table 310.16. Always consult the latest adopted NEC edition in your jurisdiction.
Voltage Drop: The Distance Trap
Ampacity tables only tell you the maximum current a wire can carry before the insulation melts. They do not account for voltage drop over distance. If your 100A subpanel is located far from the main breaker, #3 AWG copper might deliver adequate current but fail to deliver adequate voltage.
The NEC recommends a maximum voltage drop of 3% for feeders and 5% for the total system (feeder + branch). Let us run the math on a 240V, 100A feeder running 150 feet to a detached garage subpanel.
Formula: VD = 2 × Current × Length × Resistance per foot
#3 AWG Copper AC resistance at 75°C ≈ 0.245 ohms / 1,000 ft.
VD = 2 × 100A × 150 ft × (0.245 / 1000) = 7.35 Volts
Percentage: (7.35V / 240V) × 100 = 3.06%
At 3.06%, you have exceeded the 3% feeder recommendation. Your 240V tools will see 232.6V, which can cause motors to run hot and draw excess current. The fix: Upsize to #2 AWG copper. #2 AWG has a resistance of roughly 0.194 ohms/kft, dropping the voltage loss to 2.4%, safely under the 3% threshold. If you are using aluminum, you would upsize from #1 AWG to #1/0 AWG for this distance.
Derating, Bundling, and Aluminum Substitutions
The baseline assumptions fall apart the moment you change the physical environment of the wire. Here is what forces you to abandon #3 Copper / #1 Aluminum and move to a larger size.
| Condition | Why It Changes the Math | Required Action |
|---|---|---|
| More than 3 Current-Carrying Conductors | NEC Table 310.15(C)(1) requires derating. If you pull 4-6 conductors in one conduit, you must derate ampacity to 80%. | Use the 90°C column for derating math. #2 AWG Cu (130A × 0.80 = 104A) is now your minimum. |
| Ambient Temp > 30°C (86°F) | Hot attics, rooftops, or boiler rooms reduce a wire's ability to shed heat. | Apply temperature correction factors from NEC Table 310.15(B)(1). A 40°C attic requires an 88% multiplier. |
| Using Aluminum Wire | Aluminum has lower conductivity and expands/contracts more than copper under thermal load. | Use #1 AWG Al minimum. You MUST apply an anti-oxidant compound (like Noalox) to aluminum strands before terminating to prevent galvanic corrosion and high-resistance faults. |
A note on aluminum terminations: Never torque an aluminum wire lug without a calibrated digital torque screwdriver. Aluminum creeps and relaxes over time. If you under-torque it, the connection loosens, arcs, and starts a fire. If you over-torque it, you crush the softer metal strands, reducing the cross-sectional area and creating a hot spot. Check the breaker manufacturer's datasheet; a typical 100A lug requires 250 to 300 inch-pounds of torque.
The Residential Service Exception vs. Standard Feeders
If you are wiring the main service feed from the utility meter to a 100A main panel in a single-family home, you might notice electricians using #4 AWG copper or #2 AWG aluminum. This is not a violation; it is a specific carve-out.
NEC Section 310.12 (formerly 310.15(B)(7)) provides an ampacity bonus for residential services and feeders that supply the entire dwelling load. Because residential loads are highly diversified (you rarely run the oven, dryer, HVAC, and EV charger at 100% simultaneously), the code allows you to size the wire at 83% of the service rating.
- 100A Residential Service (83% rule): 100A × 0.83 = 83A. #4 AWG Copper (85A @ 75°C) is legally sufficient.
- 100A Subpanel Feeder or Commercial Load: The 83% rule does NOT apply. You must use the full 100A rating, requiring #3 AWG Copper.
Do not rely on standard tables if your installation involves continuous loads exceeding 3 hours (which require sizing the wire to 125% of the load, meaning a 100A continuous load requires wire rated for 125A), utility-owned service laterals, or complex conduit fills passing through fire-rated assemblies. In these scenarios, local Authority Having Jurisdiction (AHJ) approval or a stamped engineering drawing is mandatory.
Sizing wire for a 100A breaker is a balance of thermal limits, voltage stability, and physical installation constraints. Stick to #3 AWG copper for standard short-run feeders, upsize for distance, and always respect the 75°C termination limit regardless of the insulation printed on the wire jacket.
