For a standard 100-amp circuit, use 3 AWG copper or 1 AWG aluminum wire, protected by a 100-amp double-pole breaker. This baseline assumes standard 75°C terminations, a 30°C ambient temperature, and no more than three current-carrying conductors in the conduit.
- Material: Copper (unless aluminum is explicitly specified)
- Temperature Column: 75°C (Standard for residential breakers/lugs per NEC 110.14(C))
- Ambient Temperature: 30°C (86°F)
- Conduit Fill: Not more than 3 current-carrying conductors (no derating required)
- Insulation Type: THHN/THWN-2
The Baseline Sizing Table (Copper vs. Aluminum)
Wire sizing is not a guessing game; it is dictated by the Cerrowire Ampacity Charts and the National Electrical Code (NEC). The table below outlines the exact ampacities for wire sizes surrounding the 100-amp threshold. Notice that we rely on the 75°C column for our final breaker sizing, even though modern THHN wire insulation is rated for 90°C.
| AWG Size | Material | 75°C Ampacity (Termination Limit) | 90°C Ampacity (Insulation Limit) | Max Standard Breaker |
|---|---|---|---|---|
| 4 AWG | Copper | 85A | 95A | 80A |
| 3 AWG | Copper | 100A | 115A | 100A |
| 2 AWG | Copper | 115A | 130A | 100A / 110A |
| 2 AWG | Aluminum | 90A | 100A | 90A |
| 1 AWG | Aluminum | 100A | 115A | 100A |
| 1/0 AWG | Aluminum | 120A | 135A | 125A |
Why 3 AWG Copper and Not 4 AWG?
A common trap for DIYers and junior apprentices is looking at the 90°C column on a wire spool and assuming 4 AWG THHN (rated 95A at 90°C) is close enough for a 100-amp breaker, or relying on the "next size up" rule. This is a dangerous misinterpretation of NFPA 70: National Electrical Code.
Under NEC 110.14(C), the ampacity of a conductor is limited by the temperature rating of the equipment terminations (the lugs on your breaker and panelboard). Almost all residential and light-commercial breakers and lugs rated 100A or less are tested and listed for 75°C. Therefore, you must use the 75°C ampacity column to size your wire, regardless of the fact that the THHN insulation itself can withstand 90°C.
In the 75°C column, 4 AWG copper is only rated for 85 amps. If you protect 4 AWG wire with a 100-amp breaker, you violate NEC 240.4 (Overcurrent Protection). The breaker will not trip until the current exceeds 100 amps, but the wire's termination points will overheat, degrade, and potentially cause a fire long before the breaker trips. You must step up to 3 AWG copper, which hits exactly 100A in the 75°C column.
What Changes the Answer: Derating, Distance, and Continuous Loads
The 3 AWG copper / 1 AWG aluminum baseline only holds true under perfect conditions. In the real world, you must adjust for voltage drop, conduit bundling, and load duration.
1. Voltage Drop Over Distance
The NEC recommends a maximum voltage drop of 3% for branch circuits and 5% for the combined feeder and branch circuit. Let's run the math for a 100-amp, 240V circuit running 150 feet from the main panel to a subpanel using 3 AWG copper.
- Resistance of 3 AWG Cu: ~0.245 ohms per 1,000 feet.
- Total Loop Length: 150 ft x 2 (out and back) = 300 feet.
- Total Resistance: 0.245 x (300 / 1000) = 0.0735 ohms.
- Voltage Drop: 100A x 0.0735 ohms = 7.35 volts.
- Percentage Drop: (7.35 / 240) x 100 = 3.06%.
Because 3.06% exceeds the 3% recommendation for a branch circuit, you should upsize to 2 AWG copper for a 150-foot run to keep the drop under 3% (2 AWG yields a ~2.4% drop). If this is a feeder to a subpanel, 3.06% is technically acceptable (under the 5% total limit), but upsizing is still best practice to prevent motor burnout and dimming lights.
2. Conduit Bundling (Derating)
If you pull multiple circuits through the same conduit, the wires heat each other up. Under NEC 310.15(C)(1), if you have 4 to 6 current-carrying conductors in a raceway, you must apply an 80% derating factor to the wire's 90°C ampacity.
For 3 AWG THHN (115A at 90°C): 115A x 0.80 = 92A. This is below your 100A requirement. If you are bundling conductors, you must upsize to 1 AWG copper (130A at 90°C x 0.80 = 104A) to safely carry 100 amps.
3. Continuous vs. Non-Continuous Loads
Is your 100-amp load running for 3 hours or more continuously? (Examples: EV chargers, large HVAC units, or commercial lighting). Under NEC 210.20 and 215.2, continuous loads must be multiplied by 125%.
A 100-amp continuous load requires wire and overcurrent protection sized for 125 amps. In this scenario, 3 AWG copper (100A) is entirely insufficient. You must use 1 AWG copper (130A at 75°C) and a 125-amp breaker, or limit the continuous load to 80 amps on your 100-amp breaker.
| Condition | Required Copper Size |
|---|---|
| Standard 100A, < 100 ft, non-continuous | 3 AWG |
| Run length exceeds 125 ft (Voltage Drop) | 2 AWG |
| 4 to 6 conductors in one conduit | 1 AWG |
| Continuous load (3+ hours at full draw) | 1 AWG (with 125A breaker) |
Aluminum Wire Prep and When to Call the AHJ
If you choose 1 AWG aluminum (often used for long feeder runs to subpanels to save money), you cannot simply strip it and torque it down like copper. Aluminum oxidizes rapidly when exposed to air, creating a high-resistance layer that generates intense heat at the lug.
You must abrade the wire strands with a wire brush and immediately coat them with an antioxidant compound like Noalox or Penetrox before terminating. Furthermore, aluminum and copper expand at different rates. You must use a calibrated torque screwdriver or wrench to tighten the lugs to the exact inch-pound specification printed on the panelboard label (typically between 250 and 350 in-lbs for 100A lugs). Hand-tightening aluminum feeders is a leading cause of subpanel fires.
When an Engineer or the AHJ Must Confirm
While this guide covers standard feeder and branch circuit sizing, you must consult your local Authority Having Jurisdiction (AHJ) or a licensed professional engineer under the following conditions:
- Service Entrance Conductors: If this 100-amp wire is the main service drop from the utility meter to your main panel, NEC 310.12 and 230.42 apply, which have specific sizing and grounding rules that supersede standard feeder rules.
- High Ambient Temperatures: If the conduit runs through an attic that regularly exceeds 104°F (40°C), you must apply ambient temperature correction factors from NEC Table 310.15(B)(1), which will force you to upsize the wire.
- Local Amendments: Some municipalities have strict local amendments that mandate copper-only for all interior wiring or require voltage drop calculations to be submitted as part of the permit package.
Sizing wire correctly is about matching the weakest link in the circuit—usually the termination lug—to the thermal limits of the conductor. Stick to the 75°C column, calculate your voltage drop for runs over 100 feet, and always torque your lugs to spec.
