You need 3 AWG copper wire (THHN/THWN-2 in conduit) and a 100-amp breaker for a standard 100A feeder. If you are pulling NM-B (Romex) cable instead of individual conductors in conduit, you must upsize to 1 AWG copper due to strict 60°C insulation limits enforced by the NEC.
- Material: Copper (do not apply these sizes to aluminum)
- Temperature Column: 75°C (standard for modern 100A breakers and panel lugs)
- Ambient Temperature: 30°C (86°F) or lower
- Installation: Maximum 3 current-carrying conductors (CCCs) in a single raceway (no bundling derating)
The NEC Ampacity Table: Why 3 AWG and Not 4 AWG?
When sizing conductors, electricians reference NFPA 70 (National Electrical Code) Table 310.16. A common mistake is assuming 4 AWG copper is sufficient because it is rated for 85 amps in the 75°C column, and some try to round up. The NEC does not allow rounding up to the next standard breaker size unless the calculated load is non-standard and the ampacity is under 800A (240.4(B)). However, for a dedicated 100A feeder, you must hit the 100A mark exactly or exceed it.
| AWG Size | Insulation Type | Ampacity at 75°C | Meets 100A Requirement? |
|---|---|---|---|
| 4 AWG | THHN / THWN-2 | 85 Amps | No (Undersized) |
| 3 AWG | THHN / THWN-2 | 100 Amps | Yes (Exact Match) |
| 2 AWG | THHN / THWN-2 | 115 Amps | Yes (Oversized) |
Per NEC 110.14(C)(1)(a), termination temperature limitations dictate that we must use the 60°C column for circuits rated 100A or less unless the equipment is specifically listed and identified for 75°C terminations. Virtually all modern 100A molded-case circuit breakers (like the Square D Homeline or QO series, and Siemens EQ) and subpanel lugs are rated for 75°C. Therefore, the 75°C column applies to THHN/THWN-2 wires in conduit, making 3 AWG the legal minimum.
Decision Tree: Conduit vs. NM-B Cable
The insulation jacket completely changes the math. NM-B cable (commonly known by the brand name Romex) is governed by NEC 334.80, which mandates that the ampacity of NM-B cable must be determined using the 60°C column, regardless of the fact that the individual wires inside the jacket might technically be rated for 90°C. In the 60°C column, 3 AWG copper is only rated for 85 amps. To legally carry 100 amps with NM-B, you must jump to 1 AWG.
| Installation Method | Insulation / Cable Type | NEC Temp Column | Required Copper Size |
|---|---|---|---|
| Individual wires in conduit (EMT, PVC) | THHN / THWN-2 | 75°C | 3 AWG |
| Direct Burial Underground | USE-2 / UF-B | 60°C / 75°C* | 3 AWG (USE-2) / 1 AWG (UF-B) |
| Stapled to framing (Indoor) | NM-B (Romex) | 60°C | 1 AWG |
*Note: UF-B cable is limited to 60°C per 339.5. USE-2 is rated 75°C but local AHJ interpretations on direct burial terminations vary; 3 AWG USE-2 is generally accepted for 100A underground feeders if terminations are 75°C rated.
Voltage Drop Check: The 100-Foot Threshold
The NEC does not strictly mandate voltage drop limits for standard feeders, but Informational Note 2 to NEC 310.15(B) strongly recommends a maximum 3% drop on branch circuits and feeders to ensure equipment operates efficiently. For a 240V system, a 3% drop equals 7.2 volts.
Let us run the math for a 100A load at 240V using 3 AWG copper (Circular Mils = 52,620) and the standard copper K-factor of 12.9:
- At 100 feet: VD = (2 × 12.9 × 100A × 100ft) / 52,620 = 4.9 Volts (2.04%). This passes easily.
- At 150 feet: VD = (2 × 12.9 × 100A × 150ft) / 52,620 = 7.35 Volts (3.06%). This fails the 3% recommendation.
If your conduit run from the main panel to the subpanel exceeds 120 feet, you must upsize to 2 AWG copper. At 150 feet, 2 AWG (66,360 CM) drops the voltage loss to 5.83V (2.4%), bringing it safely back under the 3% threshold. You can verify these figures using manufacturer tools like the Southwire Voltage Drop Calculator.
What Changes the Answer: Derating and Continuous Loads
The 3 AWG baseline assumes ideal conditions. Real-world jobsite factors will force you to upsize your wire if you hit any of the following triggers:
1. Bundling (More than 3 CCCs in a Raceway)
If you are pulling two 100A circuits in the same conduit (meaning 4 current-carrying conductors), NEC Table 310.15(C)(1) requires an 80% derating factor. 3 AWG THHN in the 90°C column is 110A. Derated by 80%, it becomes 88A—no longer sufficient for a 100A breaker. You must upsize to 2 AWG (130A × 0.80 = 104A).
2. High Ambient Temperatures
If the conduit runs through an attic space or boiler room where ambient temperatures regularly exceed 86°F (30°C), you must apply the temperature correction factors in Table 310.15(B)(1). At 113°F (45°C), the correction factor for 75°C wire is 0.82. A 3 AWG wire (100A) derates to 82A. You would need to upsize to 1 AWG to maintain a 100A capacity.
3. The 125% Continuous Load Rule
If the 100A load is considered "continuous" (expected to run for 3 hours or more, such as a commercial EV charger or heavy HVAC equipment), NEC 210.20(A) requires the overcurrent protective device to be sized at 125% of the load. A 100A continuous load requires a 125A breaker and wire sized for 125A. In this scenario, 3 AWG is illegal; you must install 1/0 AWG copper (rated 125A at 75°C).
When an Engineer or AHJ Must Confirm
While the guidelines above cover 95% of residential and light commercial subpanel feeders, you must pull a permit and defer to your local Authority Having Jurisdiction (AHJ) or a licensed Professional Engineer (PE) in the following scenarios:
- Service Entrance Conductors: If this 100A feed is the main service drop from the utility meter to the primary disconnect, NEC 230.42 and utility company "Green Books" dictate specific sizing, often allowing 2 AWG copper for 100A residential services due to diversity factors. Never guess on service entrance wire.
- Aluminum Substitutions: If material costs force a switch to aluminum (e.g., 1 AWG XHHW-2), the termination lugs must be explicitly rated for aluminum (ALR/CU-AL) and treated with anti-oxidant paste. Many modern 100A breakers only accept copper. Check the breaker schematic label before buying aluminum.
- Fault Current Ratings: If the available fault current at the utility transformer exceeds 10,000 AIC (Ampere Interrupting Capacity), standard 100A breakers may violently fail during a dead short. An engineer must calculate the AIC and specify a high-AIC breaker (e.g., 22kAIC or 65kAIC).
Default Recommendation: For a standard 100-amp subpanel feeder under 120 feet in normal ambient temperatures, buy 3 AWG THHN/THWN-2 copper (black, red, white, and bare ground) and pull it through a 1-inch PVC conduit protected by a 100A dual-pole breaker. If you are forced to use NM-B cable, buy 1 AWG NM-B. Do not attempt to use 4 AWG copper or 2 AWG aluminum for a 100A copper-specific termination.
