For a 100-amp service or subpanel using THHN/THWN-2 wire in conduit, you need 3 AWG copper or 1 AWG aluminum, protected by a 100-amp breaker. If using standard SER cable, step up to 1 AWG copper or 1/0 AWG aluminum to account for insulation rating limits.
- Material: Copper (primary), Aluminum (secondary comparison)
- Temperature Column: 75°C (per NEC 110.14(C) for standard terminations)
- Ambient Temperature: 30°C (86°F)
- Conduit: PVC or EMT, max 3 current-carrying conductors (no derating required)
The Core Sizing Table: Copper vs. Aluminum for 100A
To determine the correct wire size, we reference the NFPA 70 National Electrical Code (NEC), specifically Table 310.16. The most common mistake DIYers make is looking at the 90°C column because THHN wire is rated for 90°C. However, NEC 110.14(C) dictates that unless your equipment is specifically listed and identified for 90°C terminations, you must use the 75°C column for ampacity sizing. Most residential breakers and panel lugs (like standard Square D Homeline or Siemens EQ series) are rated for 75°C.
| AWG Size | Material | Insulation Type | 75°C Ampacity | Max Breaker Size |
|---|---|---|---|---|
| 4 AWG | Copper | THHN/THWN-2 | 85A | 80A |
| 3 AWG | Copper | THHN/THWN-2 | 100A | 100A |
| 2 AWG | Copper | THHN/THWN-2 | 115A | 115A (Next size up: 125A) |
| 2 AWG | Aluminum | THHN/THWN-2 | 90A | 90A |
| 1 AWG | Aluminum | THHN/THWN-2 | 100A | 100A |
| 1/0 AWG | Aluminum | THHN/THWN-2 | 120A | 125A |
Why 3 AWG Copper (And Why Not 4 AWG or NM-B?)
Looking at the table above, 4 AWG copper is only rated for 85A at 75°C. Putting a 100A breaker on 4 AWG wire violates NEC 240.4(B) because 85A does not round up to the next standard breaker size of 100A (the rule only allows rounding up if the ampacity doesn't match a standard size, but 85A is insufficient for a 100A continuous or standard load limit here). Therefore, 3 AWG copper (100A) is the absolute minimum for THHN in conduit.
The NM-B (Romex) and SER Cable Trap
If you are running a subpanel feeder and want to use flat cable instead of pulling individual THHN wires through conduit, you cannot use standard NM-B (Romex). NEC 334.80 restricts NM-B ampacity to the 60°C column regardless of the wire's actual thermal rating. In the 60°C column, 2 AWG copper is only rated for 95A, meaning NM-B cannot legally be used for a 100A feeder.
Instead, you must use SER (Service Entrance Rated) cable. Because SER cable is often installed in thermal insulation or against framing, many AHJs (Authorities Having Jurisdiction) enforce the 60°C or 75°C column depending on the exact installation method. To guarantee compliance and avoid failed inspections, electricians universally pull 1 AWG copper SER or 1/0 AWG aluminum SER for 100A subpanel feeders. This provides a safe buffer and accounts for the higher resistance and voltage drop inherent in bundled flat cables.
Copper vs. Aluminum: The Real-World Tradeoffs
Aluminum is significantly cheaper and lighter, making 1/0 AWG aluminum SER the default choice for long subpanel runs. However, aluminum requires specific handling: you must apply anti-oxidant paste (like Noalox) to the stripped conductors and use a calibrated torque screwdriver to tighten the lugs to the manufacturer's exact spec (usually 40-50 in-lbs). Failure to torque aluminum correctly leads to thermal expansion cycles, loose connections, and eventually melted lugs or panel fires.
Variables That Change Your Wire Size (Distance, Heat, and Bundling)
The baseline sizes above assume a short run in a cool environment. Real-world jobsites rarely cooperate. Here is what forces you to upsize your wire.
Voltage Drop at Distance
NEC 310.15(B) notes that ampacity tables do not account for voltage drop. For a 240V feeder, the recommended maximum voltage drop is 3% (7.2V). Let's run a voltage drop check for a 100A load at 150 feet using 3 AWG copper.
Using the standard formula and data from the Southwire Voltage Drop Calculator:
- Formula: VD = (2 × K × I × D) / CM
- K (Copper): 12.9 ohms
- I (Current): 100A
- D (Distance): 150 ft
- CM (3 AWG Circular Mils): 52,620
VD = (2 × 12.9 × 100 × 150) / 52,620 = 7.35V.
7.35V is 3.06% of 240V. This exceeds the 3% recommendation. At 150 feet, you must upsize to 2 AWG copper (which drops the VD to roughly 2.3%) to ensure your subpanel receives adequate voltage under heavy load.
| One-Way Distance | Copper AWG Required | Aluminum AWG Required | Reasoning |
|---|---|---|---|
| Under 50 ft | 3 AWG | 1 AWG | Meets 75°C ampacity; VD is negligible. |
| 50 ft - 110 ft | 2 AWG | 1/0 AWG | Upsized to keep VD under 3% at 80A actual draw. |
| 110 ft - 180 ft | 1 AWG | 2/0 AWG | Required to maintain <3% VD at full 100A load. |
| Over 180 ft | 1/0 AWG | 3/0 AWG | Heavy upsize; consider moving the main panel closer. |
Bundling and Derating
If you pull more than three current-carrying conductors through a single conduit, NEC 310.15(C)(1) requires derating. If you have 4 to 6 conductors (for example, running two separate 100A subpanel feeders in one 2-inch PVC pipe), you must apply an 80% derating factor. A 3 AWG copper wire rated at 100A derates to 80A (100 × 0.8). This is now undersized for a 100A breaker. You would need to jump to 1 AWG copper (130A × 0.8 = 104A) to legally protect the circuit.
When an Engineer or the AHJ Must Confirm
While the tables above cover 95% of residential subpanel and service upgrades, certain conditions require professional stamping or explicit AHJ approval.
Continuous Loads (The 125% Rule)
If your 100A load is considered 'continuous' by the NEC (meaning it is expected to run at maximum current for 3 hours or more, such as a massive server rack, continuous HVAC compressor array, or commercial lighting), NEC 210.20(A) and 215.3 require the overcurrent device and conductors to be sized at 125% of the continuous load. A 100A continuous load requires a 125A breaker and wire sized for 125A (1 AWG copper minimum).
Utility Service Drops vs. Subpanel Feeders
If you are installing a 100A main service panel (where the utility meter feeds directly into your main breaker), the rules change. The utility company owns the service drop from the pole to the weatherhead. Many utilities mandate a minimum of 2 AWG copper or 1/0 AWG aluminum for the service entrance conductors, regardless of the NEC minimums, to handle fault currents and future grid upgrades. Always check with your local utility's 'Green Book' or service requirements manual before buying wire for a main service upgrade.
High Ambient Temperatures
If your conduit runs through an unventilated attic in a southern climate where ambient temperatures regularly exceed 30°C (86°F), you must apply temperature correction factors from NEC Table 310.15(B)(1). At 40°C (104°F), the correction factor for 75°C wire is 0.88. Your 3 AWG copper (100A) drops to 88A, requiring an upsize to 2 AWG or 1 AWG to maintain the 100A rating. When in doubt, or if your local inspector is known for strict thermal derating enforcement, pull 1 AWG copper as a universal baseline.
Disclaimer: This guide provides NEC-style guidance for educational purposes. Your local AHJ (Authority Having Jurisdiction) has final authority over all electrical installations. Always pull permits and schedule inspections for 100A panel work.
