To feed a 100 amp sub panel, use 3 AWG copper or 1 AWG aluminum wire, protected by a 100A double-pole breaker. This baseline assumes standard 75°C terminations, THHN/THWN-2 insulation in conduit, and an ambient temperature of 30°C (86°F).
All sizing recommendations in this guide rely on the following conditions:
• Material: Copper (Cu) or Aluminum (Al) as specified.
• Temperature Column: 75°C (per NEC 110.14(C) for standard 100A breaker lugs).
• Insulation: THHN/THWN-2 (rated 90°C, but ampacity is capped by the 75°C termination limit).
• Ambient Temp: 30°C (86°F) or lower.
• Conduit Fill: Maximum of 3 current-carrying conductors in a single raceway.
NEC-style guidance; your local AHJ (Authority Having Jurisdiction) has final authority on all installations.
Baseline Ampacity Chart (NEC Table 310.16)
Before pulling any wire, you need to understand how the National Electrical Code (NEC) rates conductors. Even though modern THHN wire is insulated for 90°C, the lugs inside your subpanel and the breaker terminals are almost universally rated for a maximum of 75°C. Therefore, we must size the wire using the 75°C column. If you use the 90°C column to size your breaker, you will overload the mechanical lugs and create a fire hazard.
| Wire Size (AWG) | Copper (75°C Col) | Copper (90°C Col) | Aluminum (75°C Col) | Aluminum (90°C Col) |
|---|---|---|---|---|
| 4 AWG | 85A | 95A | 65A | 75A |
| 3 AWG (Target Cu) | 100A | 110A | 75A | 85A |
| 2 AWG | 115A | 130A | 90A | 100A |
| 1 AWG (Target Al) | 130A | 145A | 100A | 115A |
Note: Don't forget your Equipment Grounding Conductor (EGC). Per NEC 250.122, a 100A feeder requires a minimum 8 AWG copper or 6 AWG aluminum ground wire.
Why 3 AWG Copper and Not One Size Smaller?
A common mistake on the jobsite is attempting to use 4 AWG copper to save money, assuming the 90°C insulation rating (95A) is close enough to 100A to utilize the NEC 240.4(B) 'next standard size up' rule. This is incorrect for a 100A subpanel feeder.
The 'next size up' rule only applies when your calculated load does not correspond to a standard breaker size, and the wire ampacity is sufficient for that calculated load. If your subpanel is rated for 100A and you are installing a 100A feeder breaker, the wire's allowable ampacity (in the 75°C column) must be at least 100A. Because 4 AWG copper maxes out at 85A in the 75°C column, it cannot be protected by a 100A breaker. You must step up to 3 AWG copper, which lands exactly at 100A.
If you are using aluminum, the jump is even more pronounced. 2 AWG aluminum is only rated for 90A at 75°C. You must step up to the thicker 1 AWG aluminum to legally hit the 100A threshold.
Variables That Force an Upsize: Length, Bundling, and Material
The 3 AWG Cu / 1 AWG Al baseline only holds true under perfect conditions. Real-world installations frequently require upsizing due to voltage drop, conduit derating, or material transitions.
1. Voltage Drop (The Distance Factor)
While the NEC enforces ampacity for fire safety, it only recommends a maximum voltage drop of 3% for feeders (NEC 310.15(B) Informational Note). If your subpanel is located far from the main service, 3 AWG copper will suffer from excessive voltage sag under heavy load.
| One-Way Distance | Wire Size (Cu) | Calc. Voltage Drop (at 100A / 240V) | Drop % | Verdict |
|---|---|---|---|---|
| 50 ft | 3 AWG | 2.45V | 1.02% | Pass |
| 100 ft | 3 AWG | 4.90V | 2.04% | Pass |
| 150 ft | 3 AWG | 7.35V | 3.06% | Fail (>3%) |
| 150 ft | 2 AWG (Upsized) | 5.82V | 2.42% | Pass |
If your conduit run exceeds 140 feet, upsize to 2 AWG copper or 1/0 AWG aluminum to keep your 240V tools and appliances running efficiently. You can verify your specific run using the Southwire Voltage Drop Calculator.
2. Conduit Bundling and Derating
If you are pulling more than one circuit through the same conduit, you must apply NEC derating factors. For example, if you pull two separate 240V feeders in a single PVC conduit, you have 4 current-carrying conductors. This triggers an 80% derating factor.
Here is where the 90°C column saves you: you apply the derating factor to the 90°C ampacity, then check if the result is still above your 75°C termination requirement. For 3 AWG THHN (110A at 90°C), multiplying by 0.8 yields 88A. Because 88A is less than 100A, 3 AWG fails the bundling test. You must upsize to 2 AWG THHN (130A x 0.8 = 104A), which safely clears the 100A requirement.
3. Aluminum vs. Copper Transitions
Aluminum is significantly cheaper and lighter, making 1 AWG SER (Service Entrance Cable) a popular choice for short indoor runs to a subpanel. However, aluminum requires specific preparation. You must wire-brush the conductor strands and apply an anti-oxidant compound like Noalox before terminating. Furthermore, aluminum expands and contracts more than copper under thermal load; you must use a calibrated torque screwdriver to tighten the lugs to the manufacturer's exact specification (usually printed on the breaker label, often around 40-50 in-lbs for 100A lugs) to prevent loose connections and subsequent arcing.
When an Engineer or the AHJ Must Confirm
While the rules above cover 95% of residential and light-commercial subpanel installations, certain edge cases require professional sign-off or an engineered design.
- Continuous Loads: If your subpanel will serve a continuous load (defined as running at maximum current for 3 hours or more, like a commercial kiln or heavy HVAC), NEC 210.20(A) requires the breaker and wire to be sized at 125% of the continuous load. If the continuous load alone is 85A, you need a wire rated for 106.25A, forcing an upsize to 2 AWG copper.
- High Ambient Temperatures: If your conduit runs through an uninsulated attic in a southern climate where ambient temperatures regularly exceed 104°F (40°C), you must apply temperature correction factors from NEC Table 310.15(B)(1). A 30°C baseline no longer applies, and 3 AWG copper will severely derate.
- Service Entrance vs. Subpanel: If this 100A feed is actually the main service entrance to a dwelling unit (not a subpanel to a garage or workshop), NEC 310.12 applies. This specific article often allows 4 AWG copper or 2 AWG aluminum for a 100A residential service due to the diversity of residential loads. However, mixing up 310.12 (services) and 310.16 (feeders) is a common failing point on electrical inspections. Always clarify the panel's role with your local inspector. For deeper code context, refer to the NFPA National Electrical Code documentation.
Always de-energize the main service panel, lock out the breaker, and verify the bus bars are dead with a tested non-contact voltage meter and multimeter before terminating any feeder wires. If you are unsure about your calculated loads or local amendments, hire a licensed electrician to pull the permit and perform the termination.
