The correct wire gauge for a 100 amp sub panel is 3 AWG copper or 1 AWG aluminum, protected by a 100-amp double-pole breaker. This assumes standard 75°C terminations, 30°C ambient temperature, and no more than three current-carrying conductors in the raceway.
- Material: Copper (THHN/THWN-2) or Aluminum (XHHW-2)
- Termination Rating: 75°C (Standard for 100A breakers and panel lugs per NEC 110.14(C))
- Ambient Temperature: 30°C (86°F)
- Conduit Type: PVC, EMT, or RMC (not buried directly in earth without conduit)
- Current-Carrying Conductors (CCCs): 3 or fewer (2 hots + 1 neutral for 120/240V split-phase)
The Baseline Sizing Table (And Why 3 AWG Wins)
When sizing a feeder, you must look at the ampacity table in NEC Article 310.16. The most common mistake DIYers make is looking at the 90°C column because THHN wire is rated for 90°C. However, unless your breaker and panel lugs are explicitly stamped for 90°C (they almost never are at the 100A residential tier), you are legally bound to the 75°C column for termination sizing.
| Wire Material | AWG Size | Insulation Type | 75°C Ampacity (Termination Limit) | 90°C Ampacity (Derating Baseline) | Status for 100A Feeder |
|---|---|---|---|---|---|
| Copper | 4 AWG | THHN/THWN-2 | 85A | 95A | Too Small (Fails 75°C rule) |
| Copper | 3 AWG | THHN/THWN-2 | 100A | 115A | Exact Match |
| Copper | 2 AWG | THHN/THWN-2 | 115A | 130A | Upsize for Voltage Drop / Derating |
| Aluminum | 2 AWG | XHHW-2 | 90A | 105A | Too Small |
| Aluminum | 1 AWG | XHHW-2 | 100A | 120A | Exact Match |
| Aluminum | 1/0 AWG | XHHW-2 | 120A | 135A | Upsize for Voltage Drop / Derating |
Why not use 4 AWG Copper?
A 4 AWG copper THHN wire has an ampacity of 95A in the 90°C column. It seems close enough to 100A, and some hardware stores might even tell you it's fine. It is not. Per NEC 110.14(C), the ampacity of the conductor must be chosen based on the lowest temperature rating of any connected termination. Since standard 100A breakers and subpanel lugs are rated for 75°C, you must use the 75°C column. In that column, 4 AWG copper is only rated for 85A. Putting 100A of load through an 85A-rated termination will cause the lug to overheat, melt the insulation, and potentially start a fire inside the panel.
Voltage Drop: When Distance Forces an Upsize
Ampacity tells you what the wire can handle without melting. Voltage drop (VD) tells you what the equipment at the end of the wire will actually receive. The NEC (Article 215.2 Informational Note) recommends a maximum 3% voltage drop for feeders to ensure efficiency and prevent motor burnout or breaker nuisance tripping.
Let's run the math for a 150-foot run from the main panel to a detached garage subpanel, pulling a full 100A load at 240V.
- Target Max Drop (3% of 240V): 7.2 Volts
- 3 AWG Copper (CM = 52,620): VD = (2 × 12.9 × 100A × 150ft) / 52,620 = 7.35V (3.06%)
At 150 feet, 3 AWG copper slightly exceeds the 3% recommendation. If your garage will run high-draw equipment like a plasma cutter, a large air compressor, or an EV charger, that 3% drop will cause motors to run hot and draw excess amperage.
Copper vs. Aluminum: Cost, Terminations, and Derating
Choosing between copper and aluminum isn't just about the wire gauge; it changes your entire installation strategy, conduit size, and termination prep.
| Feature | 3 AWG Copper (THHN) | 1 AWG Aluminum (XHHW-2) |
|---|---|---|
| Approximate Cost (per foot) | $3.50 - $4.50 | $1.80 - $2.50 |
| Minimum Conduit Size (3-wire + ground) | 1-inch PVC/EMT | 1.25-inch PVC/EMT |
| Termination Prep | Strip and torque | Wire brush + Noalox (anti-oxidant) required |
| Stiffness / Pulling Effort | High (requires pulling compound) | Medium (easier to bend in conduit) |
| Lug Compatibility | Universal | Must verify lugs are ALR (Aluminum Rated) |
The Bundling and Derating Trap
What happens if you decide to pull a second circuit (like a 20A multi-wire branch circuit for outdoor lights) through the same conduit as your 100A subpanel feeder? You now have more than three current-carrying conductors (CCCs) in the raceway.
Per NEC Table 310.15(C)(1), 4 to 6 CCCs require an 80% derating factor. This derating is applied to the 90°C column, but the final result must still meet your 75°C termination requirement.
- 3 AWG Copper Derated: 115A (90°C column) × 0.80 = 92A. Since 92A is less than your 100A breaker, 3 AWG fails.
- 2 AWG Copper Derated: 130A (90°C column) × 0.80 = 104A. Since 104A > 100A, 2 AWG passes.
If you share the conduit, you must automatically upsize to 2 AWG copper or 1/0 AWG aluminum, regardless of the run length. Always use a separate conduit for branch circuits if you want to stick to the minimum 3 AWG feeder size.
When an Engineer or the AHJ Must Confirm
While the 3 AWG copper / 1 AWG aluminum rule covers 90% of residential detached garage and workshop subpanels, specific load profiles and environmental factors require professional sign-off or a formal Article 220 load calculation.
1. Continuous Loads (The 125% Rule)
If your subpanel is being installed specifically to feed a continuous load—defined by the NEC as a load expected to run for 3 hours or more, such as a commercial server rack, baseboard heating, or a hardwired 80A EV charger—the math changes. You must multiply the continuous load by 125% to size the conductor and the overcurrent device. If your calculated continuous load is 85A, your feeder must be sized for 106.25A, forcing you to jump to a 110A or 125A breaker and 1 AWG copper wire.
2. High Ambient Temperatures
If your conduit runs through an unventilated attic in a southern climate where ambient temperatures routinely 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 90°C wire is 0.91. This reduces the baseline ampacity before you even apply bundling derating, frequently forcing an upsize to 2 AWG copper just to maintain a 100A rating.
3. Grounding and Bonding
Remember that a subpanel requires a strict separation of neutrals and grounds. You will need to pull a 4th wire (an Equipment Grounding Conductor). For a 100A feeder, NEC Table 250.122 requires a minimum 8 AWG copper or 6 AWG aluminum ground wire. Do not rely on the conduit as your sole ground path, and ensure the main bonding jumper (the green screw or strap) is removed from the subpanel's neutral bar. For complex grounding electrode systems at detached structures (like driving auxiliary ground rods per Article 250.32), consult your local inspector to ensure your specific soil conditions and layout meet code.
