For a standard residential 100 amp subpanel wire run under 50 feet, use 3 AWG copper or 1 AWG aluminum conductors, protected by a 100A double-pole breaker. This assumes THHN/THWN-2 insulation in a raceway, terminated at 75°C rated lugs, with no more than three current-carrying conductors in the conduit.
- Material: Copper (primary) / Aluminum (secondary)
- Temperature Rating: 75°C column (NEC 110.14(C) standard for residential terminations over 100A or standard 75°C lugs)
- Ambient Temperature: 30°C (86°F)
- Conduit Type: EMT or PVC raceway, maximum 3 current-carrying conductors (no derating required)
The Direct Answer: Sizing 100 Amp Subpanel Wire
When feeding a 100A subpanel, the National Electrical Code (NEC) requires the conductor ampacity to meet or exceed the overcurrent protective device (OCPD) rating, factoring in termination temperature limits. Most modern residential breakers and subpanel lugs are rated for 75°C. Therefore, we must look at the 75°C column of NEC Table 310.16 (formerly 310.15(B)(16)).
| Conductor Material | AWG Size | Insulation Type | 75°C Ampacity | Breaker Size |
|---|---|---|---|---|
| Copper | 3 AWG | THHN/THWN-2 | 100 Amps | 100A 2-Pole |
| Aluminum | 1 AWG | XHHW-2 / THWN-2 | 100 Amps | 100A 2-Pole |
Why 3 AWG Copper and Not One Size Smaller (4 AWG)?
A common bench mistake is assuming 4 AWG copper is sufficient because it is rated for 85A at 75°C, and NEC 240.4(B) allows the "next size up" rule for overcurrent protection. However, 240.4(B) only applies if the actual calculated load does not exceed the conductor's ampacity. If your subpanel is rated for 100A and your calculated load approaches that capacity, an 85A conductor protected by a 100A breaker is a code violation and a fire hazard. You must hit the 100A mark in the 75°C column, which lands exactly on 3 AWG copper.
Voltage Drop: When Distance Forces an Upsize
Ampacity tables assume a short run. Once your wire run exceeds 50 feet, voltage drop becomes the governing constraint. The NEC recommends a maximum 3% voltage drop on feeders (7.2V on a 240V nominal system). While a 100A panel rarely runs at 100% continuous capacity (which would require 125% sizing anyway), we calculate using an 80A realistic continuous load to ensure headroom.
Using the standard single-phase voltage drop formula: VD = (2 × K × I × D) / CM
- K (Copper resistivity) = 12.9
- I (Current) = 80A
- CM (Circular Mils for 3 AWG) = 52,620
At 50 feet: VD = (2 × 12.9 × 80 × 50) / 52,620 = 1.96V (0.8% drop). Perfectly safe.
At 150 feet: VD = (2 × 12.9 × 80 × 150) / 52,620 = 5.88V (2.45% drop). Still passes the 3% rule.
At 200 feet: VD = (2 × 12.9 × 80 × 200) / 52,620 = 7.84V (3.26% drop). Fails.
If your conduit run exceeds 175 feet, you must upsize to 2 AWG copper (CM = 66,360) to bring the voltage drop at 200 feet back down to a compliant 2.59%. Always measure your actual routing distance, including vertical drops and bends, not just the straight-line distance.
Decision Tree: Variables That Change Your Wire Size
The baseline 3 AWG copper / 1 AWG aluminum answer holds true for standard indoor, climate-controlled runs. Jobsite conditions frequently force deviations from the baseline. Use this decision matrix to adjust your materials list before heading to the supply house.
| Jobsite Condition | Impact on Sizing | Required Action |
|---|---|---|
| Run exceeds 175 feet | Voltage drop exceeds 3% at 80A load | Upsize to 2 AWG Copper or 1/0 AWG Aluminum |
| 4+ current-carrying conductors in one conduit | NEC 310.15(C)(1) requires 80% derating | Upsize to 2 AWG Copper (100A / 0.8 = 125A required) |
| Conduit routed through attic > 30°C (86°F) | Ambient temperature correction factors apply | Consult NEC 310.15(B)(1); likely upsize to 2 AWG Copper |
| Using Aluminum instead of Copper | Lower conductivity, higher expansion rate | Use 1 AWG Al, apply anti-oxidant paste, torque to spec |
Frequently Asked Questions
Can I use 2 AWG aluminum for a 100 amp subpanel wire?
No. A common misconception is that 2 AWG aluminum is sufficient because it is rated for 90A at 75°C, and some assume the "next size up" breaker rule applies. As with the copper 4 AWG scenario, NEC 240.4(B) does not permit protecting a 90A conductor with a 100A breaker if the calculated load or panel rating demands 100A. You must use 1 AWG aluminum, which is rated exactly 100A at 75°C. Additionally, when terminating aluminum, you must use a torque screwdriver to hit the exact inch-pound specification on the lug label and apply Noalox or a similar anti-oxidant compound to prevent thermal creep and arcing over time.
What size ground wire do I need for a 100 amp subpanel?
According to NEC Table 250.122, the minimum size equipment grounding conductor (EGC) for a 100A overcurrent device is 8 AWG copper or 6 AWG aluminum. However, if you were forced to upsize your current-carrying conductors for voltage drop (e.g., stepping up to 2 AWG copper for a 200-foot run), NEC 250.122(B) requires you to increase the ground wire proportionally. In that specific 2 AWG copper scenario, your ground must be upsized to 6 AWG copper to maintain the same fault-current clearing impedance.
Can I use 100 amp subpanel wire in direct burial without conduit?
Yes, but the insulation type and ampacity column change. If you use direct-burial UF-B cable, it is generally limited to the 60°C ampacity column per NEC 339.5. In the 60°C column, 3 AWG copper is only rated for 85A, meaning you would need to upsize to 1 AWG copper for direct burial UF-B. If you prefer to stay with 3 AWG copper, you must pull individual THWN-2 or XHHW-2 conductors inside a continuous, sealed PVC conduit schedule 80 underground, which allows you to utilize the wet-location 75°C ratings. Never mix bare ground wires with insulated current-carrying conductors in a wet conduit without verifying the insulation's wet rating (THWN-2 is rated for wet; standard THHN is not).
