To size wire for 100 amp panel feeders, use 3 AWG copper or 1 AWG aluminum with 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. This is your baseline before adjusting for distance.
The Baseline Assumptions
Wire sizing is not a one-size-fits-all lookup. The numbers provided above are derived from the National Electrical Code (NEC), but they rely on a strict set of environmental and material assumptions. If your jobsite deviates from these, your wire size must change.
- Material: Copper (THHN/THWN-2) or Aluminum (XHHW-2)
- Temperature Column: 75°C (Standard for 100A+ equipment per NEC 110.14(C))
- Ambient Temperature: 30°C (86°F)
- Conduit Type: EMT, PVC Schedule 80, or direct burial SER/USE-2
- Conductor Count: Maximum 3 current-carrying conductors in the raceway
NEC Ampacity Table Breakdown
When pulling wire for a 100-amp subpanel, you must reference NEC Table 310.16. A common bench mistake is looking at the 90°C column because THHN wire is rated for 90°C. However, NEC 110.14(C) dictates that unless the equipment (breakers and lugs) is explicitly marked for 90°C, you must use the 75°C column for termination limits. Almost all standard 100A residential breakers and panel lugs are rated for 75°C.
| Wire Size (AWG/kcmil) | Material | Insulation Type | 75°C Ampacity | 90°C Ampacity | Verdict for 100A Feeder |
|---|---|---|---|---|---|
| 4 AWG | Copper | THHN/THWN-2 | 85A | 95A | ❌ Too Small |
| 3 AWG | Copper | THHN/THWN-2 | 100A | 115A | ✅ Pass (Baseline) |
| 2 AWG | Copper | THHN/THWN-2 | 115A | 130A | ✅ Pass (VD Upgrade) |
| 2 AWG | Aluminum | XHHW-2 | 90A | 100A | ❌ Too Small |
| 1 AWG | Aluminum | XHHW-2 | 100A | 115A | ✅ Pass (Baseline) |
Why Not One Size Smaller?
Makers and DIYers often ask if they can use 4 AWG copper (85A at 75°C) and rely on the NEC 240.4(B) "next size up" rule to protect it with a 100A breaker. Here is why that fails for a true 100A panel feeder:
The "next size up" rule only applies if your calculated load does not exceed the wire's ampacity. If your actual load calculation is 85A, you could theoretically use 4 AWG copper and a 90A or 100A breaker. However, if you are sizing a feeder specifically to deliver the full 100A capacity of the panel to a detached garage or workshop, your calculated load capacity is 100A. An 85A wire will overheat if you actually pull 100A through it. Furthermore, 4 AWG copper at 90°C is only 95A, meaning even at the higher temperature column, it cannot legally be rounded up to a 100A breaker. You must use 3 AWG copper to safely and legally deliver 100 amps.
Voltage Drop: The Distance Factor
Ampacity tells you what the wire can handle without melting; voltage drop tells you what the wire can deliver without starving your tools. The NEC recommends a maximum 3% voltage drop for feeders. On a 240V system, 3% equals a 7.2V drop.
Using the standard voltage drop formula (VD = 2 × K × I × D / CM) and the Southwire Voltage Drop Calculator, we can find the maximum run length for 3 AWG copper carrying a full 100A load:
- K (Copper): 12.9
- I (Current): 100A
- CM (Circular Mils for 3 AWG): 52,620
- Max Distance (D): ~145 feet
If your subpanel is 145 feet or less from the main panel, 3 AWG copper is perfect. If the run is 150 feet, your voltage drop exceeds 3%, and you must bump up to 2 AWG copper to maintain proper voltage at the far end.
Decision Tree: Finalizing Your Wire Pick
Use this decision path to lock in your exact material and gauge. Do not deviate from the final column.
| Condition / Jobsite Factor | Material Choice | Final Wire Size Pick | Breaker Size |
|---|---|---|---|
| Run is under 145 ft, standard 30°C ambient, copper preferred | Copper (THHN) | 3 AWG | 100A |
| Run is 146 ft to 225 ft, copper preferred | Copper (THHN) | 2 AWG | 100A |
| Run is under 100 ft, budget-conscious, aluminum acceptable | Aluminum (XHHW-2) | 1 AWG | 100A |
| Run is 100 ft to 160 ft, aluminum acceptable | Aluminum (XHHW-2) | 1/0 AWG | 100A |
| Using pre-assembled 4-wire SER cable in an indoor crawlspace | Aluminum (SER) | 1-1-1-3 AWG | 100A |
What Changes the Answer: Derating and Bundling
The baseline assumes you have no more than three current-carrying conductors (two hots, one neutral; the ground does not count) in your conduit. If you are pulling feeders for two subpanels in the same PVC conduit, you now have four current-carrying conductors (or six, if both neutrals carry unbalanced current).
Per NEC 310.15(C)(1), 4 to 6 conductors require an 80% derating factor. You must apply this derating to the 90°C column of the wire. For 3 AWG copper, the 90°C rating is 115A. 115A × 0.80 = 92A. Because 92A is less than your 100A requirement, 3 AWG copper is now illegal for this bundled run. You would be forced to step up to 2 AWG copper (130A × 0.80 = 104A) to maintain your 100A feeder capacity.
When an Engineer or AHJ Must Confirm
While this guide covers standard residential and light-commercial 100A subpanel feeders, you must pull a permit and consult your local Authority Having Jurisdiction (AHJ) or a licensed professional engineer under the following conditions:
- Service Entrance Conductors: If this 100A feed is coming directly from the utility meter to the main service disconnect (not a subpanel feeder from an existing main), utility specifications and NEC Article 230 override standard feeder rules.
- Continuous Loads: If the subpanel will supply continuous loads (operating for 3 hours or more, like commercial lighting or server racks), NEC 210.20 requires the breaker and wire to be sized at 125% of the continuous load. A 100A continuous load requires a 125A breaker and 1 AWG copper wire.
- High Ambient Temperatures: If the conduit runs through an attic or boiler room where ambient temperatures regularly exceed 30°C (86°F), you must apply the temperature correction factors from the bottom of Table 310.16, which will likely force you to increase your wire gauge.
