For a standard 100-amp subpanel, use 3 AWG copper wire (or 1 AWG aluminum) paired with a 100-amp double-pole breaker in the main panel. This assumes THHN/THWN-2 conductors in conduit, a 75°C temperature rating, 30°C ambient temperature, and a run under 100 feet.
- Conductor Material: Copper (unless aluminum is explicitly specified)
- Insulation Type: THHN/THWN-2 in PVC or EMT conduit
- Temperature Column: 75°C (per NEC 110.14(C) for standard residential breakers and lugs)
- Ambient Temperature: 30°C (86°F)
- Conduit Fill: 3 or fewer current-carrying conductors (no derating required)
The Core Sizing Matrix: Matching AWG to Breaker Size
Sizing the wire for sub panel feeders requires cross-referencing your calculated load with NEC Table 310.16. The most common mistake DIYers make is using the 90°C column for THHN wire. While THHN is rated for 90°C, the terminals on standard residential breakers and panel lugs are almost universally rated for 75°C. Therefore, your ampacity is bottlenecked by the 75°C column.
| Subpanel Breaker Size | Copper AWG (75°C) | Aluminum AWG (75°C) | Copper Ground (Table 250.122) |
|---|---|---|---|
| 60 Amp | 6 AWG (65A)* | 4 AWG (65A)* | 10 AWG |
| 100 Amp | 3 AWG (100A) | 1 AWG (100A) | 8 AWG |
| 125 Amp | 1 AWG (130A) | 1/0 AWG (120A)** | 8 AWG |
| 200 Amp | 2/0 AWG (175A)*** | 4/0 AWG (205A) | 6 AWG |
*60A Note: 6 AWG copper is rated 65A at 75°C. Per NEC 240.4(B), you can use the next standard breaker size (60A) if the load doesn't exceed the wire ampacity. However, if your calculated continuous load is exactly 60A, you must upsize to 4 AWG.
**125A Note: Aluminum 1/0 is rated 120A. You must upsize to 2/0 AL (135A) for a full 125A continuous load.
***200A Note: Copper 2/0 is 175A. For a full 200A residential service feeder, 4/0 AL or 350 kcmil CU is typically required unless specific residential calculation allowances apply.
Why 3 AWG and Not One Size Smaller for 100A?
Looking at the 75°C column, 4 AWG copper is rated for 85 amps. The NEC 'next size up' rule (240.4(B)) allows you to protect a wire with a breaker up to the next standard size if the exact ampacity isn't a standard breaker size. The standard breaker sizes are 15, 20, 30, 40, 50, 60, 70, 80, 90, 100. The next size up from 85A is 90A. You cannot protect 4 AWG wire with a 100A breaker. Therefore, to use a standard 100A breaker, you must step up to 3 AWG, which is rated exactly 100A at 75°C.
What Changes the Answer: Distance, Bundling, and Derating
The matrix above assumes a short run (under 100 feet) in a cool environment. Real-world jobsite conditions frequently force you to upsize your wire for sub panel feeders. Here is the decision framework for when the baseline answer changes.
NEC Informational Note 210.19(A) recommends a maximum 3% voltage drop on feeders. Let's test our 3 AWG copper baseline at 150 feet carrying a full 100A load at 240V.
Formula: VD = (2 × K × I × L) / Circular Mils
Math: (2 × 12.9 × 100 × 150) / 52,620 (CM for 3 AWG) = 7.35 Volts.
7.35V / 240V = 3.06% drop.
Result: Fails the 3% recommendation. You must upsize to 1 AWG copper (CM = 83,690), which drops the VD to 4.62V (1.9%).
| Condition | NEC Rule | Action Required |
|---|---|---|
| Run exceeds 100 feet | Informational Note 210.19(A) | Calculate voltage drop; typically upsize 1 to 2 AWG sizes. |
| 4+ current-carrying conductors in one conduit | NEC 310.15(C)(1) | Apply 80% derating factor to the 90°C column, then verify against 75°C termination limits. |
| Ambient temp exceeds 30°C (86°F) | NEC 310.15(B)(1) | Multiply ampacity by temperature correction factor (e.g., 0.88 for 41-45°C). |
| Using NM-B (Romex) instead of THHN | NEC 334.80 | NM-B is strictly limited to the 60°C column. 100A requires 1 AWG copper, not 3 AWG. |
A note on Aluminum: Aluminum is highly cost-effective for long subpanel runs (often 50% cheaper than copper by foot). However, aluminum and copper are not interchangeable in sizing. Aluminum has higher resistance and requires larger cross-sections. Furthermore, aluminum requires anti-oxidant compound (like Noalox) at termination points and specific torque settings to prevent thermal creep and loose connections over time.
When an Engineer or AHJ Must Confirm Your Design
While the NEC provides the baseline, your local Authority Having Jurisdiction (AHJ) has the final say. You must pull a permit and have an inspector verify your wire for sub panel installations in the following scenarios:
- Detached Structures: Feeders to detached garages or shops require specific grounding electrode systems (ground rods) per NEC 250.32. The inspector must verify your grounding conductor size and rod placement.
- Continuous Loads: If your subpanel will serve EV chargers or heavy machinery running for 3+ hours continuously, the feeder must be sized at 125% of the continuous load. A 100A continuous load requires wire and a breaker rated for 125A.
- High-Ambient Conduit Runs: If your PVC conduit runs across a sun-baked black roof or through an uninsulated attic in a southern climate, the ambient temperature correction factors will severely derate your wire. An electrical engineer may need to calculate the exact thermal envelope.
For deeper insights on grounding and bonding requirements for separate structures, refer to the EC&M guide on NEC subpanel requirements.
Subpanel Wire Sizing FAQ
What size wire for sub panel 50 feet away?
For a 100-amp subpanel located 50 feet away, voltage drop is negligible (approx. 1.0%). You can safely use the baseline 3 AWG copper or 1 AWG aluminum THHN/THWN-2 in conduit, protected by a 100A double-pole breaker. Ensure you pull a separate 8 AWG copper equipment grounding conductor.
Can I use 4-wire or 3-wire for a sub panel?
You must use a 4-wire system (two hots, one neutral, one ground) for all new subpanel installations. The NEC eliminated the 3-wire exception for detached structures in the 2008 code cycle. The neutral and ground bars in the subpanel must be kept strictly isolated; never bond the neutral to the ground bar in a subpanel, as this creates a parallel neutral path and poses a severe shock hazard.
Does the sub panel ground wire need to be the same size?
No. The equipment grounding conductor (EGC) is sized based on the breaker rating, not the current-carrying conductors, per NEC Table 250.122. For a 100A breaker, the minimum copper ground wire is 8 AWG. If you upsized your hot wires to 1 AWG copper to mitigate voltage drop over a long distance, NEC 250.122(B) requires you to proportionally upsize the ground wire as well (typically to 6 AWG in that scenario).
How do I calculate wire for sub panel with mixed 120V and 240V loads?
Perform a standard NEC Article 220 load calculation. Sum the wattage of all 240V loads (water heaters, EV chargers). For 120V loads, divide the total wattage by 2 to balance them across the two hot legs. Add the larger 120V leg's amperage to the 240V amperage. Multiply any continuous loads (on for 3+ hours) by 1.25. The final amperage dictates your breaker and wire size. If the math yields 88 amps, you must use wire rated for at least 88A (3 AWG copper) and a 90A or 100A breaker.
