To correctly size wire for 100 amp sub panel feeds, the strict NEC minimum is 3 AWG copper or 1 AWG aluminum on a 100A breaker. This assumes THHN/THWN-2 insulation, 75°C terminations, and 30°C ambient. However, most electricians upsize to 2 AWG copper or 1/0 aluminum to mitigate voltage drop.
- Conductor Material: Copper (primary), Aluminum (secondary comparison)
- Insulation Type: THHN/THWN-2 (rated 90°C, but ampacity limited by termination ratings)
- Temperature Column: 75°C (Standard for almost all 100A breakers and panel lugs per NEC 110.14(C))
- Ambient Temperature: 30°C (86°F)
- Installation Method: Raceway (conduit), maximum 3 current-carrying conductors
Safety Note: Always de-energize the main panel, lock out the breaker, and verify dead with a tested CAT III/IV multimeter before working on feeder connections. NEC-style guidance is provided here; your local AHJ has final authority.
The Baseline: NEC Ampacity Rules for 100A Feeds
When sizing a feeder, the overcurrent protection device (the breaker) dictates the minimum wire ampacity. According to NEC Table 310.16, we must look at the 75°C column. Even though THHN wire has a 90°C insulation rating, the lugs on your 100A breaker and subpanel are almost universally rated for 75°C. The NEC requires you to use the lowest temperature rating in the circuit.
| Material | AWG Size | Circular Mils (CM) | Ampacity @ 75°C | Ampacity @ 90°C (Derating Only) |
|---|---|---|---|---|
| Copper | 4 AWG | 41,740 | 85A | 95A |
| Copper | 3 AWG (Min) | 52,620 | 100A | 115A |
| Copper | 2 AWG (Recommended) | 66,360 | 115A | 130A |
| Aluminum | 2 AWG | 66,360 | 90A | 100A |
| Aluminum | 1 AWG (Min) | 83,690 | 100A | 120A |
| Aluminum | 1/0 AWG (Recommended) | 105,600 | 120A | 135A |
Why this size and not one smaller?
You might wonder why 4 AWG copper (85A at 75°C) cannot be used on a 100A breaker. NEC 240.4(B) allows you to round up to the next standard breaker size only if the wire's ampacity does not match a standard breaker size. Because 100A is a standard breaker size (NEC 240.6), you cannot use the "next size up" rule. The wire must be rated for the full 100A. Therefore, 4 AWG is a code violation for a 100A feeder, regardless of the actual connected load.
Voltage Drop: When to Upsize Your Wire
Ampacity tells you what size wire will prevent a fire; voltage drop tells you what size wire will actually run your appliances correctly. The NEC recommends (via Informational Note to 215.2) a maximum 3% voltage drop on feeders. For a 240V circuit, 3% is 7.2 volts.
Let's calculate the voltage drop for a continuous 80A load (the practical max for a 100A panel) using the standard formula: VD = (2 × K × I × L) / CM, where K=12.9 for copper.
| One-Way Distance | 3 AWG Copper (52,620 CM) | Voltage Drop % | 2 AWG Copper (66,360 CM) | Voltage Drop % |
|---|---|---|---|---|
| 50 feet | 3.92V | 1.63% (Pass) | 3.11V | 1.29% (Pass) |
| 100 feet | 7.84V | 3.26% (Fail) | 6.22V | 2.59% (Pass) |
| 150 feet | 11.76V | 4.90% (Fail) | 9.33V | 3.88% (Fail*) |
*Note: At 150 feet, even 2 AWG exceeds the strict 3% recommendation. For runs this long, you must upsize to 1 AWG copper or calculate based on your actual non-continuous peak load rather than the full 80A. For a deep dive on these calculations, refer to EC&M's guide on voltage drop calculations.
Derating Factors: What Changes the Minimum Size?
The baseline sizes above assume perfect conditions. Real-world jobsites rarely cooperate. Here is what forces you to buy thicker wire:
- Bundling (Conduit Fill): If you pull two separate 100A feeders through the same PVC conduit, you have 8 current-carrying conductors. Per NEC Table 310.15(C)(1), you must apply a 70% derating factor. You must use the 90°C column for derating math. 2 AWG THHN (130A × 0.70 = 91A) fails. You must upsize to 1/0 AWG copper (170A × 0.70 = 119A) to maintain a 100A rating.
- High Ambient Temperature: If your conduit runs through an attic that reaches 40°C (104°F), you apply an 87% correction factor. 3 AWG copper (115A at 90°C × 0.87 = 100.05A) barely scrapes by, but 2 AWG provides a much safer margin against nuisance tripping.
- Aluminum Terminations: If using aluminum, you must apply an anti-oxidant compound (like Noalox) to the wire strands before torquing the lugs. Aluminum creeps and oxidizes; failing to use paste and a torque screwdriver set to the manufacturer's spec (usually 35-45 in-lbs for 1/0) will result in a melted lug within a year.
When an Engineer or AHJ Must Confirm
While this guide covers standard residential and light-commercial feeders, you must pull a permit and consult your local Authority Having Jurisdiction (AHJ) or a licensed professional engineer if:
- The subpanel is fed directly from the utility transformer (Service Entrance conductors fall under NEC Article 230, which has different grounding and physical protection rules than Article 215 feeders).
- The ambient temperature consistently exceeds 45°C (113°F).
- You are paralleling conductors (running multiple smaller wires per phase to achieve 100A, which is generally not permitted for feeders under 400A anyway).
FAQ: Sizing Wire for 100 Amp Sub Panels
Can I use 4 AWG copper for a 100 amp sub panel if my calculated load is only 80 amps?
No. There is a critical distinction between sizing wire for a specific calculated load and sizing wire for a panel's feeder rating. If you are feeding a subpanel rated at 100A with a 100A breaker, the overcurrent device protects the feeder. NEC 240.4 requires the conductor ampacity to be at least the rating of the breaker. 4 AWG copper is rated 85A at 75°C, which is less than the 100A breaker. The only exception is if you install an 80A breaker at the main panel to feed the subpanel, in which case 4 AWG is perfectly legal.
Does a 100 amp sub panel need a ground wire, and what size should it be?
Yes. Modern code requires a 4-wire feeder (two hots, one neutral, one equipment grounding conductor). The neutral and ground must remain strictly separated in the subpanel. Per NEC Table 250.122, the minimum Equipment Grounding Conductor (EGC) for a 100A breaker is 8 AWG copper or 6 AWG aluminum. If you upsized your ungrounded (hot) conductors for voltage drop, you are technically required by 250.122(B) to proportionally increase the ground wire size as well, though many local inspectors accept the baseline 8 AWG for short runs.
Why do I need 4 wires instead of 3 for a 100 amp sub panel?
Prior to the 2008 NEC, a 3-wire feed (two hots and a neutral) was permitted for detached buildings if a grounding electrode system was established at the second building. This was eliminated due to the danger of neutral currents traveling on grounding paths, creating shock hazards and stray voltage. Today, you must run a dedicated 4th wire (the EGC) back to the main panel's ground bus, and the subpanel's neutral bus must be isolated from the metal enclosure.
How does running wire in an attic change the wire size for a 100 amp sub panel?
Attics get significantly hotter than the 30°C (86°F) baseline used in standard ampacity tables. If your attic reaches 40°C (104°F) in the summer, you must apply a 0.87 temperature correction factor to the 90°C column of your wire. As noted in the derating section, 3 AWG copper barely survives this math (100.05A). To ensure safety and prevent the breaker's thermal trip mechanism from acting up due to heat radiating into the wire insulation, you should absolutely use 2 AWG copper or 1/0 aluminum for any attic or hot-roof runs.
