To safely size wire for 100 amp breaker protection, use 3 AWG copper or 1 AWG aluminum as the absolute NEC minimum. However, 2 AWG copper or 1/0 AWG aluminum is the practical jobsite standard to mitigate voltage drop and accommodate standard lug sizes.
- Material: Copper (THHN/THWN-2) or Aluminum (XHHW-2)
- Temperature Column: 75°C (per NEC 110.14(C) termination limits)
- Ambient Temperature: 30°C (86°F)
- Installation Method: 3 current-carrying conductors in a single raceway/conduit
Warning: De-energize and verify dead with a tested meter before working on any panel. Local AHJ amendments may override general NEC guidance.
The Baseline: Sizing Wire for 100 Amp Circuits
When sizing conductors for a 100-ampere overcurrent protection device (OCPD), the National Electrical Code (NEC) requires the wire's allowable ampacity to equal or exceed the breaker rating. According to NFPA 70 (NEC) Table 310.16, we must look at the 75°C column because nearly all 100A breakers and panel lugs are rated for 75°C terminations, even if the wire insulation itself is rated for 90°C.
| Conductor Material | AWG Size | 75°C Ampacity | 90°C Ampacity | Breaker Size |
|---|---|---|---|---|
| Copper (THHN) | 3 AWG | 100A (Code Min) | 115A | 100A |
| Copper (THHN) | 2 AWG | 115A (Recommended) | 130A | 100A |
| Aluminum (XHHW) | 1 AWG | 100A (Code Min) | 115A | 100A |
| Aluminum (XHHW) | 1/0 AWG | 120A (Recommended) | 135A | 100A |
Why This Size (And Why Not One Smaller)?
You might wonder why you cannot use 4 AWG copper (rated 85A at 75°C) if your actual continuous load is only 80 amps. The answer lies in NEC Article 240.4, which governs overcurrent protection. The breaker's primary job is to protect the wire, not just the load.
If a fault occurs or a motor seizes, pulling 95 amps through a 4 AWG copper wire, the wire's insulation will begin to degrade and melt long before the 100-amp breaker's thermal-magnetic trip curve engages. By mandating a minimum of 3 AWG copper (100A ampacity), the code ensures the breaker will always trip before the conductor reaches its thermal failure point. Never interchange aluminum and copper ampacities; aluminum has higher resistance and requires a larger physical cross-section (1 AWG) to carry the exact same 100A safely.
What Changes the Answer: Length, Bundling, and Material
The baseline numbers above assume a short run in a cool environment. Real-world jobsite conditions frequently force you to upsize to 2 AWG copper or 1/0 AWG aluminum. Here is the decision framework for when the baseline fails.
1. Voltage Drop Over Distance
The NEC recommends a maximum 3% voltage drop for feeders. Let's run the math for a 240V, 100A load at a distance of 150 feet using the formula VD = (2 × K × I × D) / CM (where K=12.9 for copper, I=100A, D=150ft).
- 3 AWG Copper (CM = 52,620): Voltage drop is 7.35V. That is a 3.06% drop, which fails the 3% recommendation.
- 2 AWG Copper (CM = 66,360): Voltage drop is 5.83V. That is a 2.43% drop, which passes safely.
If your run exceeds 100 feet, 2 AWG copper is practically mandatory to prevent dimming lights and motor overheating at the subpanel.
2. Conduit Bundling and Derating
If you pull more than three current-carrying conductors in a single conduit (for example, two separate 240V circuits sharing a pipe), NEC Table 310.15(C)(1) requires derating. With 4-6 conductors, you must multiply the 90°C ampacity by 80%. While you can use the 90°C column for derating math, the final derated ampacity must still meet or exceed 100A. Bundling almost always forces an upsize to 2 AWG or 1 AWG copper.
| Jobsite Condition | Copper Recommendation | Aluminum Recommendation |
|---|---|---|
| Short run (<50 ft), 3 conductors | 3 AWG | 1 AWG |
| Medium run (50-150 ft) | 2 AWG | 1/0 AWG |
| Long run (>150 ft) | 1 AWG or 1/0 AWG | 2/0 AWG |
| Ambient temp > 40°C (104°F) | Apply Table 310.15(B)(1) correction factors (usually requires 2 AWG+) | Apply correction factors (usually requires 1/0 AWG+) |
When an Engineer or AHJ Must Confirm
While the rules above apply to standard feeders and branch circuits, specific scenarios require professional validation:
- Service Entrance Conductors: If this 100A wire is the main service drop from the utility meter to your main panel, NEC Article 310.12 mandates specific sizing based on the calculated load of the entire dwelling, which may allow 4 AWG copper in specific residential calculations. Always defer to the utility's service requirements and the local Authority Having Jurisdiction (AHJ).
- High Ambient Environments: If the conduit runs across a hot roof or through an unventilated attic exceeding 50°C, an electrical engineer must calculate the exact thermal derating curve.
- Continuous Loads: If the 100A load is 'continuous' (on for 3 hours or more, like commercial lighting or EV chargers), NEC 210.20 requires the breaker to be sized at 125% of the load (125A breaker), which completely changes the wire sizing requirement to 1 AWG copper minimum.
Frequently Asked Questions
Can I use 4 AWG wire for a 100 amp breaker if the actual load is only 80 amps?
No. The breaker protects the wire, not just the connected load. A 100A breaker will allow up to 100 amps to flow indefinitely without tripping. Since 4 AWG copper is only rated for 85 amps at 75°C, a sustained 95A fault will overheat and melt the wire insulation while the breaker remains closed, creating a severe fire hazard. You must match the wire ampacity to the breaker size (minimum 3 AWG copper).
What size ground wire do I need for a 100 amp subpanel feeder?
According to NEC Table 250.122, the minimum equipment grounding conductor (EGC) for a 100-amp breaker is 8 AWG copper or 6 AWG aluminum. However, if you upsized your current-carrying conductors to mitigate voltage drop (e.g., using 1 AWG copper instead of 3 AWG), NEC 250.122(B) requires you to increase the ground wire proportionally. In practice, many electricians simply pull 6 AWG copper bare ground for all 100A subpanel feeds to ensure compliance and mechanical strength.
Why do some electricians recommend 1/0 AWG copper for a 100 amp service?
This is a legacy habit and a supply-chain workaround. First, 1/0 AWG is the standard size for 150A panels, so electrical suppliers often stock it in bulk while 3 AWG and 2 AWG can be special-order items. Second, using 1/0 AWG copper guarantees negligible voltage drop on almost any residential run and makes future upgrades to 125A or 150A easier. While it is overkill and more expensive, it is perfectly legal to use a larger wire than the minimum required, provided it physically fits into the breaker lugs.
Does running 100 amp wire underground in PVC conduit change the sizing?
Running wire underground in Schedule 40 or 80 PVC conduit does not inherently change the base ampacity sizing, as the earth acts as a decent heat sink and standard ampacity tables assume typical installation conditions. However, underground trenches often involve long distances. If your trench run from the main house to a detached garage exceeds 100 feet, you must upsize to 2 AWG copper or 1/0 AWG aluminum strictly to manage voltage drop. Additionally, ensure you use wet-location rated insulation like THWN-2 or XHHW-2, as condensation inside underground conduit is guaranteed.
