For a standard 100 amp non-continuous load, you need 3 AWG copper or 1 AWG aluminum wire, assuming standard 75°C terminations and an ambient temperature of 30°C (86°F). If the load is continuous (expected to run for 3 hours or more), you must multiply the load by 125%, requiring 1 AWG copper or 1/0 AWG aluminum. These values are dictated by the 75°C column of NEC Table 310.16.

Safety & Code Caveat: Sizing conductors for a 100A feeder or service entrance involves local AHJ (Authority Having Jurisdiction) interpretations of continuous vs. non-continuous loads. Always verify dead with a tested meter before working in a panel. This guide provides NEC-style guidance; your local inspector has final authority.

The 100 Amp Breaker Wire Size Chart (NEC Table 310.16)

To use this chart correctly, you must understand the temperature columns. While modern THHN/THWN-2 wire is physically insulated to withstand 90°C, NEC 110.14(C) restricts your final ampacity to the lowest temperature rating of any connected component. Because nearly all residential breakers and panel lugs are rated for 75°C, the 75°C column is your baseline for termination ampacity. The 90°C column is only used as a starting point for derating calculations (explained below).

Wire Sizing for 100A Circuits (Source: NFPA 70 / NEC Table 310.16, 2023 Edition)
AWG Size Material 60°C Ampacity 75°C Ampacity (Termination) 90°C Ampacity (Derating Base) Application for 100A Breaker
4 AWG Copper 70A 85A 95A Undersized (Max 70A-85A)
3 AWG Copper 85A 100A 110A Min. for 100A non-continuous
2 AWG Copper 95A 115A 130A Allowed (Next size up rule)
1 AWG Copper 110A 130A 145A Min. for 100A continuous (125A)
1/0 AWG Copper 125A 150A 170A Oversized / Voltage drop mitigation
2 AWG Aluminum 75A 90A 100A Undersized for 75°C lugs
1 AWG Aluminum 100A 120A 135A Min. for 100A non-continuous
1/0 AWG Aluminum 120A 135A 150A Min. for 100A continuous
Pro-Tip for Aluminum: If you are using aluminum wire (like XHHW-2 or USE-2) for a subpanel feeder, always apply an anti-oxidant compound (like Noalox) to the stripped conductor ends before torquing the lugs to the manufacturer's spec. Aluminum expands and contracts differently than copper and can loosen over time without proper prep.

Derating, Voltage Drop, and What the Chart Cannot Tell You

The chart above assumes perfect conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway. Real-world jobsites rarely match this. Here is how derating and distance modify your base wire size.

How Derating Modifies the Base Value

When you pull more than three current-carrying conductors through a single conduit, the wires heat each other up. Per NEC Table 310.15(C)(1), you must apply an adjustment factor. This is where the 90°C column becomes useful.

  • Scenario: You are pulling two hots, a neutral, and a ground (4 current-carrying conductors) for a 120/240V multi-wire branch circuit or feeder. The adjustment factor is 80%.
  • Calculation: You cannot use 3 AWG Copper. Its 90°C rating is 110A. 110A × 0.80 = 88A. An 88A derated ampacity cannot be protected by a 100A breaker.
  • The Fix: Step up to 1 AWG Copper. Its 90°C rating is 145A. 145A × 0.80 = 116A. Since 116A > 100A, 1 AWG copper passes the derating requirement and can legally terminate on 75°C lugs.

What the Table Cannot Tell You: Voltage Drop

NEC Table 310.16 only prevents the wire from melting; it does not guarantee your equipment will receive adequate voltage. The Electrical Safety Foundation International (ESFI) and NEC Informational Notes recommend keeping voltage drop under 3% on feeders.

If you are running a 100A subpanel 150 feet away from the main panel, 3 AWG copper will experience roughly a 3.8% drop on 120V circuits at full load. To maintain a strict 3% drop at maximum capacity over that distance, you must ignore the minimum chart requirements and step up to 1/0 AWG copper or 2/0 AWG aluminum.

Frequently Asked Questions

Can I use 2 AWG wire for a 100 amp breaker?

Yes, 2 AWG copper (rated 115A at 75°C) is perfectly legal for a 100A breaker under the NEC "next standard size up" rule (NEC 240.4(B)), provided your calculated load does not exceed 100A. However, 2 AWG is an awkward middle ground: it is more expensive than 3 AWG, but lacks the extra thermal headroom of 1 AWG for continuous loads or voltage drop mitigation. Most electricians will jump straight from 3 AWG to 1 AWG to avoid stocking and pulling the less common 2 AWG size.

Does a 100 amp subpanel need a ground wire, and what size?

Yes. A feeder to a subpanel requires four wires: two hots, one neutral, and one Equipment Grounding Conductor (EGC). Per NEC 250.122, for a 100A overcurrent device, the minimum EGC size is 8 AWG copper or 6 AWG aluminum. Note that the neutral and ground must remain strictly isolated (unbonded) at the subpanel.

Can I use 90°C THHN wire to get a smaller wire size for 100 amps?

No. A common DIY mistake is looking at the 90°C column, seeing that 4 AWG THHN is rated for 95A, and assuming it is "close enough" or trying to use it for a 100A breaker. Because the breaker lugs are rated 75°C, NEC 110.14(C) forces you to use the 75°C column for your final termination ampacity. At 75°C, 4 AWG is only good for 85A. You must use 3 AWG minimum.

How far can I run 100 amp wire before voltage drop requires a larger size?

For a standard 240V load (like an EV charger or large compressor) pulling a full 100A, 3 AWG copper will hit the 3% voltage drop threshold (7.2V drop) at approximately 115 feet. If your run exceeds 115 feet at full continuous load, you must upsize to 2 AWG or 1 AWG copper. For 120V loads, that distance is halved to roughly 55 feet before you need to upsize.