100 Amp Breaker: Wire Size and Uses
Common uses for a 100A breaker
100A subpanels for garages, workshops, ADUs. Aluminum feeders very common at this size. 1 AWG Al at 130A supports 100A.
The 100A breaker is typically used for subpanel feeders - for example, a detached garage or workshop. It can also serve as a smaller main service for apartments or older homes. At this amperage, conductor cost differences between copper and aluminum become significant, so aluminum feeders are common and explicitly permitted by the NEC.
Wire size for a 100A breaker
The wire must have an ampacity that meets or exceeds the breaker rating at the appropriate temperature column. Here are the minimum wire sizes:
| Material | Minimum wire | Details |
|---|---|---|
| Copper | 3 AWG | Standard for residential branch circuits |
| Aluminum | 1 AWG | Common for feeders 60A and larger |
These sizes assume 75°C termination temperature per NEC 110.14(C), which is the standard for most residential breakers and devices. For long wire runs, check voltage drop - you may need to upsize the wire even though the ampacity is adequate.
Continuous load rule (80% maximum)
For continuous loads - any load expected to run for 3 hours or more - the NEC requires that the load not exceed 80% of the breaker rating (NEC 210.20(A)). On a 100A breaker, the maximum continuous load is 80A.
This is why sizing often seems counterintuitive:
- A 80A continuous load requires a 100A breaker (80 × 1.25 = 100A)
- Non-continuous loads can use the full 100A capacity
- 100% rated breakers exist but are rare in residential panels - assume 80% unless the breaker is specifically listed as 100% rated
Installation notes
Breaker type: A 100A breaker is a double-pole (240V) breaker. Double-pole breakers occupy two adjacent slots in the panel and provide 240V between the two hot conductors. Both poles must trip simultaneously - never use two single-pole breakers tied together with a handle tie for 240V circuits (except as explicitly allowed for specific multi-wire branch circuits).
Ground wire: A 100A circuit requires a minimum 8 AWG copper equipment grounding conductor per NEC Table 250.122.
Standard sizes: 100A is a standard breaker size per NEC 240.6(A). Standard residential breaker sizes are: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200. Non-standard sizes should never be used.
Why does my 100A breaker keep tripping?
A 100A breaker tripping on a subpanel feeder means the total load in the subpanel exceeds 100A, which typically happens when someone adds a high-draw circuit (like an EV charger or workshop equipment) without recalculating the panel load. Run a load calculation per NEC 220 to verify the subpanel isn't overloaded. Loose feeder connections are the other common cause - aluminum feeders at 100A are especially prone to loosening over time as the aluminum creeps under pressure.
Frequently asked questions
What wire size for a 100 amp breaker?
3 AWG copper or 1 AWG aluminum minimum. The wire must have an ampacity at the 75°C column that meets or exceeds 100A. For long runs, you may need to upsize for voltage drop.
What can a 100 amp breaker handle?
100A subpanels for garages, workshops, ADUs. For continuous loads (3+ hours), the maximum is 80A (80% of 100A). For non-continuous loads, the full 100A is available.
Can I put a 100A breaker on 3 AWG wire?
The breaker must not exceed the wire ampacity. 3 AWG copper supports a 100A breaker. Never put a larger breaker on wire that cannot handle it - this defeats the safety purpose of the breaker.
Is 100A a standard breaker size?
Yes. 100A is a standard breaker size per NEC 240.6(A). It is widely available from all major breaker manufacturers (Square D, Eaton, Siemens, GE). Always use the correct breaker brand for your panel - breakers are not interchangeable between panel brands.
Related guides
- 100A wire sizing guide - complete sizing with voltage drop
- Ground wire for 100A circuit
- 3 AWG copper ampacity - derating and conduit fill
- 3 AWG copper vs aluminum
- 100A vs 200A service comparison
- Copper vs aluminum for feeders
- NEC Article 240: Overcurrent protection
- Ampacity calculator
What to buy
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NEC 2023 references verified April 2026