If you ask electricians what the ampacity of #12 wire is, most people will immediately say 20 amps. That answer is correct in everyday work, but the real explanation is a little more interesting.
When you look at the NEC ampacity table, #12 THHN actually has a higher rating than 20 amps. The reason we normally use it on 20 amp circuits comes down to breaker rules and code limits.
The NEC ampacity for #12 THHN
THHN wire is rated for 90°C insulation.
If you look at NEC Table 310.16, the ampacity for #12 copper in the 90°C column is:
- #12 copper THHN = 30 amps
That surprises a lot of people the first time they see it.
Why we usually protect it with a 20 amp breaker
Most branch circuits in homes follow the small conductor rules in the NEC.
Those rules limit the breaker size for common conductor sizes.
- #14 copper → 15 amp breaker
- #12 copper → 20 amp breaker
- #10 copper → 30 amp breaker
So even though #12 THHN can technically carry more current based on temperature ratings, the breaker protecting the circuit is usually limited to 20 amps.
Where you see #12 THHN used
Because of that 20 amp breaker limit, #12 wire shows up constantly in residential electrical work.
Common examples include:
- Kitchen small appliance circuits
- Bathroom receptacle circuits
- Garage receptacles
- Dedicated appliance circuits
It is basically the standard conductor for 20 amp branch circuits.
Temperature rating still matters
Even though the breaker may limit the circuit to 20 amps, the insulation rating still matters for other parts of the code.
For example, the 90°C rating can be used when applying certain adjustment or correction factors for conduit fill or high temperatures.
That is why THHN insulation is so commonly used in conduit systems.
The easy way to remember it
- #12 THHN ampacity (90°C column): about 30 amps
- Typical breaker size used: 20 amps
So when someone asks about the ampacity of #12 THHN, the real answer depends on whether you are talking about the wire's thermal rating or the breaker protecting the circuit.
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