What is the maximum overcurrent protection rating for a circuit of 12 AWG THHN conductors that originate from a circuit breaker in a panelboard and terminate directly on a 20-ampere receptacle, assuming no temperature ratings are marked?

• A. 10A
• B. 20A
• C. 15A
• D. 25A

Answer: (B)

The key idea is matching overcurrent protection to the conductor’s ampacity. For a 12 AWG copper conductor with THHN insulation, if there’s no temperature rating marked, you use the 60°C ampacity. In that column, 12 AWG copper is rated for 20 A. So the overcurrent protection (the breaker) cannot exceed 20 A. Even though THHN insulation can handle higher temperatures, the lack of a marked rating means you default to 60°C, which gives a maximum of 20 A. A higher protection like 25 A would overcurrent the conductor. The fact there’s a 20-amp receptacle aligns with this limit, but doesn’t allow a larger OCPD. Therefore, the maximum protection is 20 A.