Master Electrician Exam (NEC)Overcurrent ProtectionEasy
A master electrician is inspecting a new residential service installation. The main service disconnect consists of a circuit breaker. What is the minimum interrupting rating required for this circuit breaker if the available short-circuit current at the service entrance is calculated to be 15,000 amperes?
- A15,000 amperes
- B5,000 amperes
- C10,000 amperes
- D22,000 amperes
Show answer & explanationAnswer & explanation
Correct answer: C. 10,000 amperes
For residential and light commercial services with an available fault current of 10,000 A or less, a standard 10,000 A interrupting rating (AIC) breaker is generally acceptable by NEC 240.86(A) and 230.65. If the available short-circuit current is 15,000 A, the breaker must at least meet that, but 10,000 A is the minimum readily available for residential.
Why the other options are wrong
- A. While a 15,000 A interrupting rating would be appropriate, the question asks for the *minimum* required, and 10,000 A is the common minimum for residential applications.
- B. This is too low and would not safely interrupt the fault current.
- D. This rating is higher than necessary for the given available fault current and the minimum requirement.
Standard Interrupting Rating
The minimum interrupting rating for a circuit breaker in a residential service is typically 10,000 amperes (10 kAIC) unless the available fault current is calculated to be higher and explicitly requires a greater rating.
- Required to safely interrupt short-circuit currents.
- Residential standard is 10,000 AIC.
- Must be equal to or greater than the available short-circuit current.
Memory trick: Interrupting ratings 'break' the short-circuit 'current' to protect the 'home'.