Master Electrician Exam (NEC)Overcurrent ProtectionMedium
An industrial facility is upgrading its electrical system. A new motor control center (MCC) requires a circuit breaker with an interrupting rating sufficient to safely clear a fault. If the available short-circuit current at the MCC's incoming terminals is calculated to be 42,000 amperes, what is the minimum interrupting rating required for the main circuit breaker of the MCC?
- A35,000 AIC
- B42,000 AIC
- C65,000 AIC
- D22,000 AIC
Show answer & explanationAnswer & explanation
Correct answer: B. 42,000 AIC
NEC 110.9 requires that overcurrent protective devices have an interrupting rating sufficient for the maximum possible fault current available at the line terminals of the equipment. Therefore, if the available short-circuit current is 42,000 amperes, the minimum interrupting rating for the circuit breaker must be at least 42,000 amperes.
Why the other options are wrong
- A. This rating is too low and would result in the circuit breaker's catastrophic failure if a fault of 42,000A occurred.
- C. While a 65,000 AIC breaker would safely clear the fault, it is not the minimum required rating, and over-specifying can lead to unnecessary costs.
- D. This rating is too low and would result in the circuit breaker's catastrophic failure if a fault of 42,000A occurred.
Interrupting Rating (AIC)
The Interrupting Rating (IR), expressed in Amperes Interrupting Current (AIC), is the maximum fault current that an overcurrent protective device (OCPD) can safely interrupt without being destroyed or causing an unsafe condition.
- Must be equal to or greater than the available short-circuit current.
- A crucial safety parameter for OCPDs.
- Different from continuous current rating.
Memory trick: IR is like a 'Fault Fighter', it must be 'BIG'ger than the 'BANG' to survive.