CSLB C-10 Electrical ContractorEquipment and InstallationHard
An engineer calculates the available fault current at a new panelboard to be 42,000 amperes. Which of the following circuit breakers may be installed to comply with NEC 110.9 interrupting rating requirements?
- A22,000 AIC
- B42,000 AIC
- C35,000 AIC
- D10,000 AIC
Show answer & explanationAnswer & explanation
Correct answer: B. 42,000 AIC
NEC 110.9 requires that overcurrent devices intended to interrupt fault current have an interrupting rating not less than the available fault current at the line terminals of the equipment. A 42,000 AIC breaker exactly meets this minimum, while lower-rated breakers (10,000, 22,000, 35,000 AIC) are insufficient and could fail catastrophically under fault conditions.
Why the other options are wrong
- A. 22,000 AIC is still insufficient for the 42,000A available fault current.
- C. 35,000 AIC is below the calculated available fault current and non-compliant.
- D. 10,000 AIC is far below the required minimum rating.
Interrupting Rating (110.9)
Equipment intended to interrupt fault current must have an interrupting rating not less than the available fault current at its line terminals.
- Applies to breakers, fuses, and switches
- Rating must be ≥ calculated available fault current
- Undersized AIC ratings risk explosive equipment failure
Memory trick: 'Never let the breaker's muscle be smaller than the fault's punch.'