NFPA 70E Article 100 Definitions.
Many people are under the misconception that the interrupting rating of an overcurrent device such as a molded case circuit breaker is the marked value on the circuit breaker handle. The value marked on the handle is actually the ampere rating of the device.
UL 489 is the Standard For Safety For Molded-Case Circuit Breakers, Molded-Case Switches and Circuit-Breaker Enclosures. According to UL 489, the ampere rating of a circuit breaker rated 100 amperes or less shall be molded, stamped, or etched on the handle or the escutcheon area of the circuit breaker so as to be visible without removing the trim or cover of the enclosure.
The ampere rating is the lowest current, that if exceeded, will initiate the overcurrent device to trip.
The interrupting rating is commonly referred to with several terms that mean the same thing:
- Interrupting rating (IR)
- Ampere interrupting capacity (AIC)
- Ampere interrupt rating (AIR)
The interrupting rating is defined as “the highest current at rated voltage that a device is identified to interrupt under standard test conditions.”
A circuit breaker with a 200-ampere rating for example will not trip unless more than 200 amperes of current is drawn through the circuit breaker. The same 200-ampere rated circuit breaker might also have an interrupting rating or ampere interrupting capacity (AIC) of 35,000 amperes which means that if the breaker is subjected to up to 35,000 amps of current during a fault condition, the device will interrupt the fault condition without blowing up.
The following definitions can be found in Article 100 of NFPA 70E:
Below is a sample of NFPA 70E. For the complete section, see the actual NFPA 70E text at NFPA.ORG. Once there, click on the free access link to NFPA 70E.
Interrupting Rating. The highest current at rated voltage that a device is identified to interrupt under standard test conditions.
Informational Note: Equipment intended to interrupt current at other than fault levels may have its interrupting rating implied in other ratings, such as horsepower or locked rotor current.