Section 110.9 addresses two different types of equipment and how they must be rated:

• Equipment intended to interrupt current at fault levels such as overcurrent devices (fuses or circuit breakers).
• Equipment intended to interrupt current at other than fault levels such as switches, relays or contactors.

Before the electrician purchases or installs the items mentioned above, care should be taken to ensure that the equipment selected is properly rated for the current that must be interrupted. Overcurrent devices must have an interrupting rating at least equal to the current that is available at the line terminals. This is the maximum available fault current.

For example, if it has been determined that there is 9,900 amps of available fault current at the line terminals of a 200-amp circuit breaker, using a breaker with a 10k AIC rating (10,000 amps interrupt capacity) complies with 110.9.

Devices that do not interrupt current at fault levels (switches and contactors) just need to have an interrupt rating at least equal to the current that needs to be interrupted. For example, to interrupt 20 amps of lighting load, use a contactor that is rated for at least 20 amps.

See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).