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 30 amps of lighting load, use a contactor that is rated for at least 30 amps.

Below is a preview of the NEC^®. See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to NFPA 70.

Code Language:

110.9 Interrupting Rating. Equipment intended to interrupt current at fault levels shall have an interrupting rating at nominal circuit voltage at least equal to the available fault current at the line terminals of the equipment.

Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage at least equal to the current that must be interrupted.