Motor control centers (MCC) are required by NEC^® Section 430.98 to be marked with the short-circuit current rating (SCCR). As well, according to UL 845, the Standard for Motor Control Centers, a motor control center (MCC) must be marked “Short-circuit current rating amps - RMS symmetrical volts - maximum. Do not install on circuits with available short-circuit currents greater than the lowest short-circuit rating of any installed unit," or the equivalent.

The intent is so that the electrician and inspector can ensure compliance with NEC^® 110.9 and 110.10. These code sections require a close look at things like overcurrent device ampere interrupt capacity (AIC), circuit impedance, circuit characteristics and SCCR’s of equipment. This is to ensure that the overcurrent device protecting the equipment is rated properly so that it can eliminate the fault condition before extensive damage occurs to the equipment.

The only way to ensure the MCC is rated to handle the amount of available short circuit current (also known as available fault current or AFC) is to first find out the maximum amount of fault current that could be delivered to the MCC during a short circuit condition and then to determine the SCCR of the MCC by looking at the nameplate. If the MCC has a SCCR equal to or greater than the available short circuit current, then the installation complies with NEC^® 110.10.

In the 2017 NEC^®, the available short circuit current at the MCC was first required to be documented and made available to those authorized to inspect the installation and to include the date of the short circuit current calculation. The inspector can look at the required calculation and verify that the MCC has not been installed at a location in the circuit where the AFC is greater than the SCCR of the MCC.

Knowing the amount of available short circuit current at the MCC is the first step in ensuring that the MCC is properly rated for the worst-case fault condition that it may see.

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).

2017 Code Language:

430.98 Marking.

(A) Motor Control Centers. Motor control centers shall be marked according to 110.21, and the marking shall be plainly visible after installation. Marking shall also include common power bus current rating and motor control center short-circuit rating.

430.99 Available Fault Current. The available short circuit current at the motor control center and the date the short circuit current calculation was performed shall be documented and made available to those authorized to inspect the installation.

2020-2023 Code Language:

430.98 Marking.

(A) Motor Control Centers. Motor control centers shall be marked according to 110.21, and the marking shall be plainly visible after installation. Marking shall also include common power bus current rating and motor control center short-circuit current rating.

430.99 Available Fault Current. The available fault current at the motor control center and the date the available fault current calculation was performed shall be documented and made available to those authorized to inspect, install, or maintain the installation.