The short-circuit current rating (SCCR) is important to know when designing and installing an electrical control panel because of its role in over-current protection. The SCCR is the maximum current (in amps) that an electrical device or control panel is able to withstand for three (3) VAC electrical cycles, or during the operation of an over-current protection device such as a fuse. UL 508A, the standard for industrial control panels in the United States, provides the below instructions for determining the SCCR of a control panel:
1. Determine the SCCR of all individual power components and UL approved series ratings.
a. Examine all individual components and record their SCCRs.
b. If SCCR is not marked or known, use UL508A default Table SB4.1
c. Check and record any UL approved series ratings.
d. Note the minimum SCCR in the feeder circuit and each branch circuit.
2. Modify the SCCR of certain series combinations and record their SCCRs.
a. If current limiting fuses are used in the feeder circuit, compare the Ipeak of the fuse from UL 508A Table SB4.2 to the lowest rated SCCR component or minimum SCCR in each branch circuit.
b. The Ipeak of the fuse must be less than or equal to the lowest rated SCCR of any component or SCCR in each branch circuit.
3. The SCCR of the panel is equal to the lowest rated SCCR of any:
a Overcurrent protective device
b. UL approved series SCCR rating
c. Modified combinations from Step 2
d. Power component not affected by Step 2