Transfer equipment is already required to be marked with the short-circuit current rating (SCCR) per UL 1008 which covers transfer equipment. UL 1008 has many options for short-circuit protection of transfer switches and equipment. Transfer switches are usually marked by the manufacturer with several different combinations resulting in multiple options to choose from to ensure proper SCCR values based on the fault current available at the transfer switch.

These marked SCCR values can vary based upon the specific overcurrent protective device type, ampere rating and settings. The transfer switch in the image has a SCCR of 65,000 amps at 480 volts when protected by a Square D Type PJ circuit breaker not larger than 1200 amps. The same transfer switch has a SCCR of 200,000 amps at 600 volts when protected by a Class L fuse not larger than 1200 amps. When inspecting the specific installation, it can be confusing for an inspector to know which SCCR is valid based on the combination selected by the engineer for the specific project.

To solve this issue, in the 2017 NEC^®, the short-circuit current rating of the transfer switch, which is based on the specific overcurrent protection provided, is required to be additionally field marked on the equipment.

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).