Quite often, equipment will only have a short-circuit current rating (SCCR) when protected by a specific type of overcurrent protective device (OCPD). Using any other type of OCPD other than what is marked on the equipment will not be guaranteed to achieve the desired SCCR.

Sometimes the equipment will have a label that has blanks to be filled in by the site electrician based on the field conditions and the amount of Available Fault Current (AFC). Quite often a specific fuse will be chosen based on its current limiting characteristics and using the up-over-down method on the fuse chart.

An experienced electrician will know more than just how to hang conduit and pull wire. Basic electrical theory, impedance, resistance and overcurrent device characteristics must be understood in order to make an installation compliant with NEC^® 110.9 and 110.10.

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).

2014-2017 Code Language:

110.10 Circuit Impedance, Short-Circuit Current Ratings, and Other Characteristics. The overcurrent protective devices, the total impedance, the equipment short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit protective devices used to clear a fault to do so without extensive damage to the electrical equipment of the circuit…