Courtesy of www.MikeHolt.com
Based on the 2020 NEC. Underlined text indicates a Code change.
Q. What does the Code mean by available fault current?
See answer below.
A. As noted in Art. 100 of the 2020 NEC: Fault Current, Available (Available Fault Current). The largest amount of current capable of being delivered at a point on the system during a short-circuit condition.
Informational Note: A short circuit can occur during abnormal conditions such as a fault between circuit conductors or a ground fault. (See Informational Note Figure 100.1 in the NEC.)
Fault current is current outside the usual circuit path and with a magnitude that exceeds the normal circuit current. A fault can be line-to-line, line-to-neutral, or line-to-ground. The new NEC Informational Note, Fig. 100.1, helps explain the differences between “Available Fault Current,” “Short-Circuit Current Rating” (SCCR),” and “Interrupting Rating (AIC).” The available fault current is the largest amount of current available at that point on the circuit. It is important to note that the fault current value is not the same throughout the circuit; it becomes smaller as the impedance is increased between the point of the fault and the source of the power.
The SCCR is the maximum amount of current that equipment, other than overcurrent protective devices (OCPDs), can safely withstand. Much of the equipment supplied by the electrical system is required to be marked with this rating. The designer and installer need to make sure the fault current available at the equipment is less than what the equipment can withstand. This often needs to be addressed at the design stage of a project so specified equipment can have a suitable SCCR. Correcting an installation where the available fault current exceeds the SCCR of the equipment after it has been installed can be costly and time consuming. One very common issue is with large air conditioning equipment which often has an SCCR of 5,000A, unless a higher rating was specified at the time the equipment was purchased.
The interrupting rating of an overcurrent protective device is the maximum amount of current that an OCPD can safely interrupt. OCPDs that are subjected to currents exceeding their AIC (or KAIC) rating may fail violently. Applying equipment within their SCCR or AIC ratings is critical for a safe installation.
These materials are provided to us by Mike Holt Enterprises in Leesburg, Fla. To view Code training materials offered by this company, visit www.mikeholt.com/code.
