A key factor in protecting people and electrical equipment from electrical hazards is the proper application of overcurrent protective devices (OCPD). Section 110.10 of the National Electrical Code requires the total circuit impedance, OCPD, equipment short-circuit current ratings and other circuit characteristics to be selected and coordinated so circuit protective devices can effectively respond and operate in a fault event without extensive damage to the electrical equipment or conductors. 

OCPDs such as circuit breakers and fuses must be selected for installation, and circuits must be designed to ensure the short-­circuit current rating of any system component, including conductors, is not exceeded should a short circuit or ground fault occur. There is often science behind requirements in the NEC. This article draws attention to some information in the Code related to meeting the requirements in Section 110.10.

Wire, bus structures, switching equipment, controllers, OCPDs and distribution equipment typically have limited short-­circuit current ratings. The short-circuit current rating is the amount of fault current a component is rated to withstand without producing explosion, fire or shock hazards. If a short-circuit or ground-fault event exceeds equipment short-circuit current ratings, the components could be severely damaged or destroyed.

The effective paths for ground-fault current are protective safety circuits that facilitate OCPD operation during abnormal events such as ground faults and short circuits, and they are essential to protecting equipment and conductors of the system. In a ground-fault event, the fault is between any ungrounded circuit conductor and the earth, equipment grounding conductor (EGC), enclosing metal raceway or other grounded metal equipment. 

The greatest damage often occurs in the first half-cycle during a short circuit or ground fault. Excessive heat from the high current can cause rapid deterioration of insulation or annealing conductors, and can even cause conductors to melt or vaporize. An effective ground-fault current path is required in electrical designs and installations to ensure fast operation of OCPDs during ground-fault or short-circuit conditions. One method of achieving additional safety in electrical systems is to reduce let-through current so there is less incident energy during a short-circuit or ground-fault event. Remember that “equipment” is a broad term and includes the conductors in the system. 

During a ground-fault or short-circuit event, the time a conductor can withstand these higher levels of current is significantly less. The higher the OCPD rating, the less time a conductor can safely withstand the current. 

Important guidance addressing conductor insulation abilities has been established through engineering research by the Insulated Cable Engineers Association (ICEA). The ICEA developed this information to prevent damage to conductor insulation and created a chart that shows that currents present for the times indicated in the graph produce maximum safe operating temperatures for each conductor size. The short-circuit current, conductor cross-sectional area and OCPDs in the circuit should be applied in a manner that does not exceed the maximum short-circuit time ratings in the ICEA chart. 

The informational note to NEC 240.4 references ICEA P-32-382-2018 for the maximum allowable short-circuit currents for insulated copper and aluminum conductors. More information is available at www.ICEA.net. 

Protecting conductor integrity is essential for safe electrical systems, and this applies to ungrounded circuit conductors, grounded conductors and wire-type EGCs. Improper sizing of wire-type EGCs or use of improper OCPD types can result in their annealing, melting or vaporizing before the circuit OCPD can clear the fault. Wire-type EGCs are typically much smaller than the associated circuit conductors, usually about 25% of the size. Refer to Table 250.122 for the minimum sizes required based on the rating or overcurrent protection for the circuit. 

Selecting proper OCPDs is a critical step in providing fast clearing times that will protect EGCs within the limits of the conductor withstand capabilities. Consider the entire system of protection, including the size of wire-type EGCs, their withstand ratings, the magnitude of ground-fault currents and the operating characteristics of the OCPDs. 

Protective devices that do not operate fast enough might leave EGCs vulnerable to severe damage during a ground-fault event. There is a mandatory note following Table 250.122 that references performance requirements for the ground-fault current paths addressed in 250.4(A)(5) and 250.4(B)(4). Protecting the integrity of wire-type EGCs often part of the effective ground-fault current path is implied in these requirements.