Available short-circuit current requirements were either added or changed in a number of locations in the 2017 National Electrical Code (NEC). To understand and deal with the issue of available short-circuit current in the existing text, as well as the new text for the 2017 NEC, design engineers, electrical contractors, electricians and electrical inspectors must recognize the purpose of this marking.

The documentation for the calculation required for the short-circuit current marking must be made available to the facility owner and to anyone designing, installing, inspecting, maintaining or operating the electrical system. Too often, this information is not present at the site and no one knows what or where it is.

A thorough understanding of available short-circuit current ratings starts with the definition in Article 100: “The prospective symmetrical fault current at a nominal voltage to which an apparatus or system is able to be connected without sustaining damage exceeding defined acceptance criteria.” 

In other words, all electrical equipment has a withstand rating, which is the point at which the equipment can withstand a certain amount of fault current without undue damage. This component withstand capability is further enhanced by the overcurrent protective device design and operation during bolted and arcing fault conditions.

Section 110.10, Circuit Impedance, Short-Circuit Current Ratings, and Other Characteristics, emphasizes not permitting damage to the electrical equipment: “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.”

A technical subcommittee assigned to study high-capacity electrical systems inserted this text into the 1965 NEC. The 1965 NFPA Electrical Code Handbook provided the following explanatory text: “[The study] was based on a survey of the available currents of utility systems throughout the United States and reports of damage caused by non-clearing faults. It was realized by this Committee that proper protection of electrical systems involves design engineering and the coordination of the specific systems with the characteristics of the overcurrent protective devices.

“It was also realized that where arcing faults occur on high capacity low voltage systems, the actual value of fault current may be below the response value of the overcurrent devices. In such systems, some other form of protection [during arcing faults], such as ground-fault protection, would be necessary to ensure the desired protection.

“It was concluded by the Technical Subcommittee that although the industry was not ready for the mandatory application of elaborate ground-fault protection requirements, they should take the first step of alerting design engineers to their responsibilities in this area. It was felt that every system could be designed so that the selected overcurrent protective devices would properly clear any bolted faults placed on the systems either between the phase conductors or between any conductor and the enclosing raceway or grounding conductor. The addition of a separate grounding conductor cabled with the circuit conductors can serve to reduce both the resistance and reactance of the fault current return path through ground.

“The available current from the supply system, the actual characteristics of the building electrical system, such as conductors, transformers, the time-delay characteristics of the overcurrent protective devices, and the contribution of the spinning load [such as motors], can all be part of ‘other characteristics’ mentioned in the Code wording [in 110.10]. The need for better protection is recognized but will need to go hand in hand and will be dependent on a better understanding of its function by industry and the availability of equipment which will provide the desired function.”

The historical intent or content of a section could be lost but for the recording of the committee or panel’s work in the substantiation during the NEC process.

For many years, I worked as an EC in the Phoenix area, where the City of Phoenix Electrical Plan Reviewers and the electrical utilities were very active in requiring proper equipment protection based on the utility company delivery of the available fault current at a building’s service. The reviewers emphasized bracing equipment to withstand the available fault current and the let-through current of circuit breakers and fuses so that equipment damage was minimized. Recognition of the potential damage, expansion of equipment available fault-current markings and documentation of equipment fault-current ratings in the 2017 NEC is long overdue.