Service Entrance and Short Circuit Currents
Service entrance equipment can present unique challenges when it comes to proper application of electrical equipment. The service point is the point of demarcation between utility and premise wiring. Our discussion today will springboard off of a previous article that focused on calculating short-circuit currents. Let’s focus on the proper application of the equipment at the service entrance location in the power distribution system. We’ll reference the National Electric Code® (NEC) but with an understanding that this is also the point of demarcation between the National Electrical Safety Code® (NESC) and the NEC. Regardless of the code jurisdiction under which the application falls, when you drop back to the basics of electrical principles, we gain an understanding to help in the proper application of electrical equipment at this and any other location in the power distribution system. Attention to detail is warranted for safety.
A service consists of conductors and other equipment essential for delivering power from the utility to the premise. The NEC provides the requirements for services in Article 230, titled the same. Although not always the case, the point of common coupling with the utility is typically the point most likely to present the highest level of short circuit current in the facility. This equipment must be adequate for the installation in both rating and listing. The ratings we’ll focus on today include Short Circuit Current Rating (SCCR) and Interrupting Rating (IR).
SHORT CIRCUIT CURRENT RATING
If you confuse SCCR with IR, you are not alone. I have heard these terms abused. To understand SCCR, consider it the ability of a piece of equipment to hold together while fault current flows through it on its journey to the downstream faulted portion of the circuit. The equipment has to be able to let this fault current pass through without damaging itself.
The equipment between the utility transformer and the faulted portion of the circuit must let the fault current pass through without experiencing an unintended rapid disassembly. The performance of the distribution equipment will depend upon many factors, one of which is the magnitude of fault current and the other is, of course, the capability of the equipment installed. The meter socket, for example, is not designed to stop the flow of current but rather serve its primary function of measuring the amount of energy being delivered to the facility. It must also serve its secondary function of delivering current to the load. The current it delivers is normally a load current that should not exceed its continuous current rating. The current could also be a temporary overload that should not exceed its maximum amp rating. Then again, the current could be a very high short-circuit current on the order of many times the magnitude of the normal load current, which should not exceed its SCCR. The meter socket will be equipped with an SCCR, which tells us how much current it can let through safely without causing damage to itself. Of course, there are many other types of equipment that may be located at the service entrance location, including power distribution blocks, busway, conductors, and more. The possible configurations are many but the basic principles remain the same, and each component that must deliver current to the load must be appropriately rated. They must have an adequate IR and SCCR.
Once we have the basic understanding of what an SCCR label means for electrical equipment and why it is important, it should be evident that all of the equipment that delivers normal load currents may be called upon to deliver high short-circuit currents and will need to have an adequate SCCR rating to do so without creating a hazard. Short-circuit currents place extreme for