The National Electrical Code (NEC) requires that all electrical devices intended to interrupt a circuit at fault levels must have an interrupting rating that is equal to or greater than the maximum available short-circuit current at the point of installation. This equipment must also have a minimum short-circuit current rating that is greater than the available short-circuit current, and an arc fault rating that is higher than the available short-circuit current. These ratings are marked on equipment such as panelboards, switchboards, busways, contactors, and starters. Listed products which have been tested under fault conditions are considered to have met the requirements of 110.10.
Circuit Impedance, Short-Circuit Current Ratings, and Other Characteristics
The purpose of overcurrent protection is to open the circuit before any damage is done to conductors or conductor insulation when an overcurrent condition occurs. An overcurrent condition can be caused by an overload, a ground fault, or a short. This protection ensures that the circuit will be safely disconnected if any of these conditions occur.
When selecting overcurrent protective devices such as fuses and circuit breakers, it is important to make sure that the short-circuit current rating of the system components is not exceeded in the event of a short circuit or high-level ground fault. This will ensure that the system is safe and will be able to handle the current without damaging any components.
All wires, bus structures, switching, protection, and disconnect devices, and distribution equipment have limited short-circuit ratings and would be damaged or destroyed if those ratings are exceeded. Therefore, it is important to select overcurrent protective devices with sufficient interrupting ratings and make sure that the available short-circuit current does not exceed the short-circuit current rating of any electrical component. This will ensure that the overcurrent protective device will be able to limit the let-through energy to within the rating of the electrical component and provide adequate short-circuit protection.
Utility companies usually provide their customers with information about the available short-circuit current levels at the service equipment. To calculate the short-circuit currents at each point in the distribution system, customers can contact the manufacturers of overcurrent protective devices or refer to the IEEE 141-1993 (R1999) publication, the IEEE Recommended Practice for Electric Power Distribution for Industrial Plants (Red Book).
Adequate short-circuit protection can be provided by fuses, molded-case circuit breakers, and low-voltage power circuit breakers, depending on the specific circuit and installation needs. These devices can help ensure that the available short-circuit current does not exceed the short-circuit current rating of any electrical component.
Application Example
For a typical one-family dwelling with a 100-ampere service, the available short-circuit current would be approximately 6000 amperes. In multifamily structures, where the transformers are located close to the multimetering location, the available short-circuit current could be as high as 22,000 amperes. The short-circuit current rating of utilization equipment should be known, as it will be different depending on the load rating. For example, HVAC equipment is tested at 3500 amperes through a 40-ampere load rating, and 5000 amperes for loads rated more than 40 amperes.
