Short Circuit Studies

A Short Circuit Study is performed to calculate the available fault current for electrical equipment. Article 110.9 of the NEC requires fuses and circuit breakers to have an interrupting rating which exceeds the calculated fault current. Article 110.10 of the NEC requires busses and cables to be protected from an electrical fault.

The most important number for performing a Short Circuit Study is the maximum available fault current at the point of service. After obtaining this number from the utility, the system impedances are modeled. This involves obtaining feeder lengths from the contractor. The lengths, sizes, quantities, and materials of conductors add impedance to the system which reduces the fault currents. Transformers also can also reduce fault currents. Large motors exceeding 50HP and generators can contribute fault currents in the event of power failure.

An Equipment Duty Evaluation is often performed after obtaining the fault currents at each bus. This will allow you to see if your electrical equipment complies with code. If you are a distributor, you do not want to sell insufficiently rated equipment to your client. If you are a contractor, you do not want to install insufficiently rated equipment and risk failing inspection and having to purchase & reinstall sufficiently rated equipment. A building owner would not want to risk having overdutied devices because a short circuit could cause an electrical panel to explode or cause a fire which would result in extensive property damage or injury to personnel.