The National Electric Code requires that equipment be marked with the available fault current, and that this labeling be kept up-to-date when there are changes that affect the available fault current.
What is available fault current?
The available fault current is also known as the short circuit current. It is the maximum current available should there be a short circuit, such as a ground fault, that could cause an arc flash. The labeling of available fault current is not a part of arc flash labeling and a separate label is typically used.
Before the available fault current for equipment within your facility can be determined, you need to know the available fault current at the secondary terminals on the utility transformer feeding your facility. This is usually provided by the utility and gives the maximum possible fault current.
Based on the utility's transformer available fault current the available fault current for the electrical equipment and panels throughout the facility can then be calculated. Each must be calculated separately as the available fault current varies based on the circuit impedance. The circuit impedance is unique at every point in the system. For example, circuit impedance is affected by factors such as the type and length of the wiring, as well as the other equipment that is in the circuit.
The addition or removal of electrical equipment from a circuit can change the impedance and change the available fault current in other parts of the circuit. For that reason the NEC 110 code requires that available fault current be recalculated whenever electrical changes are made.
Available Fault Current Labels
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