Summary
Highlights
Overcurrent is defined as current exceeding the rating of equipment or the ampacity of a conductor. Protection against overcurrent is crucial for both conductors (Article 240) and equipment (specific equipment articles, e.g., Article 450 for transformers).
Overcurrent can result from an overload, a ground fault, or a short circuit. True overcurrent protection must address all three. While one device often handles all three, complex scenarios like feeder taps or motors may require specialized protection.
An overload (defined in Article 100) is the operation of equipment in excess of its rating or a conductor in excess of its ampacity. This is distinct from short circuits and ground faults. Overloads typically involve a small current increase over a long duration, leading to insulation failure rather than conductor melting if unchecked.
Ground faults occur when an ungrounded conductor contacts metal parts. Short circuits happen when two ungrounded conductors, or an ungrounded and a grounded/neutral conductor, make contact. These are emergency situations involving thousands of amps for a very brief duration.
A branch circuit overcurrent device, such as a circuit breaker, provides protection against all three types of overcurrent (overloads, ground faults, short circuits). It must also have an appropriate interrupting rating, never less than 5000 amps.
Supplementary overcurrent protection devices offer limited protection in addition to the required branch circuit overcurrent device. They are often used for specific equipment like motors and do not need to be readily accessible as per sections 240.10 and 240.24.