Selecting overcurrent protection for motors is the topic for this EC&M Tech Talk. Randy Barnett first lists the three types of overcurrent conditions that can exist and explains the difference between. This leads to a logical discussion of the code rules for protecting against each of these conditions. The general rules for overcurrent protection are found in Art. 240 Overcurrent Protection with specific rules for overcurrent protection of motors found in Art. 430 Motors, Motor Circuits, and Controllers. This Tech Talk concentrates on Art. 430 for motors.

Two specific parts of Art. 430 address overcurrent protection. A discussion of Part III presents the rules for protecting motors from overload conditions. In most cases these rules will apply to continuous duty motors. So, the code requirement for classifying motors as continuous duty is presented with the excepted motor types listed. Overload settings are based on a percentage of the full load current rating found on the nameplate. The percentages of this value for overload protection can vary.

Fuses and circuit breaker ratings used for short-circuit and ground-fault protection are typically based on table values found in Art. 430, as are ampacity values for the motor branch circuit conductors. Learning when to apply the table values and when to use motor nameplate values is key to this discussion.

Tying all the above information together is accomplished using a schematic diagram of the NEC Informative Annex D8 example for a 25-hp motor.