Motor Branch-Circuit Design

Minimum safety provisions for the control of motors are set forth in the National Electrical Code (NEC). Although these minimum provisions must be met, they are no substitute for an intelligent selection of protective devices made on the basis of the motor circuit being designed. The code describes this fact in Article 430, Section IV: “Where maximum branch-circuit short-circuit and ground-fault protective device ratings are shown in the manufacturer’s overload relay table for use with a motor controller or are otherwise marked on the equipment, they shall not be exceeded even if higher values are allowed [by the code].”

NOTE: An instruction sheet included with each starter contains thermal unit selections, as well as fuse or circuit breaker selections, or both, as applicable.

The capability of industrial systems to deliver high short-circuit currents has been increasing steadily over the years. This has caused much concern about the capability of motor controllers to withstand high-current faults, without creating hazards for personnel and destruction of equipment.

NEMA standards require that contactors be able to interrupt currents up to ten times motor full-load current. Therefore, it is acceptable for the overload relay to respond before the short-circuit protector up to this level. At currents more than ten times motor full-load current, the short-circuit protective device must respond first to minimize equipment damage.

A fully coordinated system is achieved when the overload relays operate in response to motor overloads before the fuses or circuit breaker, and the fuses or circuit breaker open the circuit before the overload relays trip or burn out on short-circuit currents.

Proper coordination requires a thorough knowledge of the time-versus-current limits of all of the branch-circuit components, as well as the time-versus-current trip characteristics of the overload relay and short-circuit protective device.