A motor branch circuit is defined as starting at the circuit breaker or fused switch where the branch circuit receives its power. It includes the conductors, the conduit that provides mechanical protection, a disconnect, and the motor controller that provides motor starting and stopping control and some form of overload protection. The branch circuit's circuit breaker or fused switch is intended to protect the conductors from overheating due to short circuits and ground fault. Protecting the motor itself against overcurrent is provided by overload relays in the motor starter assembly, not the branch circuit's fuse or circuit breaker. And, the overload relay protective system controls start and stop contacts are not designed to interrupt a short-circuit.
The NEC specifies devices intended to break current (switches, circuit breakers and contractors) must have sufficient interrupting capacity for the voltage and the current to be interrupted (Section 110-9). The devices must be capable of clearing all possible faults without causing damage to the electrical components of the circuit (Section 110-10). The size of wire used must have an ampacity at the operating temperature which is not less than 125 percent of the motor full-load rating (Section 430-22). For purposes of applying this rule, full-load currents are defined by tables 430-147 through 430-150.
A switch or circuit breaker may serve both as a motor controller and disconnecting means (Section 430-111). However, for this use, it must be located within sight of the motor and within 50 feet. If this is not possible, a provision for locking the controller in the open position must be made, or a manual switch must be provided at the motor location (Section 430-86).
It is considered good practice to select the branch circuit fuse or circuit breaker rating as small as feasible. This suggests dual-element fuses rated at approximately 125 percent of the motor full-load current. This circuit fuse configuration provides short-circuit protection and good back-up running overload protection. This usually allows the motor to start properly while coordinating fuse operation with the motor running overload relays.
The manual or automatic starter can be located anywhere in the branch circuit between the circuit disconnect and the motor. The motor starter contains the relays or fuses for overload and locked rotor protection. It does NOT provide short-circuit protection unless it is a combination starter which contains a circuit breaker or a fused disconnect switch. These combination starters include a starting contactor, the overload relays and either a circuit breaker or a fused disconnect switch in one factory assembled box. These combination starters are very popular. Motor control centers are modular cabinets designed for easy addition of combination starters.
A three-phase motor will not start if the supply system loses one leg (i.e., one phase) due to a blown fuse, an open line, or some other reason. However, the motor could continue to run if it is operating within the motor's torque capability. This situation will cause currents in the two operating phases to be excessive, and the motor will overheat unless the single-phase condition is either promptly corrected or the motor is turned off. The overload relays in the starter should operate before the temperature reaches damaging levels, but these overload relays are sometimes misapplied or sized incorrectly (especially if capacitors are switched with the motor). That is why you may want to improve your motor protection by placing a single-phase detector relay at the service entrance or at the terminals of the transformer supplying the motor feeders.
Nema Code Letters
Short Taps From A Feeder
Feeders For Motor Circuits
Motor Branch Circuits - Example 1
Motor Feeder Circuit - Example 2
Motor Overcurrent Protection - Example 3
Design Of Motor Branch Circuits: Several Motors
