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The best method for providing overcurrent protection for most circuits is to use a circuit breaker that combines overcurrent protection with short-circuit and ground-fault protection. However, this isn't usually the best choice for motors. With rare exceptions, the best method for providing overcurrent protection in these cases is to separate the overload protection devices from the short-circuit and ground-fault protection devices (Fig. 1 at right).
Motor overload protection devices like heaters protect the motor, the motor control equipment, and the branch-circuit conductors from motor overload and the resultant excessive heating (430.31). They don't provide protection against short-circuits or ground-fault currents. That's the job of the branch and feeder breakers, which don't provide motor overload protection. This arrangement makes motor calculations different from those used for other types of loads. Let's look at how to apply Art. 430, starting at the motor.
Overload protection
Motor overload devices are often integrated into the motor starter. But you can use a separate overload device like a dual-element fuse, which is usually located near the motor starter, not the supply breaker.
If you use fuses, you must provide one for each ungrounded conductor (430.36 and 430.55). Thus, a 3-phase motor requires three fuses. Keep in mind that these devices are at the load end of the branch circuit and that they don't provide short-circuit or ground-fault protection.
Motors rated more than 1 hp without integral thermal protection and motors rated 1 hp or less that are automatically started [430.32(C)] must have an overload device sized per the motor nameplate current rating [430.6(A)]. You must size the overload devices no larger than the requirements of 430.32. Motors with a nameplate service factor (SF) rating of 1.15 or more must have an overload protection device sized no more than 125% of the motor nameplate current rating.
Let's look at Fig. 2 on page C28 and work through a sample calculation.
Example No. 1: Suppose you use a dual-element fuse for overload protection. What size fuse do you need for a 5-hp, 230V, single-phase motor with a service factor of 1.16 if the motor nameplate current rating is 28A?
(a) 25A
(c) 35A
(b) 30A
(d) 40A
The overload protection...