Juyandrew
Member
- Location
- Reston, Virginia
Does a 15 amp inverse circuit breaker meet code requirement for protection of a 3 phase, 460 volt, 0.5 HP motor with 1.1 FLA?
Background information:
NEC 1999 is followed by this concerned project. The circuit breaker is located in a MCC compartment for supplying power to a 3 phase, 460 volt, 0.5 HP coiling up door motor. There is a non-fused disconnect switch located close to the coiling up door. There is a thermal rely or some types of thermostat type protector in the motor for protection. When the motor is heated to the preset point, it opens the contactor coil circuit to stop the motor. It will automatically reset when motor cools down. Following is the statement in a design correction document changing the original installed 20 amp breaker to 15 amp breaker.
Table 430-152 states a 0.5 HP motor (1.1 full load amperage) requires a 15 amp inverse time circuit breaker (250% x 1.1 = 2.75) or a 3 amp or 6 amp fuse depending if it is non-time delay or time-delay fuses (non-time delay 300% x 1.1 = 3.3 or time-delay 175% x 1.1 = 1.925).
It is believed that Exception No. 1 in section 430-52 (c) (1) was used to determine that the next higher standard circuit breaker size for the 2.75 amp maximum calculated rating of a 0.5 HP motor with 1.1 FLA is 15 amp. Since 15 amp breaker is the smallest standard circuit breaker size listed in section 240-6 (a).
In section 430-52 (c)(1) Exception 2 (c), it states that where the rating specified in Table 430-152, as modified by Exception 1, is not sufficient for the starting current of the motor, the rating of an inverse time circuit breaker shall be permitted to be increased but shall in no case exceed 400 percent for full-load current of 100 amperes or less .... 15 amp breaker is approximately 1360 % of the motor FLA.
Background information:
NEC 1999 is followed by this concerned project. The circuit breaker is located in a MCC compartment for supplying power to a 3 phase, 460 volt, 0.5 HP coiling up door motor. There is a non-fused disconnect switch located close to the coiling up door. There is a thermal rely or some types of thermostat type protector in the motor for protection. When the motor is heated to the preset point, it opens the contactor coil circuit to stop the motor. It will automatically reset when motor cools down. Following is the statement in a design correction document changing the original installed 20 amp breaker to 15 amp breaker.
Table 430-152 states a 0.5 HP motor (1.1 full load amperage) requires a 15 amp inverse time circuit breaker (250% x 1.1 = 2.75) or a 3 amp or 6 amp fuse depending if it is non-time delay or time-delay fuses (non-time delay 300% x 1.1 = 3.3 or time-delay 175% x 1.1 = 1.925).
It is believed that Exception No. 1 in section 430-52 (c) (1) was used to determine that the next higher standard circuit breaker size for the 2.75 amp maximum calculated rating of a 0.5 HP motor with 1.1 FLA is 15 amp. Since 15 amp breaker is the smallest standard circuit breaker size listed in section 240-6 (a).
In section 430-52 (c)(1) Exception 2 (c), it states that where the rating specified in Table 430-152, as modified by Exception 1, is not sufficient for the starting current of the motor, the rating of an inverse time circuit breaker shall be permitted to be increased but shall in no case exceed 400 percent for full-load current of 100 amperes or less .... 15 amp breaker is approximately 1360 % of the motor FLA.