Protection of a Motor with 1.1 FLA by a 15 amp Inverse Bkr

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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.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
It is only for short circuit protection of the conductors, and yes, 15A would be an acceptable CB rating for this motor.

Overload protection is a separate requirement.
 
NEC Defination Clarification

NEC Defination Clarification

Per Figure 430.1, the concern breaker is "Motor Branch-circuit Short-Circuit and Ground-fault protection" Breaker (Part IV). Per definition in section 430.51, General - Part IV specifies devices intended to protect the motor branch-circuit conductors, the motor control apparatus, and the motors against overcurrent due to short circuits or ground faults. The statement "It is only for short circuit protection of the conductors" does not a code compliance statement.

If it is acceptable by NEC, the following condition will meet Code requirement but totally leave the load without protection.


A 15 A inverse type breaker installed for protection of a 3 phase, 460 V, 0.5 HP roll-up door motor without overload protection as permitted by section 430.33 meets code requirement although the motor is totally not protected. The lock-rotor current of the motor is approximately 8.8 amp which will not activate the 15 A inverse type breaker at all till it is damaged by overheat if the door is jammed.

430.33 Intermittent and Similar Duty. A motor used for a condition of service that is inherently short-time, intermittent, periodic, or varying duty, as illustrated by Table 430.22(E), shall be permitted to be protected against overload by the branch-circuit short-circuit and ground-fault protective device, provided the protective device rating or setting does not exceed that specified in Table 430.52.



















































 

kwired

Electron manager
Location
NE Nebraska
Per Figure 430.1, the concern breaker is "Motor Branch-circuit Short-Circuit and Ground-fault protection" Breaker (Part IV). Per definition in section 430.51, General - Part IV specifies devices intended to protect the motor branch-circuit conductors, the motor control apparatus, and the motors against overcurrent due to short circuits or ground faults. The statement "It is only for short circuit protection of the conductors" does not a code compliance statement.

If it is acceptable by NEC, the following condition will meet Code requirement but totally leave the load without protection.


A 15 A inverse type breaker installed for protection of a 3 phase, 460 V, 0.5 HP roll-up door motor without overload protection as permitted by section 430.33 meets code requirement although the motor is totally not protected. The lock-rotor current of the motor is approximately 8.8 amp which will not activate the 15 A inverse type breaker at all till it is damaged by overheat if the door is jammed.

430.33 Intermittent and Similar Duty. A motor used for a condition of service that is inherently short-time, intermittent, periodic, or varying duty, as illustrated by Table 430.22(E), shall be permitted to be protected against overload by the branch-circuit short-circuit and ground-fault protective device, provided the protective device rating or setting does not exceed that specified in Table 430.52.

So if you do use provisions in 430.33, for a polyphase induction motor table 430.52 is going to limit the overload protection to 250% of motor full load which in your case is going to be 2.75A. A 15 amp circuit breaker still fits the bill for short circuit and ground fault protection but not for overload protection.
 
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