# Am I sizing these Motor OCPD's right?



## Huntxtrm (Apr 3, 2012)

I have 5 motors, 3 phase 480 volt. 3 on one feeder, 2 on the other. I'll just do my math on one feeder so you can tell me where I am screwing up or not. My conclusions are in Bold.

All on one feeder:
40hp 3ph 480
5hp 3 h 480
10hp 3ph 480

1. 40hp=52FLA(430.250)x 1.25 for Branch circuit size=65A*= #6THWN-2*
52FLA(430.250) x 1.75(430.52)=91A next larger fuse is *100 amp*.
EGC = #8thwn-2 (250.122)

2. 5hp=7.6FLA(430.250)x1.25 for Branch circuit size=9.5A=*#12THWN-2*
7.6FLA x 1.75=13.3A next larger fuse is *15 amp*.
*EGC=#12*

3. 10hp=14FLA x 1.25 for branch circuit size=17.5A= *#12 Thwn-2*
14FLA x 1.75 = 24.5A next larger fuse size is *25A*
EGC= *#12 Thwn-2*

*Feeder*
Largest motor FLA is 52A.
52A x 1.25=65
65+7.6+14=86.6A = *#3 Thwn Feeder*
*Here is where I get confused!*
Largest ocpd is a 100 amp fuse, add FLA of the rests.
100A + 14 + 7.6 = 121.6A next smaller breaker is 110A for my feeder OCPD. 
*EGC is #8*


Here is my delima. If I use fused disconnects feed from a breaker, sometimes my feeder ocpd is smaller than my largest motor fuse. Should I be sizing my feeder ocpd as if my largest motor had a breaker on it, instead of a fuse. Just for the calculation? Makes sense to me.

Just to explain. These three motors are supplied by 1 feeder, from a distribution panel, into a gutter with fusible disconnects on it, feeding to the motors.

Please somebody clarify this for me, and give me comfort in my math.


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## don_resqcapt19 (Jul 18, 2010)

Huntxtrm said:


> ...
> Here is my delima. If I use fused disconnects feed from a breaker, sometimes my feeder ocpd is smaller than my largest motor fuse. Should I be sizing my feeder ocpd as if my largest motor had a breaker on it, instead of a fuse. Just for the calculation? Makes sense to me.
> ...


Yes, if you are using breakers for the feeder OCPD, then you assume the use of breakers for the motor OCPDs.


> 430.62 Rating or Setting — Motor Load
> (A) Specific Load. A feeder supplying a specific fixed motor load(s) and consisting of conductor sizes based on 4 30.24 shall be provided with a protective device having a rating or setting not greater than the largest rating or setting of the branch-circuit short-circuit and ground-fault protective device for any motor supplied by the feeder [*based on* the maximum permitted value for *the specific type of a protective device* in accordance with 430.52, or 440.22(A) for hermetic refrigerant motor-compressors], plus the sum of the full-load currents of the other motors of the group. Where the same rating or setting of the branch-circuit shortcircuit and ground-fault protective device is used on two or more of the branch circuits supplied by the feeder, one of the protective devices shall be considered the largest for the above calculations.


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## Huntxtrm (Apr 3, 2012)

don_resqcapt19 said:


> Yes, if you are using breakers for the feeder OCPD, then you assume the use of breakers for the motor OCPDs.


Thanks, that is what I thought. So, I am doing it right. I just got confused by that lawyer talk they have in the code book.:thumbsup: I appreciate the info.


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