multiple motors

[stjohnbarleycorn]

I can't seem to find where you round down on the feeder circuit for the next standard size breaker. Thanks for any help.

 

[charlie b]

I'm not sure what you are asking, and don't understand the connection between having multiple motors and rounding down the breaker sizes. Can you clarify your question?

 

[stjohnbarleycorn]

sorry, for a single motor when you figure out the OCPD size from the table, 430.52 430.52(c) states you can't go over the amperage in the table, and then in 420.52(C) ex. 1 it states you can go to the next standard size up. If I am reading it correctly.
If you are feeding multiple motors you take 125% of the first and then the sum of FLC of the rest. I think there is a reference that says you can't round up to the next size in this case because of the multiple motors on the circuit. But I may be wrong. thanks.

 

[elohr46]

sorry, for a single motor when you figure out the OCPD size from the table, 430.52 430.52(c) states you can't go over the amperage in the table, and then in 420.52(C) ex. 1 it states you can go to the next standard size up. If I am reading it correctly.
If you are feeding multiple motors you take 125% of the first and then the sum of FLC of the rest. I think there is a reference that says you can't round up to the next size in this case because of the multiple motors on the circuit. But I may be wrong. thanks.

Perhaps you are thinking of 430.52(C)(1)(ex.2)(a)

eg. if you have ocpd of 28 amps X 400% = 112 amps. You have to round down to 110 because 400% is your maximum allowed.

 

[stjohnbarleycorn]

that might be what I am thinking of.
So there is no difference in feeding a single motor or multiple motors on a feeder, as far as the rating of the OCPD, You can go with the next size up standard breaker, unless it exceeds 400%? thanks for the help.

 

[Hameedulla-Ekhlas]

sorry, for a single motor when you figure out the OCPD size from the table, 430.52 430.52(c) states you can't go over the amperage in the table, and then in 420.52(C) ex. 1 it states you can go to the next standard size up. If I am reading it correctly.
If you are feeding multiple motors you take 125% of the first and then the sum of FLC of the rest. I think there is a reference that says you can't round up to the next size in this case because of the multiple motors on the circuit. But I may be wrong. thanks.

I think 125% for the largest and sum of the other is for sizing a feeder and 250% for the largest and sum of the other is for sizing the breaker.

 

[KP2]

I can't seem to find where you round down on the feeder circuit for the next standard size breaker. Thanks for any help.

I have seen in a few books that recomend using a lower size OPCD for example; {if the calculation equals 72 amps then try a 70 amp breaker}. The code allows us to round up to an 80 amp breaker. Art.240.4.

One example of rounding down first is the WOLF method, of which I'm not a fan of.


However the OCPD must allow the motor to start, so we can exceed the 250 percent (for an inverse time breaker) and not exceed 400%.

 

[augie47]

H-E makes a good point on your post. You mention the 125% which is the number used in sizing feeder conductors. You would use the appropriate number in 430.52 for sizing your ground-fault/short circuit device.

 

[stjohnbarleycorn]

I was reading over the info, I think what I am getting at is that for a feeder for multiple motors , and using a IT breaker, you take the largest motor FLC, times 250%, then the other motors FLC added to that. The final number you come up with should be rounded down, not up as with a branch circuit.

 

[augie47]

I was reading over the info, I think what I am getting at is that for a feeder for multiple motors , and using a IT breaker, you take the largest motor FLC, times 250%, then the other motors FLC added to that. The final number you come up with should be rounded down, not up as with a branch circuit.

I agree...........

 

[KP2]

In article 430.62(A) ex 1 it does say for instantaneous trip circuit breakers that we can not exceed the Values of T430.52.
But for all other fuse or standard breakers, (Inverse Time) we can still round up on the feeder.
Is that what your looking at?

 

[augie47]

In article 430.62(A) ex 1 it does say for instantaneous trip circuit breakers that we can not exceed the Values of T430.52.
But for all other fuse or standard breakers, (Inverse Time) we can still round up on the feeder.
Is that what your looking at?

I'm sorry, but I don't read it that way. I may be missing it but I can find no reference in 430.62 for feeders to allowing "a higer size"as I do in 430.52 for branch circuits.

 

[elohr46]

that might be what I am thinking of.
So there is no difference in feeding a single motor or multiple motors on a feeder, as far as the rating of the OCPD, You can go with the next size up standard breaker, unless it exceeds 400%? thanks for the help.

No, that section only applies to an individual motor.

 

[KP2]

I'm sorry, but I don't read it that way. I may be missing it but I can find no reference in 430.62 for feeders to allowing "a higer size"as I do in 430.52 for branch circuits.

I see that too.

Do you think that in the term they use in exception 1 "or motor short circuit protectors" applys to the other 3 types from 430.52?

And why single out Inst. trip?

 

[Stopmoving]

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
430.24 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.

Disregarding exceptions, this is the code you're looking for. You take the largest OCPD and add the full load current of the remaining motors. then you go down to the next standard size. (See bolded portion).

 

[kwired]

when sizing a feeder for multiple motors code just tells you how to calculate the minimum size of feeder needed. you can run however large you wish over the minimum.

 

[leepatzius]

I can't seem to find where you round down on the feeder circuit for the next standard size breaker. Thanks for any help.

Is this what you're asking about?

430.62 Rating or Setting

(A) Specific Load. A feeder supplying a specific fixed motor load(s) and consisting of conductor sizes based on 430.26 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 [ """ ] plus the sum of the full load currents of the other motors of the group.

EDIT: Whoops! Sorry "stopmoving", I didn't see your post.

 

[kwired]

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
430.24 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.

Disregarding exceptions, this is the code you're looking for. You take the largest OCPD and add the full load current of the remaining motors. then you go down to the next standard size. (See bolded portion).

That would allow for feeder with overcurrent protection sized larger than the feeder. I think the feeder is required to supply motor loads only to be able to use this rule, so be careful not to supply lighting and appliance type loads from the feeder or everything changes.

 

[stew]

the direction in 430.62 say to use the values determined in accordance with 430.52. That being said it seems to me that the exception 2 would allow the next larger size but only when the load will not start and only when the 400% maximum is maintained correcet?