# Motor feeder OCPD calculations



## erics37 (May 7, 2009)

> *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
> ...


Am I interpreting this correctly if, in a situation where your OCPD calculation leads to a non-standard OCPD rating (per 240.6), you would have to size your motor feeder overcurrent device DOWN instead of UP to the next standard size, if applicable?


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## Bulldog1 (Oct 21, 2011)

erics37 said:


> Am I interpreting this correctly if, in a situation where your OCPD calculation leads to a non-standard OCPD rating (per 240.6), you would have to size your motor feeder overcurrent device DOWN instead of UP to the next standard size, if applicable?


Yes that is correct.


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## BBQ (Nov 16, 2010)

I think we should ask Harry. :thumbsup:


But I am not seeing it like you are. 



> 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*


I don't see how 240.6 comes into it at all, but I am really not sure on this.


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## erics37 (May 7, 2009)

BBQ said:


> I think we should ask Harry. :thumbsup:
> 
> 
> But I am not seeing it like you are.
> ...


Well I just mean, say you calculate out a feeder load to be 104 amps. You'll probably have a tough time finding a 104 amp circuit breaker, but the way I'm interpreting 430.62, you wouldn't be allowed to use a 110 amp breaker (the next standard size up). If you could find a 104 amp breaker then you could use that, as long as it's not greater than the calculated feeder load. Otherwise you'd probably just go with a 100 amp breaker. See what I mean?


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## Bulldog1 (Oct 21, 2011)

BBQ said:


> I think we should ask Harry. :thumbsup:
> 
> 
> But I am not seeing it like you are.
> ...



I was taught the largest breaker plus FLA of all the other motors and to always round down. Maybe 240.6 does not come into play.


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## chicken steve (Mar 22, 2011)

iirc, the history of motor OCPD's has had it's 'round' ups & downs

~CS~


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## erics37 (May 7, 2009)

Bulldog1 said:


> I was taught the largest breaker plus FLA of all the other motors and to always round down. Maybe 240.6 does not come into play.


No I don't mean it comes into play with the feeder OCPD calculation itself, just as a guide to help find standard breaker sizes.

I suppose if you were feeding a bunch of motors though you'd have an adjustable breaker anyway.


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## BBQ (Nov 16, 2010)

This is what I see.

If you have a feeder that supplies four motors

Each motor has the following branch-circuit short-circuit and ground-fault protective devices

1 - 30 amps device 23 FLA

2 - 25 amp device 18 FLA

3 - 20 amp device 12 FLA

4 - 15 amp device 8 FLA


So motor 1 has the largest branch-circuit short-circuit and ground-fault protective device at 30 amps.

Now more from the section



> plus the sum of the full-load currents of the other motors of the group.


So it is 30 + 18 + 12 + 8 = 68 amps. 

The feeder device cannot exceed 68 amps if the conductors were sized using Article 430. (In other words the conductor ratings are below the device rating)

If you sized the conductors using 'normal' methods you could use any size feeder breaker you wanted as long as the conductors were larger than the device.


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## erics37 (May 7, 2009)

BBQ said:


> If you sized the conductors using 'normal' methods you could use any size feeder breaker you wanted as long as the conductors were larger than the device.


Okay I see, that makes sense.


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## BBQ (Nov 16, 2010)

erics37 said:


> Okay I see, that makes sense.


I think to really see how it works you would have to do a full calculation from start to finish.

Find some real motor figures, max out each of their individual 'short circuit ground fault protection' and then run the smallest feeder conductors Article 430 would allow for the group and at that point I think the purpose of 430.62 would become clear. I just don't have the patience to cruise the Grainger web site to get some example motor data and do all the steps.


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## erics37 (May 7, 2009)

BBQ said:


> I think to really see how it works you would have to do a full calculation from start to finish.
> 
> Find some real motor figures, max out each of their individual 'short circuit ground fault protection' and then run the smallest feeder conductors Article 430 would allow for the group and at that point I think the purpose of 430.62 would become clear. I just don't have the patience to cruise the Grainger web site to get some example motor data and do all the steps.


No I see what you're getting at. 430.24 tells you how to size minimum feeder conductors, 430.62 tells you how to protect mimimum size feeder conductors. But there's nothing stopping you from sizing the conductors over that and protecting accordingly outside of 430.


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## BBQ (Nov 16, 2010)

erics37 said:


> No I see what you're getting at. 430.24 tells you how to size minimum feeder conductors, 430.62 tells you how to protect mimimum size feeder conductors. But there's nothing stopping you from sizing the conductors over that and protecting accordingly outside of 430.


That is how I see it but I can't tell you I found that section easy or that I am as confident about it as I normally am. 

As a side note I don't think 240.6 would be a factor, you could use non standard breakers so in my example above of 68 amps you could use 65 amp fuses or a 68 amp breaker if you could find one. :jester:


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## Bulldog1 (Oct 21, 2011)

BBQ said:


> That is how I see it but I can't tell you I found that section easy or that I am as confident about it as I normally am.
> 
> As a side note I don't think 240.6 would be a factor, you could use non standard breakers so in my example above of 68 amps you could use 65 amp fuses or a 68 amp breaker if you could find one. :jester:



As far as testing purposes go NC tests are based off this...

Feeder wire size Article 430.24 / 310.16

Feeder Short Circuit Article 430.62 / 240.6

If you round up for the feeder short circuit your answer could be wrong.


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## BBQ (Nov 16, 2010)

Bulldog1 said:


> As far as testing purposes go NC tests are based off this...
> 
> Feeder wire size Article 430.24 / 310.16
> 
> ...


I am not following you.


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## pstout44 (Dec 23, 2011)

*OCPD re: your ?*

If using inverse-time breakers multiply your largest motor's FLA by 2.5, then add the FLA's of all the other motors (based on 430.247 through 430.250). If the calculation yields a standard size breaker, use it; if not, use the next one down on the list.


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## don_resqcapt19 (Jul 18, 2010)

BBQ said:


> ... I just don't have the patience to cruise the Grainger web site to get some example motor data and do all the steps.


Why would you look there? The calculations are required to be based on the information in the code book.


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## Bulldog1 (Oct 21, 2011)

BBQ said:


> I am not following you.




:001_huh:
Feeder base your wire size off 310.16
Feeder OCPD round down.


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## BBQ (Nov 16, 2010)

Bulldog1 said:


> :001_huh:
> Feeder base your wire size off 310.16
> Feeder OCPD round down.



Still not getting it.


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## erics37 (May 7, 2009)

BBQ said:


> Still not getting it.


Geez Harry I don't think he can break it down any further :laughing:


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## BBQ (Nov 16, 2010)

erics37 said:


> Geez Harry I don't think he can break it down any further :laughing:


Huh? :thumbsup:


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