multiple motor feeder OCPD size

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bnz

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How do you size a feeder suppling several motors when you only have the MCA and MOCP values?
 
bnz said:
How do you size a feeder suppling several motors when you only have the MCA and MOCP values?
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Find a wire size that meets the MCA and OCP that meets the MOCP? Or is your question not that simple?

What are you feeding?
 
Cow said:
Find a wire size that meets the MCA and OCP that meets the MOCP? Or is your question not that simple?

What are you feeding?
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He wants to run several units off of one feeder and he only has the MCA and MOCPD info.
 
I said motors to keep the question simple but in actuality it's (3) Roof Top Air Conditioners and (2) Make-Up Air Units.
 
bnz said:
I said motors to keep the question simple but in actuality it's (3) Roof Top Air Conditioners and (2) Make-Up Air Units.
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I figured that since motors would be required to have FLA info on the nameplate.
 
You size feedres by the motor listing FLA in the NEC,,,,,so it doesn't matter what info you have on those motors.....you need to go by the ned list anyway,,,,,,and it states you size 125% of the LARGEST motor and the 100% after that,,,,,all you need is voltage and horspower ratings to figure this.
 
mcclary's electrical said:
You size feedres by the motor listing FLA in the NEC,,,,,so it doesn't matter what info you have on those motors.....you need to go by the ned list anyway,,,,,,and it states you size 125% of the LARGEST motor and the 100% after that,,,,,all you need is voltage and horspower ratings to figure this.
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agree..see 430.24 for feeders and 430.53 for SCGF ptotection.
 
Only problem for feeders is I don't think we have explicit permission to upsize the overcurrent device like we do in a branch circuit. That could be a problem if you have a huge hard starting motor along with a few small ones. The little ones won't up the ampacity requirement much, and the feeder OCPD may not be large enough. So you may need to oversize the feeder conductors if the feeder OCPD trips on startup.
 
mcclary's electrical said:
You size feedres by the motor listing FLA in the NEC,,,,,so it doesn't matter what info you have on those motors.....you need to go by the ned list anyway,,,,,,and it states you size 125% of the LARGEST motor and the 100% after that,,,,,all you need is voltage and horspower ratings to figure this.
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How can you do this when he said that he only has the MCA and MOCPD sizes?
 
infinity said:
How can you do this when he said that he only has the MCA and MOCPD sizes?
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might be a bit risky, but from 440.32 can one not deduce that the MCA is 125% of the actual current and work backward from there ?
 
augie47 said:
might be a bit risky, but from 440.32 can one not deduce that the MCA is 125% of the actual current and work backward from there ?
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The MCA contains 125% of the largest motor, usually the compressor, plus the other loads, fan, controls etc. You could take an educated guess I suppose. :)
 
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