motor feeder

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CARZEP

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My question that no instructor has been able to answer correctly is as follows:
When sizing a motor feeder that has 8 motors but only 5 of the motors will be installed and three will be installed in the future.
How do you size the feeder for the 8 motors and use only the OCPD of the largest and the sum of the FLC of the other 4 and in the future change the OCPD to protect the other three or size it for all 8 motors.
 
You could size the feeder conductors for the 8 motors and then see if the OCPD for the 5 motors provides feeder protection per 430.62(B).

If it doesn't, you could size the OCPD for the eight motors, and then size the feeder conductor to be protected by the OCPD (instead of sizing per 430.24.)

(Or you could just skip option 1 and go straight to option 2.)
 
feeders

feeders

david luchini said:
You could size the feeder conductors for the 8 motors and then see if the OCPD for the 5 motors provides feeder protection per 430.62(B).

If it doesn't, you could size the OCPD for the eight motors, and then size the feeder conductor to be protected by the OCPD (instead of sizing per 430.24.)

(Or you could just skip option 1 and go straight to option 2.)
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" OK thanks very much I will check that out. I already ran the feeder for the eight motors but used OCPD on the feeder for only five motors and later change it to handle the other 3 motors."
 
CARZEP said:
" OK thanks very much I will check that out. I already ran the feeder for the eight motors but used OCPD on the feeder for only five motors and later change it to handle the other 3 motors."
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Whats the conductor size for the 8 motors and what is the OCPD size for the 5 motors?
 
CARZEP said:
My question that no instructor has been able to answer correctly is as follows:
When sizing a motor feeder that has 8 motors but only 5 of the motors will be installed and three will be installed in the future.
How do you size the feeder for the 8 motors and use only the OCPD of the largest and the sum of the FLC of the other 4 and in the future change the OCPD to protect the other three or size it for all 8 motors.
Click to expand...


Are you running the feeder to a starter rack or MCC? Either way, each motor should have its own combination starter (magnetic only CB and overloads) that will protect each individual motor branch circuit. Therefore the feeder and overcurrent protection upstream can be based on total load of the eight.
 
Dale001289 said:
Are you running the feeder to a starter rack or MCC? Either way, each motor should have its own combination starter (magnetic only CB and overloads) that will protect each individual motor branch circuit. Therefore the feeder and overcurrent protection upstream can be based on total load of the eight.
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But the right combination of motors could (for example) only require 200 amp conductor but allow more then 200 amp feeder overcurrent protection. If only five of the eight motors are present though, that overcurrent device may not be allowed to be over the 200 amp ampacity of the conductors. That said an inspector may let that go, or if you had a start on the branch circuits (like a breaker and a motor controller already connected and ready for motor conductors) for the additional three motors maybe even takes that into consideration.
 
kwired said:
But the right combination of motors could (for example) only require 200 amp conductor but allow more then 200 amp feeder overcurrent protection. If only five of the eight motors are present though, that overcurrent device may not be allowed to be over the 200 amp ampacity of the conductors. That said an inspector may let that go, or if you had a start on the branch circuits (like a breaker and a motor controller already connected and ready for motor conductors) for the additional three motors maybe even takes that into consideration.
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I would suggest the feeder CB have an adjustable trip - say 400AF w/200AT. It could be set higher later as long as feeder was sized for all 8 motors.


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Dale001289 said:
I would suggest the feeder CB have an adjustable trip - say 400AF w/200AT. It could be set higher later as long as feeder was sized for all 8 motors.


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How much are those compared to my example of needing 200 amp conductor but maybe 250 amp breaker and just go ahead and use 250 amp conductor instead of 200 amp conductor?

Long run? maybe more worth it then for short run.
 
kwired said:
How much are those compared to my example of needing 200 amp conductor but maybe 250 amp breaker and just go ahead and use 250 amp conductor instead of 200 amp conductor?

Long run? maybe more worth it then for short run.
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Those types of molded case CB’s are very expensive. You’d have to do a cost analysis - case by case - but voltage drop would really spike cable cost on longer runs.


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Dale001289 said:
Those types of molded case CB’s are very expensive. You’d have to do a cost analysis - case by case - but voltage drop would really spike cable cost on longer runs.


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I thought they might be. My 200 amp is just something I pulled out of a hat. We don't know if what size of motors OP is dealing with. He might be looking at only a 30-60 amp feeder conductor, might be looking at a 340 amp feeder conductor, might be looking at a 1000 amp plus feeder conductor, then length may or may not be a factor as well.
 
I'm with david in post #4. There is nothing that prohibits you from over sizing the feeder to the existing motors so size it for the 8.
Then select a breaker based on the existing motors or on the conductor size. Once motors are added you can upsize your breaker if needed.
 
augie47 said:
I'm with david in post #4. There is nothing that prohibits you from over sizing the feeder to the existing motors so size it for the 8.
Then select a breaker based on the existing motors or on the conductor size. Once motors are added you can upsize your breaker if needed.
Click to expand...
Again, we don't know what size motors are involved. If we are only talking 30-60 amp feeder conductor being involved - maybe not a big deal to change the breaker later. If we are talking about needing a 400 amp frame breaker now but a 600 amp frame breaker later - it starts to be more significant cost.
 
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