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Thread: Motor Feeder Breaker Size

  1. #11
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    Quote Originally Posted by texie View Post
    Something to consider: Max OCPD based on 430.62 only applies if your feeder conductors are based on 430.24. If you size the feeder conductors to match the feeder breaker you can use any size feeder/breaker you wish.
    Good Point. Thank You!

    However, we do want to base our feeder conductor ampacity on 430.24 --- that would be 862 Amps. Feeder conductor size to match the feeder breaker would require 1600 Amp ampacity. That is almost twice the number/size of conductors (5 x 400 kcmil vs 3 x 350 kcmil). Quite a difference in expense for a 75 ft - 100 ft run.

  2. #12
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    Quote Originally Posted by dickricker View Post
    Good Point. Thank You!

    However, we do want to base our feeder conductor ampacity on 430.24 --- that would be 862 Amps. Feeder conductor size to match the feeder breaker would require 1600 Amp ampacity. That is almost twice the number/size of conductors (5 x 400 kcmil vs 3 x 350 kcmil). Quite a difference in expense for a 75 ft - 100 ft run.
    You mentioned in the OP that both motors are soft started. Good chance you would never have any trouble tripping 1000 or 1200 amp breaker because you are limiting the starting current which is the reason you are allowed to have breaker set at a higher point than the conductor ampacity in the first place.

  3. #13
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    Quote Originally Posted by dickricker View Post
    Good Point. Thank You!

    However, we do want to base our feeder conductor ampacity on 430.24 --- that would be 862 Amps. Feeder conductor size to match the feeder breaker would require 1600 Amp ampacity. That is almost twice the number/size of conductors (5 x 400 kcmil vs 3 x 350 kcmil). Quite a difference in expense for a 75 ft - 100 ft run.
    I didn't think you would want to do his and this illustrates why the 430.24 rule can be such a help. I was just mentioning it as a clarification.

  4. #14
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    Quote Originally Posted by kwired View Post
    You mentioned in the OP that both motors are soft started. Good chance you would never have any trouble tripping 1000 or 1200 amp breaker because you are limiting the starting current which is the reason you are allowed to have breaker set at a higher point than the conductor ampacity in the first place.
    Yes. Yes. I agree 100%

    However, the 430.62 question remains. Using 430.24 feeder conductor ampacity, following 430.62, using a 1600 Amp (or 1200 Amp) motor branch circuit breaker to calculate maximum feeder circuit breaker size and coming up with 1600 Amps (or 1200 Amps), can I then reduce the size of the motor branch circuit breaker without recalculating and reducing the size of the feeder circuit breaker?

    What I would like to do is use a 1000 Amp or 1200 Amp motor branch circuit breaker and a 1400 or 1600 Amp feeder circuit breaker, and still use feeder conductor ampacity based on 430.24

    I this in line with NEC 2011?

    Or, will I need to use the same size breakers in both locations and adjust their trip points to achieve coordination?

  5. #15
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    Quote Originally Posted by texie View Post
    I didn't think you would want to do his and this illustrates why the 430.24 rule can be such a help. I was just mentioning it as a clarification.
    Yes. Thank You. The 430.24 rule can be quite a money and work saver.

  6. #16
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    Quote Originally Posted by dickricker View Post
    Yes. Yes. I agree 100%

    However, the 430.62 question remains. Using 430.24 feeder conductor ampacity, following 430.62, using a 1600 Amp (or 1200 Amp) motor branch circuit breaker to calculate maximum feeder circuit breaker size and coming up with 1600 Amps (or 1200 Amps), can I then reduce the size of the motor branch circuit breaker without recalculating and reducing the size of the feeder circuit breaker?

    What I would like to do is use a 1000 Amp or 1200 Amp motor branch circuit breaker and a 1400 or 1600 Amp feeder circuit breaker, and still use feeder conductor ampacity based on 430.24

    I this in line with NEC 2011?

    Or, will I need to use the same size breakers in both locations and adjust their trip points to achieve coordination?
    Since 240.6 lists the next standard size breaker above 1200 as 1600, you could use a 1200 branch and a 1600 mp feeder OCP device.
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  7. #17
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    Quote Originally Posted by dickricker View Post
    Yes. Yes. I agree 100%

    However, the 430.62 question remains. Using 430.24 feeder conductor ampacity, following 430.62, using a 1600 Amp (or 1200 Amp) motor branch circuit breaker to calculate maximum feeder circuit breaker size and coming up with 1600 Amps (or 1200 Amps), can I then reduce the size of the motor branch circuit breaker without recalculating and reducing the size of the feeder circuit breaker?

    What I would like to do is use a 1000 Amp or 1200 Amp motor branch circuit breaker and a 1400 or 1600 Amp feeder circuit breaker, and still use feeder conductor ampacity based on 430.24

    I this in line with NEC 2011?

    Or, will I need to use the same size breakers in both locations and adjust their trip points to achieve coordination?
    430.24 is conductor minimum ampacity and you must have conductor with ampacity no less than what comes from 430.24. Anything larger is acceptable.

    430.62 is maximum short circuit and ground fault protection.
    Anything smaller is acceptable.


    A feeder overcurrent device does not have to be higher setting than a branch circuit it supplies, even though it is typical to see that.

  8. #18
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    Feb 2013
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    Quote Originally Posted by augie47 View Post
    Since 240.6 lists the next standard size breaker above 1200 as 1600, you could use a 1200 branch and a 1600 mp feeder OCP device.
    Thank You, Augie47!

    Just to be 100% clear:

    Do you mean that I could use a 1200 Amp branch and a 1600 Amp feeder OCP device with feeder conductor ampacity following 430.24?

    Do you interpret 430.62 to mean that the maximum motor feeder breaker size is calculated using the maximum permitted branch-circuit breaker value for the largest motor, rather than the actual branch-circuit breaker value for the largest motor?

  9. #19
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    Feb 2013
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    Quote Originally Posted by kwired View Post
    A feeder overcurrent device does not have to be higher setting than a branch circuit it supplies, even though it is typical to see that.
    Good Point! Thank You!

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