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Thread: Simple motor conductor sizing question

  1. #1
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    Simple motor conductor sizing question

    Prepping for my master's exam and not sure the given answer on this question is correct.

    What size THHN conductor is required for a 5hp 230v, single phase motor?

    I am given an answer of #10

    My thinking is:

    Using table 430.248 the motor full load current is 28A

    430.22(A) tells me 125% of motor full load current rating = 35A

    Number 10 THHN is good for 35A at the 75 degree terminal rating but....you technically can only put #10 on a 30A breaker....so therefore would one not have to upsize the wire to #8 in order to protect it at 35A?

    They did not ask about overcorrect protection but it would be stupid not to consider it if it affects your wire size.

    Thoughts?

  2. #2
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    The 30 amp limit in Article 240 for #10 conductors does not apply to motor circuits.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  3. #3
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    Quote Originally Posted by Mustwin351 View Post
    Prepping for my master's exam and not sure the given answer on this question is correct.

    What size THHN conductor is required for a 5hp 230v, single phase motor?

    I am given an answer of #10

    My thinking is:

    Using table 430.248 the motor full load current is 28A

    430.22(A) tells me 125% of motor full load current rating = 35A

    Number 10 THHN is good for 35A at the 75 degree terminal rating but....you technically can only put #10 on a 30A breaker....so therefore would one not have to upsize the wire to #8 in order to protect it at 35A?

    They did not ask about overcorrect protection but it would be stupid not to consider it if it affects your wire size.

    Thoughts?
    Motor OCPD does not fall under 240.4(D), it is covered by 240.4(G), which sends you straight to 430.
    "Electricity is really just organized lightning." George Carlin


    Derek

  4. #4
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    Quote Originally Posted by jumper View Post
    Motor OCPD does not fall under 240.4(D), it is covered by 240.4(G), which sends you straight to 430.
    thank you guys

  5. #5
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    Quote Originally Posted by Mustwin351 View Post
    thank you guys
    Applies to 440 installs also. MCA could be 22A and MOCD is 40A. #12 MC or piped THHN and a 40A breaker.

    Breaker is for SC/GF protection, Heaters or internal thermal protection for OL.
    "Electricity is really just organized lightning." George Carlin


    Derek

  6. #6
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    Quote Originally Posted by Mustwin351 View Post
    Prepping for my master's exam and not sure the given answer on this question is correct.

    What size THHN conductor is required for a 5hp 230v, single phase motor?

    I am given an answer of #10

    My thinking is:

    Using table 430.248 the motor full load current is 28A

    430.22(A) tells me 125% of motor full load current rating = 35A

    Number 10 THHN is good for 35A at the 75 degree terminal rating but....you technically can only put #10 on a 30A breaker....so therefore would one not have to upsize the wire to #8 in order to protect it at 35A?

    They did not ask about overcorrect protection but it would be stupid not to consider it if it affects your wire size.

    Thoughts?
    Others already answered about small conductor rule not applying to motor circuits, as well as some other specific circuits it doesn't apply to either. But motor circuits do have two different overcurrent protection schemes that must apply to them, overload protection (which is typically slower reacting protection) and short circuit and ground fault protection (which is quick reacting protection). Most of the time two different devices are used to provide each type of protection. The short circuit/ground fault protetion can be up to 80 amps for this 5 hp motor circuit, before it has to fail to start the motor - in which case it possibly can still be increased.

    I typically put a 5 HP single phase motor on at least a 50 but usually a 60 amp breaker and almost never have starting issues with that level of protection. Put it on a 30 and it may not hold long enough to start the motor.
    I live for today, I'm just a day behind.

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