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Thread: Not More than 600V: Transformer Primary Conductors

  1. #11
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    Okay, let's try this again with an example. 500kVA, 480:208/120V. Suppose I decide to set the primary OCPD to be 1000A.

    What is the code minimum feeder size (THWN in conduit) to the primary side of the transformer?

  2. #12
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    Quote Originally Posted by publicgood View Post
    Okay, let's try this again with an example. 500kVA, 480:208/120V. Suppose I decide to set the primary OCPD to be 1000A.

    What is the code minimum feeder size (THWN in conduit) to the primary side of the transformer?
    Are you going to have primary and secondary protection?

    JAP>

  3. #13
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    Quote Originally Posted by jap View Post
    Are you going to have primary and secondary protection?

    JAP>
    Yes - both primary and secondary protection. Required due to wye connection on the secondary.

  4. #14
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    Quote Originally Posted by publicgood View Post
    Yes - both primary and secondary protection. Required due to wye connection on the secondary.
    You can protect the transformer primary only, if you wanted. Secondary conductor protection is a separate issue.

  5. #15
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    Quote Originally Posted by publicgood View Post
    Okay, let's try this again with an example. 500kVA, 480:208/120V. Suppose I decide to set the primary OCPD to be 1000A.

    What is the code minimum feeder size (THWN in conduit) to the primary side of the transformer?
    3 sets of 400 mcm.

  6. #16
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    Quote Originally Posted by publicgood View Post
    Okay, let's try this again with an example. 500kVA, 480:208/120V. Suppose I decide to set the primary OCPD to be 1000A.

    What is the code minimum feeder size (THWN in conduit) to the primary side of the transformer?
    It sounds like you're asking if the 1000 amp OCPD with 1000 amp conductors and the OCPD is not large enough to compensate for the inrush current can you use a larger OCPD with the same 1000 amp conductors, is this accurate?
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  7. #17
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    Quote Originally Posted by infinity View Post
    It sounds like you're asking if the 1000 amp OCPD with 1000 amp conductors and the OCPD is not large enough to compensate for the inrush current can you use a larger OCPD with the same 1000 amp conductors, is this accurate?
    I wanted to size the conductors at 125% all the time no matter the primary OCPD...like is done for motors. I’m hearing this can’t be done by code. Even though you only upsized the primary OCPD for inrush, the conductors also need to be upsized to stay protected.

    It is too bad above 600V tells us it is okay to do this in 215.

  8. #18
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    Quote Originally Posted by publicgood View Post
    I wanted to size the conductors at 125% all the time no matter the primary OCPD...like is done for motors. I’m hearing this can’t be done by code. Even though you only upsized the primary OCPD for inrush, the conductors also need to be upsized to stay protected.

    It is too bad above 600V tells us it is okay to do this in 215.
    215 doesn't say that for above 600V. The conductor must still be protected. Of course the rules for protecting conductors above 600V are different.

  9. #19
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    Quote Originally Posted by jap View Post
    240.21 (1) (2) (3) (4) and (5) are rules for Feeder "Taps".

    What does this have to do with sizing an OCPD for a feeder to a transformer ?


    JAP>
    Well if he has overcurrent protection higher then the conductor ampacity then he must have a "feeder tap".

    If not allowed from the transformer's perspective then it is pointless to even have 240.21(B)(3) because it only applies to tap conductors supplying a transformer. Overcurrent protection on secondary side of transformer is a part of this rule though.

  10. #20
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    Quote Originally Posted by kwired View Post
    Well if he has overcurrent protection higher then the conductor ampacity then he must have a "feeder tap".

    If not allowed from the transformer's perspective then it is pointless to even have 240.21(B)(3) because it only applies to tap conductors supplying a transformer. Overcurrent protection on secondary side of transformer is a part of this rule though.
    Well that's true.

    JAP>

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