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Thread: Transformer tap

  1. #1
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    Transformer tap

    I have two sets of secondary conductors from 75kva three phase 480v to 208/120v feeding two panels. Each panel main circuit breaker is 225A. So 225x2 = 450a which is 216% of transformer secondary full load amps.

    The primary breaker is sized within 125% of full load amps.

    Customer says transformer is protected by primary breaker. No need for secondary breakers at all in terms of transformer protection. The secondary breakers provided for secondary conductor and in accordance with tap rules. The secondary two sets feeding panels demand load is less than 75kva rating now.

    The demand load of both panels is less than transformer 75kva rating at present but can exceed if in future they add load to panels to exceed that of the 75kva transformer since btoh panels have room and can add in future.

    Is the above allowed in code?
    Last edited by hhsting; 12-18-18 at 05:20 PM.

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    As described, your installation meets the code requirements of articles 240, 408, and 450.
    Just because you can, doesn't mean you should.

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    Quote Originally Posted by jim dungar View Post
    As described, your installation meets the code requirements of articles 240, 408, and 450.
    Yes but what about the part where demand load can exceed transformer capacity in future?

    Sent from my SM-G935U using Tapatalk

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    Quote Originally Posted by hhsting View Post
    Yes but what about the part where demand load can exceed transformer capacity in future?
    The NEC does not address the sizing of transformers.
    Just because you can, doesn't mean you should.

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    Quote Originally Posted by jim dungar View Post
    The NEC does not address the sizing of transformers.
    So any transformer secondary conductors and secondary side panelboard size can be big as one wants, no limit so as long as the nec 2014 articles 450, 240.21 are met?
    Nothing in code to prevent overloading the transformer.

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    Quote Originally Posted by hhsting View Post
    So any transformer secondary conductors and secondary side panelboard size can be big as one wants, no limit so as long as the nec 2014 articles 450, 240.21 are met?
    Yes.
    Just because you can, doesn't mean you should.

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    Quote Originally Posted by jim dungar View Post
    The NEC does not address the sizing of transformers.
    It does address overcurrent protection of transformers though. I don't see anything that allows 216% protection of this secondary, I do see it can be 125%, then allows next higher standard size overcurrent device.
    I live for today, I'm just a day behind.

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    Quote Originally Posted by kwired View Post
    It does address overcurrent protection of transformers though. I don't see anything that allows 216% protection of this secondary, I do see it can be 125%, then allows next higher standard size overcurrent device.
    I see that the secondary protection is allowed to be "not required."

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    OK, note 2 - where secondary protection is required.... where is it required?

    What protects a multiwire secondary should you have severe imbalance if you have primary only protection?
    I live for today, I'm just a day behind.

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    Quote Originally Posted by kwired View Post
    OK, note 2 - where secondary protection is required.... where is it required?
    Secondary protection is required when the primary protection exceeds the table value for primary protection only, such as 125% for a 480V primary and 208V secondary.
    Just because you can, doesn't mean you should.

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