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Thread: Panel Size

  1. #1

    Panel Size

    There is an existing single phase 480/240 15KVA Transformer. At this time it is primary only protected. All that was being fed by it was two 120 volt receptacles for two small heater motors. Is there any issue installing a 125 Amp panel? Or should it be no larger than a 50 amp panel? The transformer secondary is 62.5 Amperes. At this time the load will only be a couple amperes. But no telling what might be added later as this is in an industrial plant.

  2. #2
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    Quote Originally Posted by tpd View Post
    There is an existing single phase 480/240 15KVA Transformer. At this time it is primary only protected. All that was being fed by it was two 120 volt receptacles for two small heater motors. Is there any issue installing a 125 Amp panel? Or should it be no larger than a 50 amp panel? The transformer secondary is 62.5 Amperes. At this time the load will only be a couple amperes. But no telling what might be added later as this is in an industrial plant.
    How are the secondary conductors being protected now?

  3. #3
    There is a small disconnect that the secondary conductors go to just below the transformer that is mounted on the wall. So the protection for these conductors is the 30 amp breaker in the MCC feeding the primary side. This disconnect will be removed.

  4. #4
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    Quote Originally Posted by tpd View Post
    There is a small disconnect that the secondary conductors go to just below the transformer that is mounted on the wall. So the protection for these conductors is the 30 amp breaker in the MCC feeding the primary side. This disconnect will be removed.
    Is that existing small disconnect fused?

    You don't have to limit the amperage rating of the panel you're intending to install to the output amperage of the X-fmr as long as it's at or above the output.
    You could install a 400 amp rated panelboard on the secondary side of that transformer as long as the secondary conductors from the transformer are protected in a manner that they should be.


    JAP>

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    Quote Originally Posted by tpd View Post
    There is an existing single phase 480/240 15KVA Transformer. At this time it is primary only protected. All that was being fed by it was two 120 volt receptacles for two small heater motors. Is there any issue installing a 125 Amp panel? Or should it be no larger than a 50 amp panel? The transformer secondary is 62.5 Amperes. At this time the load will only be a couple amperes. But no telling what might be added later as this is in an industrial plant.
    Look at 450.3(B) Table 450.3(B)

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    Quote Originally Posted by tpd View Post
    There is a small disconnect that the secondary conductors go to just below the transformer that is mounted on the wall. So the protection for these conductors is the 30 amp breaker in the MCC feeding the primary side. This disconnect will be removed.
    The secondary conductors need to be protected in accordance with 240.21(C).

  7. #7
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    Quote Originally Posted by tpd View Post
    There is a small disconnect that the secondary conductors go to just below the transformer that is mounted on the wall. So the protection for these conductors is the 30 amp breaker in the MCC feeding the primary side. This disconnect will be removed.
    Usually we cant protect the secondary conductors with the primary OCPD, but this is one of the cases when you can. See 240.21(C)(1).

    Edit: Your OP was about "panel size" so for completeness, see 408.36(B) exception.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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    Quote Originally Posted by electrofelon View Post
    Usually we cant protect the secondary conductors with the primary OCPD, but this is one of the cases when you can. See 240.21(C)(1).

    Edit: Your OP was about "panel size" so for completeness, see 408.36(B) exception.

    He mentioned both 120v and 240v, so I don't think 240.21(C)(1) applies.

  9. #9
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    Quote Originally Posted by packersparky View Post
    He mentioned both 120v and 240v, so I don't think 240.21(C)(1) applies.
    ok yup, missed that.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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