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Thread: Transformer 480 3phase to 120/208 3 phase

  1. #1
    Join Date
    Jul 2009
    Posts
    24

    Transformer 480 3phase to 120/208 3 phase

    have a 480v 3 phase to 120/208 3 phase 30KVA transformer. tranformer to feed a 120/208 100 amp panel with 15 amp load. Engineer wants to feed the transformer with a 3/90 amp breaker which is 250% using NEC 450.3(B). my concern is the note #3 which states the transformer to have a thermal overload protection and arranged to interrupt the primary current. the transformer not equiped with overload protection or a way to interrupt the incoming primary current.so if the transformer does not have these do we use the 250% or do we use 125% for the breaker size

  2. #2
    Join Date
    Oct 2011
    Location
    Collegeville, PA, USA
    Posts
    1
    Note 3 is used if you want to protect the transformer at greater than 250%, namely 600 or 400% depending on transformer impedence. Using a primary rating of 250% permits the transformer inrush current that occurs anytime power is turned off then back on to pass the breaker without tripping it (power outage). If the primary is protected at 250% the secondary should also be protected at 125% by either a single device or a combination of upto 6 devices that total no more than 125%.

  3. #3
    Join Date
    Feb 2003
    Location
    Seattle, WA
    Posts
    17,138
    Take a closer look at the note. We can use up to 250% without further considerations. But if there is thermal overload protection, we can go as high as 400% or even 600%. That said, I always go with 125% on both primary and secondary.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  4. #4
    Join Date
    Feb 2003
    Location
    Seattle, WA
    Posts
    17,138
    I see that I was typing too slowly again, and was beaten to the draw.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

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