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Thread: Sizing Feeders To Transformer.

  1. #1
    Join Date
    Dec 2009
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    Sizing Feeders To Transformer.

    I have a warehouse with the service voltage of 277v/480v. I want to install a 200A 120v/208v sub panel to accommodate radiant heaters. I looked at the chart and I see that a 75KVA transformer is good for 206A. I'm thinking that on the secondary side of the transformer I can pull 3/0 wire to the new sub panel. I'm not sure about sizing on the primary side.

    What size circuit breaker and wiring do I install in the stand up section to feed the disconnect? What size fuses in the disconnect? And what size wiring to the primary side of the transformer from the disconnect.

    A nod in the proper direction will be greatly appreciated.

  2. #2
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    Feb 2003
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    This is how I handle transformers: I take the primary current (90.3 amps), multiply by 125% to get 113 amps, select the next higher standard overcurrent device (125 amps, though in this case I might pick 110 instead, since it is so close), and pick a wire with at least 113 amps of ampacity (#2). I do the same on the secondary side: Rated current (208 amps) times 125% gives you 260 amps. Select 250 amp breaker, and select 250 MCM conductors.
    Last edited by charlie b; 12-29-09 at 03:21 PM.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  3. #3
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    May 2004
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    Just to add to Charlie’s post the primary side of the xfmr is protected by the requirements of 450.3(B). Take note that if you limit the Pri OCP to 125% of the pri FLA then 450 is done. Then the secondary conductors are installed to meet the requirements of 240.21(C). This is somewhat contentious in that the general rule limits the secondary conductor length to a max of 10 feet. unless other conditions are employed per the rest of that section.

    You could run your 3/0 Cu 75 Deg C and supply a 200 A main in the panel see art 408. 36.


    Charlie

  4. #4
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    Dec 2009
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    10

    xfmr.

    Both are correct and it is not uncommon to tap the primary off a 225amp or 250amp feeder with 1/0 in raceway with the same secondary as you mention as long as the tap plus secondary are less than 25ft....NEC 240.21(B)(3) and 450.3(B).

  5. #5
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    Jul 2005
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    As noted by W-EI, you need to check two Code sections. Art 450 will help you size the protection for the transformer itself (you can follow the chart for maximum primary and secondary protection). Once you have selected your protection, check 310.16 for ampacities and 240.21(C) for conductor protection. A lot of folks get confused between the two, but Art 450 is for transformer protection and 240 for conductor protection and you must consider both.
    You mentioned a primary disconnect. You are not required to have a primary disconnect for a transformer. Your OCP device for you primary branch circuit can be your transformer disconnect and it can be located anywhere (in the same building)
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  6. #6
    Join Date
    Dec 2009
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    3

    Sizing Feeders To Transformers.

    This is my first day and I want to sincerely thank the four replies I received. This is a wonderful forum and I most definitely will be calling on your expertise from time to time.

    It's good to know that when you get stuck, there are good people out there more than happy to lend a helping hand and show you the way. The feedback is MUCH appreciated.

  7. #7
    Join Date
    Feb 2003
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    Wisconsin
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    Remember that 3ph transformers with a 3ph4w secondary and a 1ph transformer with 1ph3h secondary must have secondary protection. Should you provide secondary protection as allowed by 450 then 450 allows you to size the Pri OCPD at 250% as I understand the NEC.
    Charlie B gave a good explanation of sizing the primary cable.
    I always have looked at the secondary OCPD as providing overload protection for the transformer. Since the transformer already has overload protection then the primary OCPD protects the distribution system by taking the transformer off line should there be a transformer failure due to an internal fault, I.e. short circuit protection. What you don't want is to size the pri OCPD to small such that it nuisance trips when the transformer is energized which often occur with 45kva transformer and smaller. Even sizing a pri OCPD at 125% can lead to nuisance tripping with smaller transformers.

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