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Thread: 225 Kva transfomer Wire Size

  1. #1
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    225 Kva transfomer Wire Size

    i have a 225 Kva transformer (delta) supplying it with 480/277 (primary) the out put will be (Secondary)240/120
    what size wire should the supply to the transformer Be ?

  2. #2
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    Look at 450.3(B) and the corresponding table. Without more information it is impossible to answer you directly. What size OCPD do you have on the primary side of the transformer?

    Pete
    Last edited by pete m.; 01-30-12 at 02:03 PM. Reason: question
    Experience is something thats gained shortly after you really needed it.

  3. #3
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    i have NO OCPD that is what i need to know what size should it be and the wire size needed

  4. #4
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    The FLI for the primary side would be 271 amps but you have decisions to make in what protection scheme you are going to use based upon the loads served by the transformer. Table 450.3(B) gives you many options.

    The primary OCPD could be set as high as 250% of the primary FLI if primary and secondary OCP are provided.

    Thats why I said it comes down to the loads being served and the NEC isn't going to design the system. The NEC only dictates the parameters of your design.

    Pete
    Experience is something thats gained shortly after you really needed it.

  5. #5
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    so its either 125% Or 250% of 271 and that is what i size the wire on ?

  6. #6
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    For protection of the transformer that is correct. Pay particular attention to the notes following table 450.3(B)

    Pete
    Experience is something thats gained shortly after you really needed it.

  7. #7
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    so 125% x 271 = 338.75
    that would be a 350 kcmil conductor
    and a 350 amp Breaker

  8. #8
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    To use the 350 kcmil conductor from a 350 amp OCPD you will have to do a load calculation. 350 kcmil is only rated at 310 amps at 75 degrees Celcius but per 240.4(B) you MAY be able to do this.

    Pete
    Experience is something thats gained shortly after you really needed it.

  9. #9
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    Quote Originally Posted by Part-Time View Post
    i have a 225 Kva transformer (delta) supplying it with 480/277 (primary) the out put will be (Secondary)240/120
    what size wire should the supply to the transformer Be ?
    It is a 240V single phase, or 240V 3 phase delta high leg?
    Lady Engineer, P.E. (From NJ)

  10. #10
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    Quote Originally Posted by Lady Engineer View Post
    It is a 240V single phase, or 240V 3 phase delta high leg?
    Oops, I did not ready this carefully. I would say at 271 primary, your wire size would be 271 X 1.25 = 338A @ 480V. I think you'll have to go with 500Kcmil @ 75 degrees C. I would not use 350KCmil even though it's close.
    Lady Engineer, P.E. (From NJ)

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