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Thread: 277/480 Step dowm 120/208 425 feet

  1. #1
    Join Date
    Aug 2013
    Location
    new york
    Posts
    11

    277/480 Step dowm 120/208 425 feet

    Hey all

    Thanks in advance

    I have to re-feed a store and run new conduit over the roof .
    I have to run approx 425 feet to re-feed an existing transformer. 277/480 Step down transformer to feed to and 150 amp 120/208 panel all 3 phase
    There is and existing transformer and 120/208 panel I have only seen panel

    I am running across the roof
    My calculations are
    2 inch PVC
    4 - 1/0 conductors
    #2 ground

    Do these sizes sound correct

    I will re-feed existing transformer
    What size step down from 277/480 to feed a 120/208 150 amp panel . 75kva not sure next size down
    Not sure the next size down
    going to see this morning

  2. #2
    Join Date
    Jun 2003
    Location
    NE (9.1 miles @5.07 Degrees from Winged Horses)
    Posts
    8,952
    PVC on a roof will be fun. It will LLS by the end of the first summer and go downhill from there, even if you use all the expansion fittings required. A 2 EG is more than adequate. Ref to table 250.122


    A 75 KVA is more than adequate for a 150 amp panel. IMO a 45 could be used depending on the primary and secondary protection of the transformer along with the calculated load. You need to know the transformer size, its existing primary protection etc. first.
    Tom
    TBLO

  3. #3
    Join Date
    Dec 2007
    Location
    NE Nebraska
    Posts
    33,749
    is your run primary or secondary conductors of the transformer?

    Primary should be able to be smaller, like as little as 2 AWG (before factoring any voltage drop), maybe even less depending on calculated load and selected primary overcurrent device, it also should not need a neutral conductor in the supply.

    If you are running secondary conductors for 425 feet, you possibly need to increase conductors for voltage drop reasons - knowing calculated load is important here.

  4. #4
    Join Date
    Aug 2013
    Location
    new york
    Posts
    11
    I was unable t see the transformer today.
    I did see disconnect. It was a 200 fused.
    I didn't opened however the load side disconnect was 1-1/2 conduit.
    Assuming #1 or #2. Have to wait till Monday.
    I do know the existing store panel is 3 phase 150 main breaker.
    It was a sub shop now a small bank
    I currently will assume 75 Kva
    prob fed with smaller wire. The original run
    was shorter being underground.
    I will see if it is delta to Wye. Save some copper
    So at 425 feet 75 kva I only need 150 amps load side 120/208
    I might be able to do #1???

  5. #5
    Join Date
    Aug 2013
    Location
    new york
    Posts
    11

    Transformer Feed

    At 425 feet I am going to feed a 75 kva transformer with 1/0. Which at that distance will deliver 150 amp at 480

    ( Isn't it close to 2-1 meaning I could possibly get 300 amps 120/208 )

    My load side is 3 phase 120/208 150 amp main breaker

    What size wire can I feed the step down transformer line side... to get 120/208 load side

    I am thinking I might be able to go to #2 wire @ 480 line side.












    Quote Originally Posted by zodiack View Post
    Hey all

    Thanks in advance

    I have to re-feed a store and run new conduit over the roof .
    I have to run approx 425 feet to re-feed an existing transformer. 277/480 Step down transformer to feed to and 150 amp 120/208 panel all 3 phase
    There is and existing transformer and 120/208 panel I have only seen panel

    I am running across the roof
    My calculations are
    2 inch PVC
    4 - 1/0 conductors
    #2 ground

    Do these sizes sound correct

    I will re-feed existing transformer
    What size step down from 277/480 to feed a 120/208 150 amp panel . 75kva not sure next size down
    Not sure the next size down
    going to see this morning

  6. #6
    Join Date
    Jun 2003
    Location
    NE (9.1 miles @5.07 Degrees from Winged Horses)
    Posts
    8,952
    You need to read/study Article 450 and Tables 450.3(A) & (B). Those will tell you the max primary and secondary protection for the transformer.

    Use 240.21... to determine the location and size of overcurrent protection of the conductors.

    You need to consider both.
    Tom
    TBLO

  7. #7
    Join Date
    Jan 2016
    Location
    PA
    Posts
    4,177
    480 xfmr prim feed
    208/480 x 150 = 65 A
    use #2 (3.5% vd at 65 A), so oversized due to v drop temp derating should not impact
    125 cb (perhaps larger to allow energization of xfmr)

    150 120/208 pnl feed
    down size fuse to 150
    use 1/0 (not sure how long the run is)



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