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    Xfrmr Feeds

    If you have ATS feeding xfmr that feeds panel what are the primary and secondary conductors sized to? I have transformer schedule with prim/sec wire sizes that don’t match the ATS schedule. Thanks.

    The ATS is 200A at 480v and call for it feeding a 400a 120/208 panel. The ATS schedule calls for 3#1 aluminum. In between them is a 75kva xfrmr. The 75kva transformer schedule primary(480v) calls for 3#2/0 & #4G aluminum and 3#500 with neutral secondary(120/208).
    Last edited by mstrlucky74; 10-09-19, 10:49 AM.

    #2
    Sounds like you needed a 150 KVA transformer.. but to make it all work with 75KVA transformer
    75KVA: 90.2A potential on Primary, size the breaker in 200A ATS to 125A (3) #1/0+(1)#6GND--> Secondary 208.2A xfmr potential, main breaker in 400A 120/208 panel = 250A, (4) 250 kcmil + (1) #4G

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      #3
      Originally posted by mstrlucky74 View Post
      If you have ATS feeding xfmr that feeds panel what are the primary and secondary conductors sized to? I have transformer schedule with prim/sec wire sizes that don’t match the ATS schedule. Thanks.

      The ATS is 200A at 480v and call for it feeding a 400a 120/208 panel. The ATS schedule calls for 3#1 aluminum. In between them is a 75kva xfrmr. The 75kva transformer schedule primary(480v) calls for 3#2/0 & #4G aluminum and 3#500 with neutral secondary(120/208).
      What size OCPD is in the panel?
      Rob

      Moderator

      All responses based on the 2017 NEC unless otherwise noted

      Comment


        #4
        Originally posted by infinity View Post

        What size OCPD is in the panel?
        secondary of xfrmr feeding a 400a breaker panel.

        Comment


          #5
          Originally posted by mstrlucky74 View Post

          secondary of xfrmr feeding a 400a breaker panel.
          So this panel has a 400 amp main with no other OCPD on the secondary of the transformer ahead of the panel? If this is the case then the 500 kcmil is too small. You need a 400 amp conductor which for one set would be 600 kcmil.
          Rob

          Moderator

          All responses based on the 2017 NEC unless otherwise noted

          Comment


            #6
            Originally posted by infinity View Post

            So this panel has a 400 amp main with no other OCPD on the secondary of the transformer ahead of the panel? If this is the case then the 500 kcmil is too small. You need a 400 amp conductor which for one set would be 600 kcmil.
            Correct. Actually riser says the panel is 300A but schedule says 400A. So you size the xfrmr secondary conductors based on the OCPD the xfrmr is feeding?

            Comment


              #7
              Originally posted by mstrlucky74 View Post

              Correct. Actually riser says the panel is 300A but schedule says 400A. So you size the xfrmr secondary conductors based on the OCPD the xfrmr is feeding?
              The secondary conductors require protection and the OCPD cannot be larger than the conductor ampacity. If you have 500's you cannot feed a 400 amp OCPD.
              Rob

              Moderator

              All responses based on the 2017 NEC unless otherwise noted

              Comment

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