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Transformer Primary Conductors Calculation

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    #16
    Originally posted by TheElectrician View Post
    Yes, initially we didnt have a primary protection for the transformer, then we added it later. But on the same topic, Do I have to have a primary protection for the transformer like a service disconnect designed using the table 430.30 even if I connect the primary of the transformer directly to 4kV line of the utility via manhole?
    Well I am still not clear where the service point and service disconnect are, and it's hard to establish Much of anything without knowing that. You can't connect a customer transformer directly to the utility. A transformer isn't listed in 230 as something that can be on the line side of the service disconnect.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

    Comment


      #17
      Originally posted by electrofelon View Post
      No, I mean the service disconnect, but the service disconnect and the primary protection could be the same
      Since the power is being generated on site, where does the service disconnect get placed?

      Comment


        #18
        Originally posted by oldsparky52 View Post
        Since the power is being generated on site, where does the service disconnect get placed?
        The requirements are not any different for a "generation only" electrical service.
        Ethan Brush - East West Electric. NY, WA. MA

        "You can't generalize"

        Comment


          #19
          Originally posted by TheElectrician View Post
          The 4kV is directly connected to the grid which is the service voltage from the POCO, this is an underground run. Hope this helps. Please check attached
          [ATTACH=CONFIG]22982[/ATTACH]
          Seems odd that there is no meter or 4kV disconnect shown on the 4kV side of the transformer.

          How do they get reimbursed for the generation or disconnect to maintain the transformer and balance of system?

          If the utility wants the wire sized per their standards, we are just guessing how they come up with the size they do because they do not follow the NEC.
          Ron

          Comment


            #20
            Originally posted by TheElectrician View Post
            Hey Guys,

            We have a Solar system installed with the following specs:
            - Voltage: 480 V
            - Current: 1080 A

            This is fed into the secondary of the 1000 kVA transformer with 4kV on the primary whose spec. is:
            kVA= (1.732 x 480 x 1080)/1000 = 897.895 kVA (1000 kVA is used)

            From this the primary current will be:
            I = 897.995*1000/1.732*4000 = 129.6 A

            The POCO asked us to use a 2/0 wire for the primary, but I wanted to verify this so, Can someone please provide a step by step process to calculate the conductor size for the primary according to NEC?

            Thanks in Advance
            So assuming there is a 4kv service disconnect somewhere, note that 215.2(B) requires the transformer primary conductors to have an ampacity of at least the nameplate rating of the transformer, so you need at least 144 amps of conductor. The size if your 450.3(A) protection nay come into play too.... You also need the extra 25% on the 130 for continuous load, so your 2/0 looks a hair small, unless it is mv-105.

            Edit: just want to say that I am not very experienced in mv design, so I could well be missing something or have something wrong. There always seems to be some weird things with MV that we are not used to in the LV world.
            Ethan Brush - East West Electric. NY, WA. MA

            "You can't generalize"

            Comment


              #21
              Originally posted by electrofelon View Post
              Well I am still not clear where the service point and service disconnect are, and it's hard to establish Much of anything without knowing that. You can't connect a customer transformer directly to the utility. A transformer isn't listed in 230 as something that can be on the line side of the service disconnect.
              Originally posted by oldsparky52 View Post
              Since the power is being generated on site, where does the service disconnect get placed?
              Originally posted by ron View Post
              Seems odd that there is no meter or 4kV disconnect shown on the 4kV side of the transformer.

              How do they get reimbursed for the generation or disconnect to maintain the transformer and balance of system?

              If the utility wants the wire sized per their standards, we are just guessing how they come up with the size they do because they do not follow the NEC.
              I would like to clarify here that initially we did not have a disconnect on the primary side (4kV side) of the transformer. But we realized we needed a primary protection according to NEC 450.30 and the one line was later updated. We also have a main disconnect for the solar system (89L) on the secondary side (480V side) of the transformer. So the setup is basically Inverters -> 52IT -> Meter -> 89L -> Transformer -> Utility

              Comment


                #22
                Originally posted by TheElectrician View Post
                I would like to clarify here that initially we did not have a disconnect on the primary side (4kV side) of the transformer. But we realized [color=red]we needed a primary protection according to NEC 450.30 [/color] and the one line was later updated. We also have a main disconnect for the solar system (89L) on the secondary side (480V side) of the transformer. So the setup is basically Inverters -> 52IT -> Meter -> 89L -> Transformer -> Utility
                Think you meant 450.3...also add to the red "I also realized I needed a service disconnect
                Ethan Brush - East West Electric. NY, WA. MA

                "You can't generalize"

                Comment


                  #23
                  Originally posted by electrofelon View Post
                  Think you meant 450.3...also add to the red "I also realized I needed a service disconnect
                  Yeah, that is right! Thank you!

                  Comment


                    #24
                    Originally posted by electrofelon View Post
                    So assuming there is a 4kv service disconnect somewhere, note that 215.2(B) requires the transformer primary conductors to have an ampacity of at least the nameplate rating of the transformer, so you need at least 144 amps of conductor. The size if your 450.3(A) protection nay come into play too.... You also need the extra 25% on the 130 for continuous load, so your 2/0 looks a hair small, unless it is mv-105.

                    Edit: just want to say that I am not very experienced in mv design, so I could well be missing something or have something wrong. There always seems to be some weird things with MV that we are not used to in the LV world.
                    Yes it is MV-105. Yeah that is true, MV is so weird lol!

                    Comment


                      #25
                      Originally posted by electrofelon View Post
                      [COLOR=#ff0000]You also need the extra 25%[/COLOR] on the 130 for continuous load, so your 2/0 looks a hair small, unless it is mv-105.
                      I don't see any requirement for the above.

                      You would need to use the 90deg ampacity unless the terminations are identified for the higher temperature (110.40.) Much the same way as the 75/90 deg rating for LV.
                      Last edited by david luchini; 05-28-19, 01:18 PM.

                      Comment


                        #26
                        Originally posted by david luchini View Post
                        I don't see any requirement for the above.
                        So there is no continuous load adder for feeders over 1000 volts?
                        Ethan Brush - East West Electric. NY, WA. MA

                        "You can't generalize"

                        Comment


                          #27
                          Originally posted by electrofelon View Post
                          So there is no continuous load adder for feeders over 1000 volts?
                          I don't see it called for in the Code...unless I am missing it somewhere.

                          Comment


                            #28
                            Originally posted by electrofelon View Post
                            So there is no continuous load adder for feeders over 1000 volts?
                            Please refer to IEEE Std 885 for standard power cable ampacity tables.

                            Comment


                              #29
                              Originally posted by david luchini View Post
                              I don't see it called for in the Code...unless I am missing it somewhere.
                              I didn't look but I'm sure you are correct. Interesting!
                              Ethan Brush - East West Electric. NY, WA. MA

                              "You can't generalize"

                              Comment

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