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    Transformer Taps

    Standard dry type 300KVA 480V delta to 208V wye transformer.

    Shop drawings say "Taps 6 - 2.5% 2+4- Taps"

    I would assume these are secondary taps, and that means there are six taps at -2.5%? So I could set the secondary at -2.5% nominal, or at -5% .... clear down to -15% nominal voltage (177Y/102 volts)?

    And there are two taps at +4%? That would allow +4% or +8% voltage on the secondary?

    Do those both sound correct?

    I'm also wondering why anyone would even need to drop the output voltage by 15%? It would be better to have more positive + taps to help make up for voltage drop.

    #2
    Hmmm, is that exactly how it was formatted? My first thought was 6 taps total, 2 and 4 each direction?
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

    Comment


      #3
      ...Also if I can clear this up. I have always been uncertain: What is a - tap vs a + tap? Seems like a dumb question but bear with me: does a negative tap mean "for a primary voltage that is -x% of nominal this will give rated voltage at secondary" Or is it "-x% tap will reduce secondary voltage x% for rated primary voltage" ?
      Ethan Brush - East West Electric. NY, WA. MA

      "You can't generalize"

      Comment


        #4
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          #5
          It seems kinda ambiguous, but if I had a gun to my head and had to choose, Id say after the 6 is a dash, not a negative sign, and its 6 total, 2.5% each step, 2+, 4-
          Ethan Brush - East West Electric. NY, WA. MA

          "You can't generalize"

          Comment


            #6
            Originally posted by electrofelon View Post
            It seems kinda ambiguous, but if I had a gun to my head and had to choose, Id say after the 6 is a dash, not a negative sign, and its 6 total, 2.5% each step, 2+, 4-
            Yes, not that you say that I would agree - 6 total at 2.5% each, with 2 up and 4 down. That makes more sense

            Now, does the 4- mean it lowers the secondary voltage, or its for a lower primary voltage, so it raises the secondary V?

            In other words, do I have 4 taps to raise the Voltage or just 2?

            Comment


              #7
              I think the taps would be on the primary....similar to:

              502
              491
              480
              469
              458
              442
              431

              Comment


                #8
                I have always understood the taps to be on the primary side. Thus, a tap of +2.5% would mean more windings on the primary, with no change to the number of secondary windings. The result would be a drop in the secondary voltage of 2.5%. It would help if the manufacturer would be clearer on this question.

                My interpretation matches others that have been given. But to be explicit, the tap options are:
                • +5% (result is a lower secondary voltage by 5%)
                • +2.5% (result is a lower secondary voltage by 2.5%)
                • 0% (no change in secondary voltage)
                • -2.5% (result is a higher secondary voltage by 2.5%)
                • -5% (result is a higher secondary voltage by 5%)
                • -7.5% (result is a higher secondary voltage by 7.5%)
                • -10% (result is a higher secondary voltage by 10%)
                Charles E. Beck, P.E., Seattle
                Comments based on 2017 NEC unless otherwise noted.

                Comment


                  #9
                  Originally posted by charlie b View Post
                  I have always understood the taps to be on the primary side. Thus, a tap of +2.5% would mean more windings on the primary, with no change to the number of secondary windings. The result would be a drop in the secondary voltage of 2.5%. It would help if the manufacturer would be clearer on this question.

                  My interpretation matches others that have been given. But to be explicit, the tap options are:
                  • +5% (result is a lower secondary voltage by 5%)
                  • +2.5% (result is a lower secondary voltage by 2.5%)
                  • 0% (no change in secondary voltage)
                  • -2.5% (result is a higher secondary voltage by 2.5%)
                  • -5% (result is a higher secondary voltage by 5%)
                  • -7.5% (result is a higher secondary voltage by 7.5%)
                  • -10% (result is a higher secondary voltage by 10%)
                  Sounds right. I tried googling transformer taps earlier and came up empty. But a search for "changing taps on a transformer, was more productive, and I found this;

                  https://www.hammondpowersolutions.co...how-taps-work/

                  It seems to agree with you and David. +2.5% means more windings on the primary, and a lower secondary voltage.

                  Comment


                    #10
                    Good to get that squared away. So we can just think of it as the percentage referring to a change in the turns ratio. + Means increased turns ratio, - means less.
                    Ethan Brush - East West Electric. NY, WA. MA

                    "You can't generalize"

                    Comment


                      #11
                      Originally posted by steve66 View Post
                      Standard dry type 300KVA 480V delta to 208V wye transformer.

                      Shop drawings say "Taps 6 - 2.5% 2+4- Taps"

                      I would assume these are secondary taps, and that means there are six taps at -2.5%? So I could set the secondary at -2.5% nominal, or at -5% .... clear down to -15% nominal voltage (177Y/102 volts)?

                      And there are two taps at +4%? That would allow +4% or +8% voltage on the secondary?

                      Do those both sound correct?

                      I'm also wondering why anyone would even need to drop the output voltage by 15%? It would be better to have more positive + taps to help make up for voltage drop.
                      I guess tap 4 is your nominal voltage tap (480/208V). Your unit's taps 5, 6 as your + taps while tap nos. 3, 2, and 1 is used as the unit's -taps! All taps are in the primary. There will be more -taps to boost the secondary voltage.

                      Comment


                        #12
                        Originally posted by steve66 View Post
                        Standard dry type 300KVA 480V delta to 208V wye transformer.

                        Shop drawings say "Taps 6 - 2.5% 2+4- Taps"

                        I would assume these are secondary taps, and that means there are six taps at -2.5%? So I could set the secondary at -2.5% nominal, or at -5% .... clear down to -15% nominal voltage (177Y/102 volts)?

                        And there are two taps at +4%? That would allow +4% or +8% voltage on the secondary?

                        Do those both sound correct?

                        I'm also wondering why anyone would even need to drop the output voltage by 15%? It would be better to have more positive + taps to help make up for voltage drop.
                        Apart from special applications I've only come across taps on the HV side, usually 11kV in our case.

                        Comment


                          #13
                          Originally posted by topgone View Post
                          I guess tap 4 is your nominal voltage tap (480/208V). Your unit's taps 5, 6 as your + taps while tap nos. 3, 2, and 1 is used as the unit's -taps! All taps are in the primary. There will be more -taps to boost the secondary voltage.
                          Tap "5" would be the nominal voltage.

                          1,2,3 & 4 would be the "-" taps and 6 & 7 would be the "+" taps

                          There are 4 below nominal and 2 above nominal.

                          Comment


                            #14
                            Originally posted by topgone View Post
                            I guess tap 4 is your nominal voltage tap (480/208V). Your unit's taps 5, 6 as your + taps while tap nos. 3, 2, and 1 is used as the unit's -taps! All taps are in the primary. There will be more -taps to boost the secondary voltage.
                            I'm not sure, but I think the transformer nameplate will explain that in detail. Here is the nameplate from a different xformer that has 7 taps;

                            Click image for larger version

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                            Looking at a couple of different nameplates, the #1 tap seems to usually be the most windings on the primary side (lowest output voltage).


                            Originally posted by Besoeker3 View Post
                            Apart from special applications I've only come across taps on the HV side, usually 11kV in our case.
                            Yes, from the few nameplates I've looked at, it looks like taps are usually (or maybe always) on the primary side.

                            Comment


                              #15
                              Originally posted by steve66 View Post
                              I'm not sure, but I think the transformer nameplate will explain that in detail. Here is the nameplate from a different xformer that has 7 taps;

                              [ATTACH=CONFIG]23322[/ATTACH]

                              Looking at a couple of different nameplates, the #1 tap seems to usually be the most windings on the primary side (lowest output voltage).
                              Looks to me like #1 is highest secondary voltage, lower overall turns ratio, yes?
                              Ethan Brush - East West Electric. NY, WA. MA

                              "You can't generalize"

                              Comment

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