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    OCPD Brain Freeze!

    Guys(Gals),
    I'm having "engineer's block" so to speak. We have (3) line commutated solar inverters that kick out 12kW AC each. Each inverter is rated single phase 240V, so there's one phase and a neutral (and EGC, not shown on image). Inverter A lands on phase A, B on B, and C on C inside a panelboard, so as to create a three phase system.
    My struggle is sizing the inverter OCPD in the panelboard. Logic tells me I have true 12kW per phase, so 12,000/240=50A*1.25=62.5A=70A 3-pole breaker. My mind is drifting towards a calculation that involves 416V (240*sqrt(3)=416), but it doesn't feel right.
    I've attached a little sketch detailing. Thanks ahead of time.
    Attached Files

    #2
    I'm guessing this is not a US based installation due to the 240V Phase to neutral reference.

    The 70A breakers seems correct.
    Ron

    Comment


      #3
      These look like separate single phase inverters, are they slaved together?
      Our comedian shamelessly joked about a blackout. Talk about dark humor.

      Comment


        #4
        Originally posted by ron View Post
        I'm guessing this is not a US based installation due to the 240V Phase to neutral reference.

        The 70A breakers seems correct.
        This is a US installation.

        Comment


          #5
          Originally posted by mbrooke View Post
          These look like separate single phase inverters, are they slaved together?
          Slaved together in that they are "line commutated". Simply means each inverter syncs it's voltage and phase automatically with the grid.

          Comment


            #6
            Originally posted by fossil112 View Post
            .... Logic tells me I have true 12kW per phase, so 12,000/240=50A*1.25=62.5A=70A 3-pole breaker.
            [COLOR=#ff0000]Yes, works fine[/COLOR]

            My mind is drifting towards a calculation that involves 416V (240*sqrt(3)=416), but it doesn't feel right.
            [COLOR=#ff0000]Also should feel fine[/COLOR]
            [COLOR=#ff0000](36kw/(416))/sqrt(3) = 50A

            Do the algebra, it's the same
            [/COLOR]
            ice
            Without data you’re just another person with an opinion – Edwards Deming

            Comment


              #7
              Originally posted by iceworm View Post
              [COLOR=#ff0000](36kw/(416))/sqrt(3) = 50A

              Do the algebra, it's the same[/COLOR]
              I guess that is a bit cryptic.

              [COLOR=#ffffff]((3 x 12kw)/(240 x sqrt(3)))/sqrt(3) = 12/240 = 50
              [/COLOR]But if I tell you, you will

              ice
              Without data you’re just another person with an opinion – Edwards Deming

              Comment


                #8
                @ice

                It makes perfect sense -- believe me, I've already
                Silly me didn't take 36kW, I used 12kW...knew something was missing!

                Comment


                  #9
                  Originally posted by fossil112 View Post
                  Slaved together in that they are "line commutated". Simply means each inverter syncs it's voltage and phase automatically with the grid.
                  Got it. For a moment I was thinking as if it were stand alone hence my confusion.
                  Our comedian shamelessly joked about a blackout. Talk about dark humor.

                  Comment


                    #10
                    Originally posted by fossil112 View Post
                    This is a US installation.
                    [COLOR=RoyalBlue]I will have achieved my life's goal if I die with a smile on my face.[/COLOR]

                    Comment


                      #11
                      If you have a 240Y/132 (non-standard) system, then each phase to neutral will be 240V.
                      This allows you to use some models of 240V inverters without having to wire them phase to phase, which might be undesirable for some reason.

                      Other inverters are designed or configured for 208 volt output and wired line to line in a standard 208Y/120 system.

                      An inverter with a 416 volt output would be harder to find and if transformerless would require a higher DC string voltage.

                      The 240 volt configuration of a GTI is more commonly used with a 120/240 three wire single phase output, and not all such inverters are rated for 240 above ground/neutral in the same way that 120/240 breakers cannot be used on a 240 to ground environment.

                      Comment


                        #12
                        Originally posted by GoldDigger View Post
                        If you have a 240Y/132 (non-standard) system, then each phase to neutral will be 240V.
                        This allows you to use some models of 240V inverters without having to wire them phase to phase, which might be undesirable for some reason.

                        ...
                        Last edited by Smart $; 03-05-14, 01:07 AM.
                        [COLOR=RoyalBlue]I will have achieved my life's goal if I die with a smile on my face.[/COLOR]

                        Comment


                          #13
                          Sorry, brain slip.
                          416Y/240.
                          And since the inverters are single phase units you could connect them either to three single phase breakers or to the four wires of a three phase wye MWBC.
                          One potential benefit of the MWBC connection would indeed be the elimination of voltage drop in the neutral when all three inverters are working.

                          Tapatalk!
                          Last edited by GoldDigger; 03-05-14, 02:01 AM.

                          Comment


                            #14
                            Originally posted by GoldDigger View Post
                            Sorry, brain slip.
                            416Y/240.
                            And since the inverters are single phase units you could connect them either to three single phase breakers or to the four wires of a three phase wye MWBC.
                            One potential benefit of the MWBC connection would indeed be the elimination of voltage drop in the neutral when all three inverters are working.
                            [COLOR=RoyalBlue]I will have achieved my life's goal if I die with a smile on my face.[/COLOR]

                            Comment


                              #15
                              Connecting to a set of 240 V inverters maybe.

                              I am not sure from what the OP showed whether this is a service voltage or a transformer secondary just for this purpose.
                              It could be that the OP has it wrong too.

                              Comment

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