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Integral Unit (Chiller w/ VFD) Input Conductor Sizing

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    Integral Unit (Chiller w/ VFD) Input Conductor Sizing

    Hello all:

    I've been an "active lurker" and infrequent poster for many years (and have attended Mike's seminars in person)

    My question deals with a VFD and motor integrated into a single chiller by a large, well-known, and reputable chiller manufacturer

    All amperage values are approximate as the actual values are irrelevant to my actual question.

    Equipment Detail:
    Motor Nameplate FLA: 392 A
    VFD Nameplate Input: 780 A

    (Note that both nameplates are mounted on the unit)

    One school of design thought is that the branch conductors led to the VFD should have an ampacity of 975A (1.25 x the VFD's "nameplate" 780) per NEC 430.122.

    However, the manufacturer insists -- after repeated queries -- that the ampacity of branch conductors need only be 490A (1.25 * 392) based on their interpretation of NEC 440.33.

    The manufacturer has also declined to "remove" the nameplate of the VFD (i.e. so have single "common" nameplate for the entire integrated chiller)

    The branch conductor design is at a bit of a quandary over which way to proceed.

    Q: What IS the "right" interpretation?

    Q: Specifically, does specific guidance by the manufacturer of an integrated unit take precedence over the individual "unintegrated" component nameplates?

    I hope that we might obtain some true "3rd party" interpretation of the NEC from this forum.

    Looking forward to any and all replies and to the discussion this question might generate.
    Few "standards" exist w/o something really bad happening first.

    #2
    Why would they have two nameplates?

    Can you post pictures of them?

    However, I would suggest they could well be right based on 440.33.

    440.33 Motor-Compressor(s) With or Without Additional
    Motor Loads. Conductors supplying one or more
    motor-compressor(s) with or without an additional load(s)
    shall have an ampacity not less than the sum of the rated load
    or branch-circuit selection current ratings, whichever
    is larger, of all the motor-compressors plus the full-load
    currents of the other motors, plus 25 percent of the highest
    motor or motor-compressor rating in the group.
    This provision says the ampacity required is based on the motor FLA.
    Bob

    Comment


      #3
      Originally posted by NoVA Comms Power View Post
      Hello all:

      I've been an "active lurker" and infrequent poster for many years (and have attended Mike's seminars in person)

      My question deals with a VFD and motor integrated into a single chiller by a large, well-known, and reputable chiller manufacturer

      All amperage values are approximate as the actual values are irrelevant to my actual question.

      Equipment Detail:
      Motor Nameplate FLA: 392 A
      VFD Nameplate Input: 780 A

      (Note that both nameplates are mounted on the unit)

      One school of design thought is that the branch conductors led to the VFD should have an ampacity of 975A (1.25 x the VFD's "nameplate" 780) per NEC 430.122.

      However, the manufacturer insists -- after repeated queries -- that the ampacity of branch conductors need only be 490A (1.25 * 392) based on their interpretation of NEC 440.33.

      The manufacturer has also declined to "remove" the nameplate of the VFD (i.e. so have single "common" nameplate for the entire integrated chiller)

      The branch conductor design is at a bit of a quandary over which way to proceed.

      Q: What IS the "right" interpretation?

      Q: Specifically, does specific guidance by the manufacturer of an integrated unit take precedence over the individual "unintegrated" component nameplates?

      I hope that we might obtain some true "3rd party" interpretation of the NEC from this forum.

      Looking forward to any and all replies and to the discussion this question might generate.
      I don't have my code with me and 440 is not something I know well. But is there something in that section that says it supersedes all other code requirements? If not, 430.22 would prevail, because it has a VFD. I would think that if they were really concerned, they would hide the VFD nameplate inside of the chiller and use an equipment nameplate on the outside that unequivocally gives you the MCA and MOCP values so that you can use those and have good cause. That's what most chiller mfrs do by the way.

      Or, if they did do that but you looked deeper into it, then you may have created your own dilemma here.
      __________________________________________________ ____________________________
      Many people are shocked when they discover I am not a good electrician...

      I'm in California, ergo I am still stuck on the 2014 NEC... We'll get around to the 2017 code in around 2021.

      Comment


        #4
        Originally posted by Jraef View Post
        I don't have my code with me and 440 is not something I know well. But is there something in that section that says it supersedes all other code requirements? If not, 430.22 would prevail, because it has a VFD. I would think that if they were really concerned, they would hide the VFD nameplate inside of the chiller and use an equipment nameplate on the outside that unequivocally gives you the MCA and MOCP values so that you can use those and have good cause. That's what most chiller mfrs do by the way.

        Or, if they did do that but you looked deeper into it, then you may have created your own dilemma here.
        440.1 Scope. The provisions of this article apply to electric
        motor-driven air-conditioning and refrigerating equipment and
        to the branch circuits and controllers for such equipment. It
        provides for the special considerations necessary for circuits
        supplying hermetic refrigerant motor-compressors and for any
        air-conditioning or refrigerating equipment that is supplied
        from a branch circuit that supplies a hermetic refrigerant
        motor-compressor.
        It appears to me that they can take advantage of 440.33, even though it seems a bit odd.
        Bob

        Comment


          #5
          Some Amplification

          Thanks to all who have replied so far:

          I suspected that this topic might generate some "active discussion"



          I'll try to get some nameplate pics posted later today.


          RE:
          "... It appears to me that they can take advantage of 440.33, even though it seems a bit odd... "

          For amplification ... in THIS particular application, the chiller uses a semi-hermetically-sealed motor and compressor assembly ... (meaning that the motor casing, while normally sealed and refrigerant-cooled while in operation, can be opened and repaired by unbolting flanges)

          ... and I'd like keep this discussion focused on this particular problem.

          (That said, depending on how this thread pans-out I might start a future thread discussing the same question as it applies to non-hermetically sealed chillers.)

          Few "standards" exist w/o something really bad happening first.

          Comment


            #6
            Other Articles

            Re: "... But is there something in that section that says it supersedes all other code requirements? If not, 430.22 would prevail, because it has a VFD ... "

            NEC 440.3 (Other Articles)
            (A) These provisions are in addition to, or amendatory of, the provisions of Article 430 and other articles in this Code, which apply except as modified in this article.
            Few "standards" exist w/o something really bad happening first.

            Comment


              #7
              Originally posted by NoVA Comms Power View Post
              Re: "... But is there something in that section that says it supersedes all other code requirements? If not, 430.22 would prevail, because it has a VFD ... "

              NEC 440.3 (Other Articles)
              (A) These provisions are in addition to, or amendatory of, the provisions of Article 430 and other articles in this Code, which apply except as modified in this article.
              Yeah, I just read that now that I'm back in my office. Looks like they are right.
              Also, the NEC goes on to make it VERY clear in 440.6 (A);
              (A) Hermetic Refrigerant Motor-Compressor. For a hermetic
              refrigerant motor-compressor, the rated-load current
              marked on the nameplate of the equipment
              in which the
              motor-compressor is employed shall be used in determining
              the rating or ampacity of the disconnecting means, the
              branch-circuit conductors, the controller, the branch-circuit
              short-circuit and ground-fault protection, and the separate
              motor overload protection
              . Where no rated-load current is
              shown on the equipment nameplate, the rated-load current
              shown on the compressor nameplate shall be used.
              Learned something new today.
              __________________________________________________ ____________________________
              Many people are shocked when they discover I am not a good electrician...

              I'm in California, ergo I am still stuck on the 2014 NEC... We'll get around to the 2017 code in around 2021.

              Comment


                #8
                Why is the VFD nameplate twice the motor nameplate? I can't believe they used a VFD twice as large as necessary on a motor that size.

                I assume everything is rated at 480 volts?

                IMO, its a packaged unit, and you should use the unit nameplate. The VFD nameplate would only apply if the VFD were installed as an individual piece of equipment.

                Comment


                  #9
                  I am guessing that the supplier got lazy and didn't provide a nameplate at all. They provided a collection of equipment each with its own nameplate.

                  Something to investigate: is the motor 'supercharged' or whatever the slang term is, where a 230/460 motor is connected as 230V, but supplied from a 480V inverter to permit operation at higher than 60Hz? If this is the case, then the inverter is _not_ oversized, but the motor would be expected to use current at the 230V levels...

                  -Jon

                  Comment


                    #10
                    Originally posted by steve66 View Post
                    Why is the VFD nameplate twice the motor nameplate? I can't believe they used a VFD twice as large as necessary on a motor that size.

                    I assume everything is rated at 480 volts?

                    IMO, its a packaged unit, and you should use the unit nameplate. The VFD nameplate would only apply if the VFD were installed as an individual piece of equipment.
                    As long as it really is a unit nameplate and not just a copy of the motor nameplate....

                    Comment


                      #11
                      Originally posted by steve66 View Post
                      Why is the VFD nameplate twice the motor nameplate? I can't believe they used a VFD twice as large as necessary on a motor that size.
                      I wondered about that too. Not only that, it's VFD input current that's given. The rated output current is usually significantly higher than that. So more than twice the motor rating.
                      Si hoc legere scis nimium eruditionis habes.

                      Comment


                        #12
                        Originally posted by steve66 View Post
                        Why is the VFD nameplate twice the motor nameplate? I can't believe they used a VFD twice as large as necessary on a motor that size.

                        I assume everything is rated at 480 volts?

                        IMO, its a packaged unit, and you should use the unit nameplate. The VFD nameplate would only apply if the VFD were installed as an individual piece of equipment.


                        I would suspect it would because the manufacturer of the chiller only wants to stock a limited supply of sizes of VFD's. I would be under the impression that since it is a package unit then you design around the equipment's nameplate and not the VFD's nameplate as there is no way that they could upsize the motors on the chiller to get more capacity, which I believe is the reason why we design around the VFD's rated input when the motor and the VFD are separate items.

                        Comment


                          #13
                          Originally posted by Besoeker View Post
                          I wondered about that too. Not only that, it's VFD input current that's given. The rated output current is usually significantly higher than that. So more than twice the motor rating.
                          Its almost like the VFD input voltage is 208V, and the output voltage (and motor voltage) are 480V.

                          Comment


                            #14
                            Originally posted by Besoeker View Post
                            I wondered about that too. Not only that, it's VFD input current that's given. The rated output current is usually significantly higher than that. So more than twice the motor rating.
                            Its almost like the VFD input voltage is 208V, and the output voltage (and motor voltage) are 480V.

                            Are there VFD's that can take either 208 or 480V as the input, and provide the correct 480V output for the motor?

                            Comment


                              #15
                              Originally posted by steve66 View Post
                              Its almost like the VFD input voltage is 208V, and the output voltage (and motor voltage) are 480V.

                              Are there VFD's that can take either 208 or 480V as the input, and provide the correct 480V output for the motor?
                              No, so far you cannot create voltage that is not there to start with without a transformer, at least not above about 1HP (using a voltage doubler circuit on the front end of the drive).

                              The other way (480V input to a drive to run a 208V motor)? Yes, although its a really bad idea.

                              Several of the Japanese mfrs, Mitsubishi for one, really only make a single drive that is a "400V class", which accepts anywhere from 380-480V input. But when they get them UL listed, they interpret the rules (or don't want to pay extra) to mean that it must be listed for a particular HP regardless of the voltage rating. So the drives end up looking over sized for our motors, which can cause issues with conductor sizing.

                              But this is too much of a difference to be that issue. I agree with Knightryder12, it must be that the OEM wants to stock one VFD for any voltage, rated for the highest voltage, sized for the lowest. It only makes sense if you are intent on trading convenience for component cost.
                              __________________________________________________ ____________________________
                              Many people are shocked when they discover I am not a good electrician...

                              I'm in California, ergo I am still stuck on the 2014 NEC... We'll get around to the 2017 code in around 2021.

                              Comment

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