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Existing switchgear has lower AIC rating than Utility contribution

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    Existing switchgear has lower AIC rating than Utility contribution

    A client has two approximately 40 year old buildings fed by one 480V, 3000A service which is located in one and feeds 2000A to the other. The main switchgear and distributions immediately downstream are all 42K rated, whereas the utility letter shows a maximum short circuit fault current contribution of 52k 3-Phase and 53k Phase to ground.

    Obviously new construction would stipulate that 65k gear be installed, but this is a pre-existing condition, and there is no documentation of what the utility contribution was at time of install. This electrical gear must have been rated to accommodate utility contribution at the time to get through plan check and inspection one would assume.

    It was recommended that they replace with appropriately rated gear as theirs has not been maintained, is underrated, mismatched and an overall mess. The client is fine with this, but is questioning if it is a NEC code violation to have everything underrated, and if it would be "grandfathered in".

    Any help in answering this would be appreciated.

    #2
    Your answer is embedded in your question;
    ...as theirs has not been maintained, is underrated, mismatched and an overall mess.
    "Grandfathering" would only apply if the equipment WAS maintained to factory specs and legal at the time of installation. Even then, if the utility changed a transformer somewhere along the way and that's why you now have 52kA available, the under rated gear would (I believe) needed to have been changed anyway. Typically that's an issue that the utility would have discussed with the client (in a perfect world) ahead of making that change. But as it is, the gear in place now would not qualify for being grandfathered anyway, and the fact that it was 42kA would not qualify as a justification to replace it with new 42kA gear no matter what.

    Now, why the quibbling? There is barely any difference at 3000A between 42kA gear and 65kA gear now. 40 years ago there may have been, but not now.
    __________________________________________________ ____________________________
    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


      #3
      Code issues aside, IMO common sense says you don't want underrated equipment. That's just asking for trouble.

      Although I have to wonder if there is really 50K+ available, or if someone is being conservative in their calculations.

      And I'm assuming you understand series ratings and such, and that the equipment really is only listed for 42KAIC. And its not something like a 65K main with 42K branch breakers.

      Comment


        #4
        Upgrading could be simple depending on what exactly you have for equipment. Any details?

        Comment


          #5
          How have you arrived at 52/53kA maximum fault current? If you’ve used “infinite bus” I wouldn’t worry about it, system impedance will drop the fault kA considerably.

          Here in the UK I’d hire a loop impedance tester, the number of times we would use one it just wasn’t economic to buy. The only worrying time was when the DNO (PoCo) installed a new 33/11kV primary substation on our doorstep. It changed all the site fault levels but not enough to cause any alarm.
          The reports of my death are greatly exaggerated.

          Comment


            #6
            I love it when utilities take two months to tell you that the available fault current is, in fact, the maximum possible number you calculated on your phone six seconds after you saw the transformer.

            Is there any possibility the utility would provide you the information necessary to do an actual calculation? Since the gear is existing, I would consider not replacing the gear simply to account for the worst possible case. Of course after 40 years it may be time for an upgrade anyway.

            Comment


              #7
              Here in PG&E (Procrastination, Graft and Extortion) country, if you call and talk to some dufus on the help desk, they give you that "could have done it on my phone" number using infinite bus and the transformer nameplate. But if you go through the official channels and wait for them to take their sweet time, they have to give you the actual values, in writing, with their calculations shown. Many people who waited too long to start the process and can't live with the delay have been burned by that; they buy HIC gear and find out later that it was unnecessary.
              __________________________________________________ ____________________________
              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
                Originally posted by steve66 View Post
                Code issues aside, IMO common sense says you don't want underrated equipment. That's just asking for trouble.

                Although I have to wonder if there is really 50K+ available, or if someone is being conservative in their calculations.

                And I'm assuming you understand series ratings and such, and that the equipment really is only listed for 42KAIC. And its not something like a 65K main with 42K branch breakers.

                Thanks Steve66 for the reply.

                Code issue is the main issue at this point because the business is being sold and it depends on if it is considered a code violation as to if work is going to be completed before the sale or not done at all.

                Gear is not 65k, it is 45k confirmed. I do know series ratings, and these breakers are mismatched manufacturers anyway so series rating would be moot even if the main was at 65k.

                I agree, underrated equipment is bad, mmm-kay?

                Comment


                  #9
                  Originally posted by Tony S View Post
                  How have you arrived at 52/53kA maximum fault current? If you’ve used “infinite bus” I wouldn’t worry about it, system impedance will drop the fault kA considerably.

                  Here in the UK I’d hire a loop impedance tester, the number of times we would use one it just wasn’t economic to buy. The only worrying time was when the DNO (PoCo) installed a new 33/11kV primary substation on our doorstep. It changed all the site fault levels but not enough to cause any alarm.

                  Tony S,

                  I arrived at 52/53kA maximum current via a letter stating such current from the utility company. Actual is less and was also calculated by them, but gear (at least anything that would be newly installed) must be above maximum levels.

                  Comment


                    #10
                    I would bet that what the utility gave you was the infinite bus short circuit current. It looks like maybe a 2500kVA transformer. I would also bet that if you obtained the utility available fault current at the riser fuses (primary transformer side), modeled from that point to the service you would be less than the 52kA probably within the rating of the existing equipment.
                    The model would be the riser cable, transformer, secondary cable with the available not infinite bus fault current.

                    I would go that route prior to buying any new equipment.

                    Comment


                      #11
                      Originally posted by Bwas View Post
                      I love it when utilities take two months to tell you that the available fault current is, in fact, the maximum possible number you calculated on your phone six seconds after you saw the transformer.

                      Is there any possibility the utility would provide you the information necessary to do an actual calculation? Since the gear is existing, I would consider not replacing the gear simply to account for the worst possible case. Of course after 40 years it may be time for an upgrade anyway.

                      Bwas,

                      It is time to upgrade, arc flash can't accurately be calculated due to the fact that it is not known if the breakers will even trip as designed since maintenance has been so poor. They have has other issues with old gear on site and want to have a fresh start.

                      I agree, there is only so much fault current that can possibly be squeezed through a particular transformer size, but try to tell the inspector that when they want to see maximum current levels and to be shown that your new gear exceeds those levels.

                      Here's a good one; inspectors are demanding that the maximum fault current level be added directly to the main bus, not figured as added at the transformer location and going through however many feet of feeder to get to the main bus. Therefore, you have a utility that shows 65k as their max, you order your gear at 65k, add just one motor and viola, you're over 65k and everything fails for being underrated.

                      Inspector AND utility are on the same page saying that the point of connection of service conductors to the customer's facility is your main gear where they lug onto your bus (and that is where max fault current is to be added). Pay no attention to the fact that there is no possible way to get that much current through the transformer. It has to be that was because they said so.

                      Comment


                        #12
                        What about 240.86?
                        "Electricity is really just organized lightning." George Carlin


                        Derek

                        Comment


                          #13
                          Originally posted by ponybetter View Post
                          It is time to upgrade, arc flash can't accurately be calculated due to the fact that it is not known if the breakers will even trip as designed since maintenance has been so poor. They have has other issues with old gear on site and want to have a fresh start.
                          I would not say a blanket upgrade is needed without knowing more details about the equipment in question. Perhaps maintenance can be performed to bring the equipment back to original condition. Do we know if it is fuses or breakers? Makes a difference on amount of maintenance.

                          Comment


                            #14
                            Originally posted by wbdvt View Post
                            I would not say a blanket upgrade is needed without knowing more details about the equipment in question. Perhaps maintenance can be performed to bring the equipment back to original condition. Do we know if it is fuses or breakers? Makes a difference on amount of maintenance.
                            Exacty, if only we had details.

                            Comment


                              #15
                              Originally posted by Jraef View Post
                              Here in PG&E (Procrastination, Graft and Extortion) country, if you call and talk to some dufus on the help desk, they give you that "could have done it on my phone" number using infinite bus and the transformer nameplate. But if you go through the official channels and wait for them to take their sweet time, they have to give you the actual values, in writing, with their calculations shown. Many people who waited too long to start the process and can't live with the delay have been burned by that; they buy HIC gear and find out later that it was unnecessary.
                              Luckily I have had a good relationship with PG&E engineers, they actually shared their Aspen model, when we were coordinating our 115 kV transmission line upgrade (we own one end, they the other).

                              Never heard of Procrastination, Graft, and Extortion..

                              Comment

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