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    #16
    Originally posted by Open Neutral View Post
    Well, the new EC [the third, alas....] has raised a major alarm. Given the dedicated 100KVA transformer PSE has spec'ed, the AIC is so large the existing four panel boards would have to be ripped out and replaced.

    This is an area I know almost nil about. Would all breakers in the house have to meet a higher AIC rating, or if there is a new Main that does, is that sufficient protection?

    (The EC has a on-site meeting scheduled to ask the PoCo why they spec'ed that transformer so it may be moot.)
    If the main and all of the feeder and branch breakers are from the same manufacturer it is possible to get a "series rating" document from them that certain combinations of high AIC main and lower AIC downstream breakers have been tested in specific combinations to survive a downstream bolted fault. When you have to series rate 3 levels instead of two it gets more complicated

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      #17
      Originally posted by Open Neutral View Post
      Given the dedicated 100KVA transformer PSE has spec'ed, the AIC is so large the existing four panel boards would have to be ripped out and replaced.
      What has led you to believe that that AIC is large? Perhaps I missed it, but I didn't see what the facility has for a voltage and phase configuration. If, for example, it is 120/240V single phase, then the fault current at the transformer secondary terminals would be around 7000 amps. That is not a "large" number, and it would be smaller after passing through the secondary conductors. If you have 120/208V three phase, the AIC will be lower still.


      Charles E. Beck, P.E., Seattle
      Comments based on 2017 NEC unless otherwise noted.

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        #18
        Originally posted by charlie b View Post
        What has led you to believe that that AIC is large?
        The new EC researched it with PSE, I'm told.

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          #19
          A good rule of thumb is that the available fault current at the secondary terminals of a transformer can be estimated by taking the full load current of the secondary side and dividing by the "per unit" impedance of the secondary windings. That later value is often on the order of 5.75%, so the value you divide by is 0.0575. Different transformers will have different values. In the example I gave earlier, the full load current for a 120/240V single phase 100 KVA transformer is 100,000/240, or 417 amps. Dividing that by 0.0575 give an AIC of 7,246 amps.

          Most panels have a minimum AIC of 10,000 amps. To get that from the transformer, it would need to have an impedance of 4.17% or lower. It is possible that the utility transformer's impedance could be lower than that. Even still, the impedance of the secondary conductors will significantly reduce the available fault current at the building's main breaker.

          So don't take the EC's statement at face value. Ask for the exact number that PSE provided to him (or her). Then you can do a calculation of the impact of the secondary conductor's impedance on the value PSE gives you. I am willing to predict that the final result is below 10,000 amps, and that you therefore do not have a problem in need of a solution.
          Charles E. Beck, P.E., Seattle
          Comments based on 2017 NEC unless otherwise noted.

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            #20
            Thank you.
            We do have a substantial secondary connection - dual 350MCM of about 80 ft in length.
            But I will find out the exact details.

            And a side question: Does Washington State L&I require a PE signoff on utility designs?
            After the Merrimack Valley Gas disaster, the NTSB came out swinging against utility exemptions to the PE requirement.


            Last edited by Open Neutral; 11-06-19, 09:55 AM.

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              #21
              Originally posted by Open Neutral View Post
              And a side question: Does Washington State L&I require a PE signoff on utility designs?
              I have no fact to offer you. But my inclination is to suspect the answer to be "no."


              Charles E. Beck, P.E., Seattle
              Comments based on 2017 NEC unless otherwise noted.

              Comment


                #22
                Originally posted by charlie b View Post
                A good rule of thumb is that the available fault current at the secondary terminals of a transformer can be estimated by taking the full load current of the secondary side and dividing by the "per unit" impedance of the secondary windings. That later value is often on the order of 5.75%, so the value you divide by is 0.0575. Different transformers will have different values. In the example I gave earlier, the full load current for a 120/240V single phase 100 KVA transformer is 100,000/240, or 417 amps. Dividing that by 0.0575 give an AIC of 7,246 amps.

                Most panels have a minimum AIC of 10,000 amps. To get that from the transformer, it would need to have an impedance of 4.17% or lower. It is possible that the utility transformer's impedance could be lower than that. Even still, the impedance of the secondary conductors will significantly reduce the available fault current at the building's main breaker.

                So don't take the EC's statement at face value. Ask for the exact number that PSE provided to him (or her). Then you can do a calculation of the impact of the secondary conductor's impedance on the value PSE gives you. I am willing to predict that the final result is below 10,000 amps, and that you therefore do not have a problem in need of a solution.
                Charlie, I find your %Z value to be to high. Mid 5's would be typical for a three phase pad of around 500KVA, but smaller single phase units are quite a bit less. I have some 15KVA single phase pads here that are a little under 2%. The utility 25KVA pole unit serving me is 2.3%. PSE's published fault current values can be found here on page 27:

                https://www.pse.com/-/media/PDFs/Com...13E4DC06D07E63

                Note also that L-N faults are higher than L-L (as they show in their tables).
                Ethan Brush - East West Electric. NY, WA. MA

                "You can't generalize"

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                  #23
                  If I recall most 'Public Utility Commissions' or NESC regulations set a maximum available fault current for any residential service point. I might be mistaken though.
                  Comments based on 2017 NEC unless otherwise noted.

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