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    Arc Flash Calc

    I have a 480V, 3Ph, 3W switchrack located in a CID2 area - Refinery. SWRK has a 200AT/250AF MCB along with 2-40Hp motors, 2-7.5Hp motors and a 30kVA transformer/panelboard feeding mainly HID lighting loads. Client wants to add two more 460V motors at 15Hp each. Is it necessary to revisit the Arc Flash study and modify it due to these two additional loads?

    #2
    Originally posted by Isaiah View Post
    I have a 480V, 3Ph, 3W switchrack located in a CID2 area - Refinery. SWRK has a 200AT/250AF MCB along with 2-40Hp motors, 2-7.5Hp motors and a 30kVA transformer/panelboard feeding mainly HID lighting loads. Client wants to add two more 460V motors at 15Hp each. Is it necessary to revisit the Arc Flash study and modify it due to these two additional loads?
    Per NFPA 70E, 130.5 (G): "The incident energy analysis shall be updated when changes occur in the electrical distribution system that could affect the results of the analysis. The incident energy analysis shall also be reviewed for accuracy at intervals not to exceed 5 years."

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      #3
      wsbeih - I really appreciate this feedback.

      Comment


        #4
        Originally posted by wsbeih View Post

        Per NFPA 70E, 130.5 (G): "The incident energy analysis shall be updated when changes occur in the electrical distribution system that could affect the results of the analysis. The incident energy analysis shall also be reviewed for accuracy at intervals not to exceed 5 years."
        True but IEEE 1584 only considers motors larger than 50 hp. But let's look at this from a practical point.

        A 460V 15hp motor will contribute about 70A of short circuit current at the motor terminals so fault current will less at MCC due to conductor impedance and last only about 1/2 to 1 cycle. So if the available bolted fault current is 15kA at the MCC, the fault current from the motor is <0.5%. This will not make a difference in the incident energy at the MCC as the arcing current clearing time is greater than 1/2 cycle and the source current is the major determiner of the incident energy.

        Comment


          #5
          Following wb's thoughts:
          Working under cumulative motor fault current,
          2 x 15hp = 30 hp >> FLA = 40A
          Induction motor fault contribution of 4 x FLA = 160A, Close to WB's estimate.

          As long as we are not getting just half the story.

          Panel loading:
          1 ea 40 hp (at 125%) = 65A
          1 ea 40 hp (100%) = 52A
          2 ea 7.5 hp = 22A
          30kva at 80% = 29A

          Panel load before addition of 15hp motors = 168A
          And if this is continuous load then 168 x 1.25 = 210A
          Adding:
          2 ea 15 hp = 42A

          Panel load after addition = 210A
          If continuous load = 210 x 1.25 = 262A

          And this is all on a panel with a 200A trip Main CB.

          What am I missing?
          Without data you’re just another person with an opinion – Edwards Deming

          Comment


            #6
            Curiosity Question:
            wb -
            ws -
            Are you guys related?
            Without data you’re just another person with an opinion – Edwards Deming

            Comment


              #7
              Originally posted by iceworm View Post
              Following wb's thoughts:
              Working under cumulative motor fault current,
              2 x 15hp = 30 hp >> FLA = 40A
              Induction motor fault contribution of 4 x FLA = 160A, Close to WB's estimate.

              As long as we are not getting just half the story.

              Panel loading:
              1 ea 40 hp (at 125%) = 65A
              1 ea 40 hp (100%) = 52A
              2 ea 7.5 hp = 22A
              30kva at 80% = 29A

              Panel load before addition of 15hp motors = 168A
              And if this is continuous load then 168 x 1.25 = 210A
              Adding:
              2 ea 15 hp = 42A

              Panel load after addition = 210A
              If continuous load = 210 x 1.25 = 262A

              And this is all on a panel with a 200A trip Main CB.

              What am I missing?
              Well, probably not all the info to determine if loading violates NEC because the OP specifically asked about the need to redo the arc flash study and that is how I answered the question. Granted there may be other considerations to be taken into account but none of those were asked.

              Comment


                #8
                Originally posted by iceworm View Post
                Curiosity Question:
                wb -
                ws -
                Are you guys related?
                nope

                Comment


                  #9
                  Originally posted by wbdvt View Post

                  Well, probably not all the info to determine if loading violates NEC because the OP specifically asked about the need to redo the arc flash study and that is how I answered the question. Granted there may be other considerations to be taken into account but none of those were asked.

                  The motor loads are connected to pumps that are intermittent duty; plus each pair of motors is a primary/backup scenario, so only one pump operates at one time. The transformer serves mainly lighting loads so this load is continuous.

                  Comment


                    #10
                    That was just curiosity on my part. It didn't add up.

                    So, as for the arc-flash, you are adding only one 15hp motor. The other is another primary/backup? If so, that only adds half, ~80A to the SCC.
                    I don't see any reason to redo the arc-flash
                    Without data you’re just another person with an opinion – Edwards Deming

                    Comment


                      #11
                      Originally posted by iceworm View Post
                      That was just curiosity on my part. It didn't add up.

                      So, as for the arc-flash, you are adding only one 15hp motor. The other is another primary/backup? If so, that only adds half, ~80A to the SCC.
                      I don't see any reason to redo the arc-flash
                      Yes, the two additional motors are also primary and backup pumps. I greatly appreciate your input.
                      Isaiah

                      Comment


                        #12
                        You need to add a LABEL to each item added indicating SCC.
                        Assuming you have the software for that, only one more step to run the Arc-Flash numbers.

                        Comment


                          #13
                          Originally posted by iceworm View Post
                          Following wb's thoughts:
                          Working under cumulative motor fault current,
                          2 x 15hp = 30 hp >> FLA = 40A
                          Induction motor fault contribution of 4 x FLA = 160A, Close to WB's estimate.

                          As long as we are not getting just half the story.

                          Panel loading:
                          1 ea 40 hp (at 125%) = 65A
                          1 ea 40 hp (100%) = 52A
                          2 ea 7.5 hp = 22A
                          30kva at 80% = 29A

                          Panel load before addition of 15hp motors = 168A
                          And if this is continuous load then 168 x 1.25 = 210A
                          Adding:
                          2 ea 15 hp = 42A

                          Panel load after addition = 210A
                          If continuous load = 210 x 1.25 = 262A

                          And this is all on a panel with a 200A trip Main CB.

                          What am I missing?
                          I think you've factored the continuous loads too many times by 125%. You only need to do this once - with the largest motor - and once again with non-motor loads, i.e. transformer (lighting loads).

                          Comment


                            #14
                            Originally posted by MyCleveland View Post
                            You need to add a LABEL to each item added indicating SCC.
                            Assuming you have the software for that, only one more step to run the Arc-Flash numbers.
                            No sir, I dont have the software, but a colleague of mine does.

                            Comment


                              #15
                              Originally posted by MyCleveland View Post
                              You need to add a LABEL to each item added indicating SCC.
                              Assuming you have the software for that, only one more step to run the Arc-Flash numbers.
                              Why? Some things don't get labels like motor terminal boxes. The short circuit current only needs to be on the service entrance equipment.

                              Comment

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