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    Raceway as EGC

    How do you do a ground-fault calculation if you are using a raceway as an equipment ground? Is there somewhere that you can find the resistance properties of a particular type of conduit?
    Ryan Jackson, Salt Lake City
    Inspector, Instructor

    #2
    Re: Raceway as EGC

    Ryan:

    When you say ground-fault calculation, I assume you mean the calculation to determine the needed AIC rating of a circuit breaker. The way I do the calculations, the size of the equipment ground doesn't matter (at least it doesn't enter the calculation anywhere).

    Why doesn't it come into play? I'm not 100% sure, but I think in the case of a line to ground fault, it is assumed that there is a perfect ground at the location of the fault. In other words, the resistance and inductance of the ground path is simply ignored. Also, if a line-line short occurs, the current doesn't travel on the equipment ground.

    On the other hand, enclosing the conductors in a metal raceway does affect the ability of current to flow during a fault, and different numbers are used for conductors enclosed in metal conduit than in PVC conduit.

    Steve

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      #3
      Re: Raceway as EGC

      This is a neat article on the topic.
      http://www.electrician.com/electa1/ground2.html
      Steve is correct, that a normal short circuit calc, does not include ground impedance.
      Once you calculate the fault current available, you can then work on the actual ground impedance, which has many many variables that are difficult to include in a calculation to get an accurate answer.
      Ron

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        #4
        Re: Raceway as EGC

        Ryan,
        You can download the GEMI software from the Steel Tube Institute. The software does conduit grounding length calculations.
        Don
        Don, Illinois
        (All code citations are 2017 unless otherwise noted)

        Comment


          #5
          Re: Raceway as EGC

          The Soares book of grounding covers this issue very well. Lots of calculations and examples.

          Comment


            #6
            Re: Raceway as EGC

            Hi guys. I agree that the AIC rating should be determined by a line-to-line fault (short), but I was just curious as to how to determine it (the ground fault). I'm no engineer by any stretch...in fact I'm just a code geek and not a theory guy, but it seems to me that the fault current generated would have to be dependant upon the length and resistance(impedance) of the grounding means. Am I off on this?

            BTW: Thanks for the links...I'll be sure to check those out.

            Edited for clarity

            [ March 19, 2004, 06:41 PM: Message edited by: ryan_618 ]
            Ryan Jackson, Salt Lake City
            Inspector, Instructor

            Comment


              #7
              Re: Raceway as EGC

              Originally posted by ryan_618:
              in fact I'm just a code geek
              If you are a code geek what the heck does that make me.

              Comment


                #8
                Re: Raceway as EGC

                Originally posted by iwire:
                Originally posted by ryan_618:
                in fact I'm just a code geek
                If you are a code geek what the heck does that make me.
                A code geek
                Ryan Jackson, Salt Lake City
                Inspector, Instructor

                Comment


                  #9
                  Re: Raceway as EGC

                  HMMMM
                  Charlie Eldridge, Indianapolis, Utility Power Guy[COLOR=red][/COLOR]

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                    #10
                    Re: Raceway as EGC

                    All right code geeks ( ), lets try this question.

                    If I have a 500' length of EMT that is being used as the EGC, with no supplemental equipment grounding conductors, and a 120-V circuit in it, have I satisfied the requirement250.4(A)(5)???
                    Ryan Jackson, Salt Lake City
                    Inspector, Instructor

                    Comment


                      #11
                      Re: Raceway as EGC

                      Darn DIYs never give enough info.

                      What is the amperage of the circuit?

                      What is the size of the EMT?

                      Comment


                        #12
                        Re: Raceway as EGC



                        Lets say.... 1/2" EMT with 10 AWG on a 20 Amp OCPD.
                        Ryan Jackson, Salt Lake City
                        Inspector, Instructor

                        Comment


                          #13
                          Re: Raceway as EGC

                          Ryan I think given that description the wire size and length will be as much of a problem as the conduit.

                          A line to line fault on this circuit would only provide 99.1 amps of fault clearing current.

                          At 100 amps of fault clearing current the 1/2" EMT is good for about 425'

                          [ March 20, 2004, 12:54 PM: Message edited by: iwire ]

                          Comment


                            #14
                            Re: Raceway as EGC

                            "If I have a 500' length of EMT that is being used as the EGC, with no supplemental equipment grounding conductors, and a 120-V circuit in it, have I satisfied the requirement250.4(A)(5)?"

                            Probably Not. This one area that may be often overlooked when designing systems. In the 96 code there is a chart that provides information regarding this question. The chart was a product of a study done by Professor A.P. Sakis Meliopoulos at Georgia Tech. I had the good fortune to talk with him on a number of occasions about the project. As mentioned, the Soares Book on Grounding has a good discussion on this subject.
                            There are several assumptions that were made in the Soares study. 1. The fault current was assumed to be 5 times the OC device. 2. The fault arc was given a value of 50 volts.
                            In making fault caculations, a problem that occurs is that the impedance of the conduit is nonlinear and affected by the magnitude of the fault. The fault magnetic field affects the iron in the conduit causing the impedance to vary.
                            In the Georgia Tech study, it was found that when using todays OC devices that arc voltage of 40 volts and a fault current of 4 times the OC device
                            provide adequate safety margins.

                            [ March 20, 2004, 02:20 PM: Message edited by: bob ]

                            Comment


                              #15
                              Re: Raceway as EGC

                              250.4 General requiremwnts fro grounding and bonding.
                              The following general requirements identify what grounding and bonding of electrical systems are requied to accomplish. The presciptive methods contained in Article 250 shall be folowed to comply with the performance requirements of this section
                              So, the rest of article 250 would allow the installation described in my previous posts.

                              Looking at the language of the above reference, does it imply that by following the rest of article 250, compliance is automatically achieved?

                              If not, is the only way to ensure compliance through resistance testing?
                              Ryan Jackson, Salt Lake City
                              Inspector, Instructor

                              Comment

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