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Interpretation of "AHEAD" for table 250.122

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    Interpretation of "AHEAD" for table 250.122

    Hello,

    I have a question regarding table 250.122. The table says(please see attached screenshot) "Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, etc., Not Exceeding(Amperes)".
    So, if I have 16A load with 20A OCPD, which rating should I use to size EGC? Does the word "ahead" mean do I size per 16A rating since the current is going towards the load?

    Thank you.
    Attached Files

    #2
    20 amp. It needs to be large enough to open the OCPD based on the rating of the OCPD.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

    Comment


      #3
      Originally posted by AungYe View Post
      Hello,

      I have a question regarding table 250.122. The table says(please see attached screenshot) "Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, etc., Not Exceeding(Amperes)".
      So, if I have 16A load with 20A OCPD, which rating should I use to size EGC? Does the word "ahead" mean do I size per 16A rating since the current is going towards the load?

      Thank you.
      Follow the path from the service point to the breaker, to the circuit/load(s). The OCPD you come to before the circuit in question, is the OCPD that is "ahead" of the circuit.

      Regardless of what sizing algorithm applies to the OCPD, you use the actual trip rating of the OCPD, as it will be installed/configured. So whether it is a non-continuous 20A load sized at 100%, or a continuous 16A load sized at 125%, or a continuous duty rated breaker sized for a 20A continuous load, it is a 20A OCPD, and your starting point EGC size (before adjustments for voltage drop if applicable) is #12Cu.

      If the only OCPD is "behind" the circuit, then the circuit doesn't classify as a feeder or branch circuit. It would either be service conductors, transformer secondary conductors, or tap conductors.

      If there is an OCPD on both sides of the circuit, then the one closer to the service point is the one that is "ahead" of the circuit, and governs EGC sizing. One example where you might see this is if you build a subpanel in a remote building with a main breaker, fed from a branch breaker on the main panel. The branch breaker in the main panel would be "ahead".

      Comment


        #4
        The context of the word "ahead" is that current flows, in order, through the overcurrent device, then the conductors, then the equipment (i.e., the load). The OCPD is "ahead" of the equipment.

        Edited to add: Typing too slowly again. Sigh.
        Charles E. Beck, P.E., Seattle
        Comments based on 2017 NEC unless otherwise noted.

        Comment


          #5
          Thank you guys for quick response. So based on the responses, this is what I understand(please see the attached image). Please direct me if I misunderstand the concept.

          Thanks again,
          Aung P.
          Attached Files

          Comment


            #6
            If there is no overcurrent protective device before the transformer then there will not be an equipment grounding conductor to the first panel. or instance the power company has a transformer and from there is no equipment grounding conductor delivered to the building-- just a grounded conductor plus 2 or 3 hot conductors depending on single or 3 phase.
            They say I shot a man named Gray and took his wife to Italy
            She inherited a million bucks and when she died it came to me
            I can't help it if I'm lucky


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            Comment


              #7
              what Dennis said !
              I try to teach if you have overcurent ahead of the conductor use Table 250.122, if there is no overcurrent AHEAD of the conductor, use 250.102
              At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

              Comment


                #8
                Originally posted by AungYe View Post
                Thank you guys for quick response. So based on the responses, this is what I understand(please see the attached image). Please direct me if I misunderstand the concept.

                Thanks again,
                Aung P.
                There is no EGC between the transformer and the first panel but there is an SSBJ {Table 250.102(C)(1)}. If it's a wire type it will usually be larger than the EGC after the OCPD.

                For example if the first CB were 400 amps your secondary conductors would be 600 kcmil which requires a minimum size SSBJ of #1/0 Cu. The EGC on the load side of that 400 mp CB would be a minimum size of #3 Cu.
                Rob

                Moderator

                All responses based on the 2014 NEC unless otherwise noted

                Comment

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