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2020 NEC Article 310 Single Family Dwelling Ampacities

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    2020 NEC Article 310 Single Family Dwelling Ampacities

    This section has been re-written a bit and the Table for SFD has been brought back and can be used if no adjustment factors are needed. Also the ampacity table number is back to T310.16 again.


    310.12 Single-Phase Dwelling Services and Feeders.

    For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, service and feeder conductors supplied by a single-phase, 120/240-volt system shall be permitted to be sized in accordance with 310.12(A) through (D).

    For one-family dwellings and the individual dwelling units of two-family and multifamily dwellings, single-phase feeder conductors consisting of two ungrounded conductors and the neutral conductor from a 208Y/120 volt system shall be permitted to be sized in accordance with 310.12(A) through (C).

    310.12(A) Services.

    For a service rated 100 amperes through 400 amperes, the service conductors supplying the entire load associated with a one-family dwelling, or the service conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the service rating. If no adjustment or correction factors are required, Table 310.12 shall be permitted to be applied.

    310.12(B) Feeders.

    For a feeder rated 100 amperes through 400 amperes, the feeder conductors supplying the entire load associated with a one-family dwelling, or the feeder conductors supplying the entire load associated with an individual dwelling unit in a two-family or multifamily dwelling, shall be permitted to have an ampacity not less than 83 percent of the feeder rating. If no adjustment or correction factors are required, Table 310.12 shall be permitted to be applied.

    310.12(C) Feeder Ampacities.

    In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.12(A) or (B).

    310.12(D) Grounded Conductors.

    Grounded conductors shall be permitted to be sized smaller than the ungrounded conductors, if the requirements of 220.61 and 230.42 for service conductors or the requirements of 215.2 and 220.61 for feeder conductors are met.

    Where correction or adjustment factors are required by 310.15(B) or (C), they shall be permitted to be applied to the ampacity associated with the temperature rating of the conductor.

    Informational Note No. 1: The service or feeder ratings addressed by this section are based on the standard ampere ratings for fuses and inverse time circuit breakers from 240.6(A).

    Informational Note No. 2: See Example D7 in Annex D.
    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

    #2
    Good. The table should have never been removed to start with.

    Comment


      #3
      Originally posted by Hv&Lv View Post
      Good. The table should have never been removed to start with.
      Yeah but the table caused problems because in the past you were not required to apply de-rating factors. Now if there are no adjustment to the conductor then you can use the table. BTW, the table was still in the NEC only hidden in the back of the book. The table is the same as 83%
      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

      Comment


        #4
        to table...or not to table...? ~RJ~

        Comment


          #5
          I didn't have a problem with the 83% rule it's simple math and you could still use the old table from a previous version.
          Rob

          Moderator

          All responses based on the 2017 NEC unless otherwise noted

          Comment


            #6
            The problem with the table is this...Everybody knows the NEC load calcs for dwellings are artificially high so the place the adjustments should be made is in the calcs, not in the allowable amps for a conductor in art. 310.
            If Billy Idol is on your playlist go reevaluate your life.

            Comment


              #7
              Originally posted by ActionDave View Post
              The problem with the table is this...Everybody knows the NEC load calcs for dwellings are artificially high so the place the adjustments should be made is in the calcs, not in the allowable amps for a conductor in art. 310.
              I don't believe load calculation or diversity* is the reasoning for this code section.


              1956 NFPA Proceedings
              Also in connection with this table, the utility people pointed out that aluminum wire for services was being modified from the 84 per cent value previously given in the Code, that is 84 per cent of the copper carrying capacity, and while they were in agreement with that because the data was the result of a study by the sub-committee, they did want to point out that in a 3-wire single phase, with only one of the wires loaded, it would not be necessary to go to the higher values.

              In 1956 Aluminum received it's own tables with a note about Edison 3-wire ampacities(table pictured below). If I remember correctly Copper didn't get a reduction allowance until around 1975. Before this table, Aluminum was sized at 84% copper (see second picture below).

              Click image for larger version  Name:	1956 aluminum edison 3 wire ampacity.png Views:	0 Size:	124.2 KB ID:	2537958


              Aluminum was sized at 84% copper in 1953

              Click image for larger version  Name:	1953 aluminum ampacity.png Views:	0 Size:	150.0 KB ID:	2537957


              Later panel comments include recognition of how our normal tables are based on 3 conductors, while a 3-wire Edison should only have the equivalent of two wires fully loaded, the neutral only carrying imbalance. This is what I have come to believe is the reasoning behind the greater allowable ampacity.

              *The term diversity is also used to describe this characteristic of the conductor group(see table below). I believe their use of the term diversity back then has been misinterpreted today as meaning load diversity (the more common use of the term today), as in we don't run all of the electric baseboard heaters, the air-conditioner and the entire range at the same time.

              Other later panel comments also recognize that the "third wire" also acts as a heat sink to an extent.


              AIEE Paper "The Heating and Mechanical Effects of Installing Insulated Conductors in Steel Raceways" 1957. (AIEE Turned into IEEE).
              Notice the column "No. Conducting", the Table is titled "Diversity Tests", I can post some more of this paper if anyone wants more verification, diversity is referenced in several spots.

              Click image for larger version  Name:	1956 AIEE Diversity.png Views:	0 Size:	154.2 KB ID:	2537960
              Last edited by Devin Hanes; 10-05-19, 06:02 PM. Reason: Trying to clarify perceived reasoning in third statement by adding the text in italics.

              Comment


                #8
                Originally posted by Devin Hanes View Post

                I don't believe load calculation or diversity* is the reasoning for this code section.

                I would have to disagree with this statement. Diversity is what the section is about. Loads in a commercial establishment are usually pretty constant- lights etc... but in a home the diversity of the loads are why we don't have tremendous loads on a building. 95% of the time loads on a 200 amp service are well below 40 or 50 amps.
                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

                Comment


                  #9
                  Originally posted by Dennis Alwon View Post

                  I would have to disagree with this statement. Diversity is what the section is about. Loads in a commercial establishment are usually pretty constant- lights etc... but in a home the diversity of the loads are why we don't have tremendous loads on a building. 95% of the time loads on a 200 amp service are well below 40 or 50 amps.
                  I agree real world residential service load calculations are very conservative and I agree with your statement here "95% of the time loads on a 200 amp service are well below 40 or 50 amps."


                  Do you believe I am misreading or misapplying the Information I posted? Or that the reasoning has changed since the allowance's inception?


                  If you look at the 1956 table I posted, that note allowance was not just for residences.

                  Comment


                    #10
                    Here is the entire table and I tried to make the notes easier to read, it is the first note. They are also attached if that makes them easier to read.


                    1956 NEC

                    Click image for larger version

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                    Click image for larger version

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                    Attached Files

                    Comment


                      #11
                      Originally posted by Devin Hanes View Post

                      I agree real world residential service load calculations are very conservative and I agree with your statement here "95% of the time loads on a 200 amp service are well below 40 or 50 amps."


                      Do you believe I am misreading or misapplying the Information I posted? Or that the reasoning has changed since the allowance's inception?


                      If you look at the 1956 table I posted, that note allowance was not just for residences.
                      Which goes to my point... We should not be using art. 310 to see what size wire should be used for dwelling service or any other service for that matter, that's what art. 220 should be for.
                      If Billy Idol is on your playlist go reevaluate your life.

                      Comment


                        #12
                        Originally posted by ActionDave View Post

                        Which goes to my point... We should not be using art. 310 to see what size wire should be used for dwelling service or any other service for that matter, that's what art. 220 should be for.
                        In my post #7 I provided some references that I believe point toward ampacity, my references are from 1953-1957 to the code sections that I see as the predecessors of our current section discussed in this thread.

                        I believe I've read most if not all of the panel comments from 1956 to present day and there are varying reasons given for this allowance, but the only ones I have come across that say this section doesn't change ampacity are more recent ones, I'm thinking 90's.This section was stagnant for a long time.

                        I have come across more recent textbooks and IAEI articles that say this section doesn't change ampacity. But from my look into the evolution of this section, that appears to be incorrect unless you can have an allowance for 40 years and then just change the reason with no substantiation and keep it in the ampacity section when if the reason changed, the location should have as well. I have not come across any statement about changing the reasoning behind this section, just forty years later someone stated a reason that was not consistent with previous ones and it appears other people just went along.

                        And if that's the case, I agree, if the reasoning is not ampacity then this is the wrong section for it. By the way I read recent articles and textbooks that discuss this section I actually think it might be better in 240, around 240.4(G). That's another reason why I believe the reason at the inception of this allowance still stands, ampacity, that's why it was and is in the ampacity section. Always glad to learn something new though if someone wants to offer something I am missing.

                        You are probably aware, a few years ago there were many proposals to move this section like you are suggesting, I believe one was from George Stolz on here.

                        Comment

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