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inconsistency in annex D load calc examples

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    inconsistency in annex D load calc examples

    One of my students pointed out an inconsistency in the load calc examples in Annex D, between a load calc for a single family dwelling [D1(a)] and a commercial space [D3].
    In the former, to calculate the number of branch circuits for the general lighting load, 4500 VA is divided by the voltage of only one phase: 120 volts, while in the latter example, the general lighting load is divided by the voltage of both phases: 240 volts. In the latter example, 210.11 (B) is cited, whereby the load is to be evenly apportioned on the load center.

    Since I do not believe 210.11 (B) only applies to commercial spaces, I am unable to explain the discrepancy to my student. Any clarification would be greatly appreciated.

    #2
    I'll agree Annex Ex D(3) is a little confusing. What they are trying to show is 3 wire 240v Lighting is allowed in the Commercial Calculation, and
    it is not in residential. So while they "assume" this in the calculation breakdown, the text below it explains the actual load could/will be different.
    Take a look at the explanation text below the "10625 VA/240=44 amps" line.

    Comment


      #3
      I think there are 2 things going on in Example D1

      The 120V division is to show the minimum number of branch circuits required but read on and notice that it does not enter into the calc.
      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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        #4
        Originally posted by Henley View Post
        I'll agree Annex Ex D(3) is a little confusing. What they are trying to show is 3 wire 240v Lighting is allowed in the Commercial Calculation, and
        it is not in residential. So while they "assume" this in the calculation breakdown, the text below it explains the actual load could/will be different.
        Take a look at the explanation text below the "10625 VA/240=44 amps" line.
        Yeah, thanks. I pointed out in class that since the advent of afci requirements we no longer use 3 wire circuits in homes. Back in the day, we used them all the time and if memory serves, often arrived at required number of general lighting circuits by dividing by 240 and not 120.

        I'm thinking that rather than constantly complaining about the vagaries of the examples in the annex, I should simply submit some examples that go through the load calc steps in a way which I believe better serves students/electricians.

        Thanks for your help

        Comment


          #5
          Originally posted by Dennis Alwon View Post
          I think there are 2 things going on in Example D1

          The 120V division is to show the minimum number of branch circuits required but read on and notice that it does not enter into the calc.

          Only insofar as regards the feeder size. It certainly is relevant as to the number of two wire circuits for general lighting required, no?

          Comment


            #6
            Originally posted by ohmboyz View Post
            Only insofar as regards the feeder size. It certainly is relevant as to the number of two wire circuits for general lighting required, no?

            Yes but not the calculation of the load. Example 3a doesn't even calculate the number of circuits so you are comparing apples and oranges.
            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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              #7
              Originally posted by Dennis Alwon View Post
              Yes but not the calculation of the load. Example 3a doesn't even calculate the number of circuits so you are comparing apples and oranges.

              Yes, apples and oranges except that D 3 (a) was not the example I used in my original question. I contrasted D 1 (a) and D 3.

              Comment


                #8
                Originally posted by ohmboyz View Post
                Yeah, thanks. I pointed out in class that since the advent of afci requirements we no longer use 3 wire circuits in homes. Back in the day, we used them all the time and if memory serves, often arrived at required number of general lighting circuits by dividing by 240 and not 120.

                I'm thinking that rather than constantly complaining about the vagaries of the examples in the annex, I should simply submit some examples that go through the load calc steps in a way which I believe better serves students/electricians.

                Thanks for your help
                I wasn't really clear, I meant 240V Lighting when I said 3 wire, not Multiwire Branch Circuit as in 14/3. Funny though you talk about AFCI, when it first came out I too was told you couldn't use a "shared Neutral " as in MWBC with AFCI devices. I am told most current AFCI devices can now be used on MWBC, although I have never personally installed them. I have never been a fan of MWBC, to many pitfalls. Sorry to side track the OP.

                Comment


                  #9
                  Pitfalls

                  MWBC's pitfalls? Where? With them you get efficient use of conductors, lower voltage drop, less material cost, proven track record of safety by virtue of the fact that there are millions of them in use right now. I'm not seeing any real pitfalls.
                  If Billy Idol is on your playlist go reevaluate your life.

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                    #10
                    Originally posted by ohmboyz View Post
                    Yes, apples and oranges except that D 3 (a) was not the example I used in my original question. I contrasted D 1 (a) and D 3.
                    Sorry my bad. If you look at D(3) it shows for a 3 wire circuit-- that is why they are using 240V. The paragraph below shows 88amps for a 2 wire and 44 amps for a 3 wire circuit so that is not inconsistent
                    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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