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Thread: Dwelling calculations - application of demand factor

  1. #1
    Join Date
    Apr 2010
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    Dwelling calculations - application of demand factor

    Hi all,

    I recently took my NC limited exam and was perplexed by the following problem. It was asking (a couple of times) what the calculated loads were for dwelling units using the optional method. To paraphrase the question:

    Given a dwelling unit with the following loads (hot water heater, dishwasher, dryer and 3 separately controlled electric heating units) with a total of 18KVA (I don't recall the exact number), what is the calculated load.

    I understood that one was supposed to take the sum of the VA/sq ft + laundry + 2 kitchen ckts, plus the sum of 4 or more fastened in place appliances, THEN apply the damand factors of 100% for the first 8KVA plus 40% of the remaining load. 220.82(B).

    Then one is supposed to add the appropriate selection from 220.82(C)...heat and air to come up with a total load.

    I don't understand how I was supposed to come up with the correct answer when they lumped the HVAC into the total given in the question. I tried to apply the demand factor to the numbers they gave, but still couldn't come up with an answer.

    They did the same thing with a multi-dwelling unit too! Perhaps I got it right on the multiple-guess.

    Am I correct in my application of the optional method? I'd like to clarify it if I can.

    BTW, I passed!

    Thanks

  2. #2
    Join Date
    Feb 2003
    Location
    Seattle, WA
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    16,889
    Could this be the problem?
    Quote Originally Posted by sbrehler View Post
    I understood that one was supposed to take the sum of the VA/sq ft + laundry + 2 kitchen ckts, plus the sum of 4 or more fastened in place appliances, THEN apply the damand factors of 100% for the first 8KVA plus 40% of the remaining load. 220.82(B). Then one is supposed to add the appropriate selection from 220.82(C)...heat and air to come up with a total load.
    You were right, until you got to "4 or more fastened in place appliances." You count these appliances, even if there is only one of them. Then you count the larger "not fastened in place" appliances, such as ranges and dryers, and you count water heaters (I don't know why they didn't lump them in with the fastened in place stuff). You didn't show your math, so I can't tell if you did the actual addition correctly. The other error in your description (which you may or may not have also gotten wrong in your math) is that you apply 100% of the first 10 KVA, not the first 8 KVA.

    Now, if you are telling us that the "correct answer" as provided by the testing agency included the heating system in the application of the 40% demand factor, then they got that part wrong.

    Congratulations on having passed!
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  3. #3
    Join Date
    Jun 2006
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    Ohio
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    12,373
    Quote Originally Posted by charlie b View Post
    Could this be the problem?
    Quote Originally Posted by sbrehler View Post
    ... apply the damand factors of 100% for the first 8KVA plus 40% of the remaining load. 220.82(B).
    ...The other error in your description (which you may or may not have also gotten wrong in your math) is that you apply 100% of the first 10 KVA, not the first 8 KVA.
    Correct. I'm only commenting be cause I believe OP'er may have been using 220.83 Existing Dwelling Unit calculation, where it is first 8kVA at 100%, remainder at 40%.

    As for the problem as posed, the OP'er is correct. The calculated load cannot be determined as written. The "first 10kVA 100%, remainder 40%" demand is applied to the non-HVAC loads: 220.82(B). HVAC load demands are per 220.82(C)...(4) in this case: 65% for than four separately-controlled units.

    It could be the OP'er is confusing 220.82 with 220.83...
    I'll never get there. No matter where I go, I'm always here.

  4. #4
    Join Date
    Jan 2009
    Location
    Easton, Maryland NEC: 2011
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    7,465
    Quote Originally Posted by Smart $ View Post

    It could be the OP'er is confusing 220.82 with 220.83...
    I have did that once or a hundred times. Irks my brain to no end.
    "Electricity is really just organized lightning." George Carlin


    Derék

  5. #5
    Join Date
    Apr 2010
    Posts
    13
    Thanks guys. I didn't show math because they didn't show individual amounts for the applicances...just a lump sum. Including the HVAC! I was a little foggy last night in my post and inadvertently used the "existing dwelling" demand factors in my question. I knew (at the time to use the 100% of the first 10KVA and 40% thereafter. Using that on the test yielded no correct answers. But as you say,"...the testing agency..." may lead me to believe that they aren't infallible in their test questions. I wonder if anyone ever challenges them, or if they are ever evaluated by someone. Those few questions were the only ones that really threw me in how they were presented. Anyway...thanks for the help!

  6. #6
    Join Date
    Dec 2010
    Location
    Tennessee NEC:2008
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    Quote Originally Posted by sbrehler View Post
    Hi all,

    I recently took my NC limited exam and was perplexed by the following problem. It was asking (a couple of times) what the calculated loads were for dwelling units using the optional method. To paraphrase the question:

    Given a dwelling unit with the following loads (hot water heater, dishwasher, dryer and 3 separately controlled electric heating units) with a total of 18KVA (I don't recall the exact number), what is the calculated load.

    I understood that one was supposed to take the sum of the VA/sq ft + laundry + 2 kitchen ckts, plus the sum of 4 or more fastened in place appliances, THEN apply the damand factors of 100% for the first 8KVA plus 40% of the remaining load. 220.82(B).

    Then one is supposed to add the appropriate selection from 220.82(C)...heat and air to come up with a total load.

    I don't understand how I was supposed to come up with the correct answer when they lumped the HVAC into the total given in the question. I tried to apply the demand factor to the numbers they gave, but still couldn't come up with an answer.

    They did the same thing with a multi-dwelling unit too! Perhaps I got it right on the multiple-guess.

    Am I correct in my application of the optional method? I'd like to clarify it if I can.

    BTW, I passed!

    Thanks
    Since you paraphrased and didn't include the actual wording of the question I can only offer a guess. You might have all the info they asked, just not word for word.
    But from what (you) posted, the question might be simplier than you would think. These tests are notorious for putting in unneeded info into their questions. Since they just gave a total of 18KVA and then asked what is the calculated load, they might have just wanted 18KVA/240V=75A.
    They didn't include the other info as it was not needed for the answer to what they actually were asking.
    Just my educated guess though.
    If you aim at nothing, you will hit it every time!

  7. #7
    Join Date
    Mar 2004
    Location
    Collier County (Naples), Florida, USA
    Posts
    55
    Quote Originally Posted by Little Bill View Post
    Since you paraphrased and didn't include the actual wording of the question I can only offer a guess. You might have all the info they asked, just not word for word.
    But from what (you) posted, the question might be simplier than you would think. These tests are notorious for putting in unneeded info into their questions. Since they just gave a total of 18KVA and then asked what is the calculated load, they might have just wanted 18KVA/240V=75A.
    They didn't include the other info as it was not needed for the answer to what they actually were asking.
    Just my educated guess though.
    I'd have to agree, since there are no specific load details included; i.e the heat and appliances.
    I was always taught to never add information to the question.
    BTW, of the 12+ certification exams I have taken in my career, I have only had to take 2 twice; Journeyman Electrician and Electrical Plans Examiner. Knock on wood for continued luck on my behalf.
    David A Engelhart
    Florida Electrical Inspector BN4045 Plans Examiner PX2057
    ICC (International Codes Council) Certified 1&2 Family Dwelling Inspector
    Florida Fire College and ICC Fire Safety Inspector I and Plans Examiner
    2011 President Joseph A Schneeberger/Florida Gulf Coast Division, Florida Chapter, Southern Section International Association of Electrical Inspectors (IAEI).

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