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Thread: calculation method for a duplex?

  1. #1
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    calculation method for a duplex?

    Hi I recently took my wa. Administrator gen. 01 exam and passed all portions except for the major load calcs. I felt thoroughly prepared until I was ambushed with duplex questions. In which none of my practice exams and books cover. nor are their any examples of a duplex calc. In the Annex D of the 2014 nec code book. My question is which calculation method does a duplex fall under? Single family or multifamily. Article 100 definitions is single family: one family. Duplex: IS Two family. and Multifamily is 3 or more units. Alittle confusing to me on which method to choose. In my exam I worked the question as single family and fell short. Can anyone help me out, thankyou.

  2. #2
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    Without getting a better idea of the nature of the questions on the test, it will be hard to answer your question. But in general, you calculate the load for each one of the two units as though it were a stand-alone single family dwelling unit. Then if they ask for the service load to the duplex as a whole, you add all the loads for both units, and apply whatever demand factors are available. For example, let's look at 220.53. If each unit has a dishwasher, a garbage disposal, and a trash compactor, then you can't take advantage of the 75% demand factor when you are calculating the unit. But when you calculate the duplex, you have a total of 6 fixed-in-place appliances, and you can apply the 75% demand factor.

    That is about as far as I can go, without more information about the test question. I hope you do better on your next attempt. It took me two attempts to pass my Administrator test. But that was (gulp! ) 15 years ago. The test will have changed since then.

    Welcome to the forum.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  3. #3
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    In addition to what Charlie said, see 220.85.

  4. #4
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    General method calc. for duplex?

    Thank you both for your earlier responses. A major Family Emergency has delayed my reply time. With your advice I have figured the optional method for a duplex but am still struggling with the general. Can you give me some pointers on where I may be going wrong here on this made up calculation? Im really looking for the proper steps to take.

    Size the minimum service for the following duplex. Each unit includes the following

    1500sq.ft. living area
    2-5 hp single phase 240v A/C
    15 kw space heat 240v
    14 kw range 120/240v
    4 kw water heater 240v
    5 kw clothes dryer 120/240
    2 kw dishwasher 120v
    1/2 hp compactor 120v
    1/3 hp disposal 120v


    my solution:
    1500 sq.ft. x 3va 4500va
    small appliance 2 x 1500 3000va
    laundry 1 x 1500 1500va
    9000va


    Table 220.42 Ltg. demand:
    first 3000va @ 100% 3000va
    remaining 6000 @ 35% 2100va
    5100va

    5hp A/C 28a x 240v=6720va x2=13440
    compare to 15kw space heat (omit a/c) 15000va

    w.h 4kw x75%= 3000va
    d.w kw x 75%= 1500va
    comp.1/2 hp 9.8a x 120v=1176 x 75%= 882va
    disp.1/3 hp 7.2a x 120v=864 x 75%= 648va

    5kwdryer 5000va


    14kw range table 220.55 column c 11x10%=12100

    largest motor comp. 1176 x25%= 294va
    total 43524va/240v=181.35a

    I feel like I should be multiplying by 2 or something somewhere to account for the other unit please help.

  5. #5
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    I quoted in red but I didn't check everything

    Quote Originally Posted by sparkwell View Post
    Thank you both for your earlier responses. A major Family Emergency has delayed my reply time. With your advice I have figured the optional method for a duplex but am still struggling with the general. Can you give me some pointers on where I may be going wrong here on this made up calculation? Im really looking for the proper steps to take.

    Size the minimum service for the following duplex. Each unit includes the following

    1500sq.ft. living area
    2-5 hp single phase 240v A/C
    15 kw space heat 240v
    14 kw range 120/240v
    4 kw water heater 240v
    5 kw clothes dryer 120/240
    2 kw dishwasher 120v
    1/2 hp compactor 120v
    1/3 hp disposal 120v


    my solution:
    1500 sq.ft. x 3va 4500va
    small appliance 2 x 1500 3000va
    laundry 1 x 1500 1500va
    9000va

    I would multiply this by 2 and then take your 100%/35%

    Table 220.42 Ltg. demand:
    first 3000va @ 100% 3000va
    remaining 6000 @ 35% 2100va
    5100va

    5hp A/C 28a x 240v=6720va x2=13440
    compare to 15kw space heat (omit a/c) 15000va

    w.h 4kw x75%= 3000va
    d.w kw x 75%= 1500va
    comp.1/2 hp 9.8a x 120v=1176 x 75%= 882va
    disp.1/3 hp 7.2a x 120v=864 x 75%= 648va

    5kwdryer 5000va


    14kw range table 220.55 column c 11x10%=12100

    This is not correct. Table 220.55 says to take 8kw as the load for any range over 12 and add 5% for each kw greater than 12. 14-12= 2kw 2x 5%= 10% 8x 1.1=8.8kw

    largest motor comp. 1176 x25%= 294va
    total 43524va/240v=181.35a

    I feel like I should be multiplying by 2 or something somewhere to account for the other unit please help.

  6. #6
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    Dec 2006
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    Did you get the mike holt spreadsheet I sent you? It has all kinds of calculations- single family, multifamily, duplex, dryer, ranges, schools, etc. Check your spam if you didn't get it.

  7. #7
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    duplex calcs

    yes I did receive the spread sheet. Thank you for that and the quotes as well. I feel these will be very helpful in my quest...your time is very much appreciated thank you.

  8. #8
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    duplex calc. range demand

    regarding your correction on my range demand. I used 11kw because I counted 2 14kw ranges due to having a duplex. should I have only counted 1 range?

  9. #9
    Join Date
    Dec 2006
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    Quote Originally Posted by sparkwell View Post
    regarding your correction on my range demand. I used 11kw because I counted 2 14kw ranges due to having a duplex. should I have only counted 1 range?

    No, my bad, you are correct... I didn't consider the duplex so you got it right

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