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Thread: apartment complex - cloths dryer load calculation?

  1. #1
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    apartment complex - cloths dryer load calculation?

    I don’t remember the exact question, but I am trying to to figure out a question I got on a test I am going to be retaking soon

    The question was something like this:

    41 unit apartment building and an electric clothes dryer will be installed in each unit, what is the calculated load on all the dryers and what is the load on the neutral.

    There was 4 answers but I forget them, all I remember is 2 were 00,000 long and 2 were 000,000


    5,000 * 41 = 205,000

    205,000 * 35% = 71750

    41 - 23 = 18

    * 0.009 (0.5%)

    i am lost been trying to figure this out for way to long

    i do know what to say about the

    "2 or more single phase dryers are supplied by a 3-phase, 4-wire feeder or service, the total load shall be calculated on the basis of twice the max number connected between any 2 phases"

    none of that is mentioned in the question at all, unless something is assumed that i dont understand.

  2. #2
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    I have a few questins, but Ill wait and let someone else chime in.

  3. #3
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    I am not sure that article 220.54


    "2 or more single phase dryers are supplied by a 3-phase, 4-wire feeder or service, the total load shall be calculated on the basis of twice the max number connected between any 2 phases"


    is applied at all because that information was not supplied in the question, unless its assumed

    And the rest of the information I got was from table 220.54


    24-42 = 35% minus 0.5% for each dryer exceeding 23

    So I started with 41 dryers * 5000 watts (volt amperes)
    205,000 * 35% - 71750
    Then 41 (units/dryers) - each dryer exceeding 23 = 18
    18 * 0.5% = 0.009

    Is my work right to this point? I am kind of lost here……

    Then what ever number I come up with I multiply times 70% for the neutral, right?

    The answers given were like this
    00,000 – 00,000
    00,000 – 00,000
    000,000 – 00,000
    000,000 – 00,000

  4. #4
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    There is an example for ranges - which would be similar- in Annex D of the NEC-- Example D5(a) -- I am cooking and don't have time to work it out.

  5. #5
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    In case you don't have the book with you

    Example D5(a) Multifamily Dwelling Served
    at 208Y/120 Volts, Three Phase
    All conditions and calculations are the same as for the multifamily dwelling
    [Example D4(a)] served at 120/240 V, single phase except as follows:
    Service to each dwelling unit would be two phase legs and neutral.
    Minimum Number of Branch Circuits Required for Each Dwelling
    Unit (see 210.11)
    Range Circuit: 8000 VA ÷ 208 V = 38 A or a circuit of two 8 AWG
    conductors and one 10 AWG conductor in accordance with 210.19(A)(3)
    Minimum Size Feeder Required for Each Dwelling Unit (see 215.2)
    For 120/208-V, 3-wire system (without ranges),
    Net calculated load of 3882 VA ÷ 2 legs ÷ 120 V/leg = 16 A
    For 120/208-V, 3-wire system (with ranges),
    Net calculated load (range) of 8000 VA ÷ 208 V = 39 A
    Total load (range + lighting) = 38.5 A + 16.2 A = 54.7 A
    Feeder neutral: (range) of 8000 VA × 70% = 5600 VA ÷ 208 V = 27 A
    Total load: (range + lighting) = 27 A + 16 A = 43 A
    Minimum Size Feeders Required from Service Equipment to Meter
    Bank (For 20 Dwelling Units — 10 with Ranges)
    For 208Y/120-V, 3-phase, 4-wire system,
    Ranges: Maximum number between any two phase legs = 4
    2 × 4 = 8.
    Table 220.55 demand = 23,000 VA
    Per phase demand = 23,000 VA ÷ 2 = 11,500 VA
    Equivalent 3-phase load = 34,500 VA
    Net Calculated Load (total):
    40,590 VA + 34,500 VA = 75,090 VA
    75,090 VA ÷ (208 V)(1.732) = 208 A

  6. #6
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    Quote Originally Posted by HeyItsMe View Post
    I am not sure that article 220.54

    ...
    24-42 = 35% minus 0.5% for each dryer exceeding 23
    ...
    You are applying the demand factor incorrectly.

    For each unit over 23, the demand factor decreases by 0.5%...

    24 units at 34.5%
    25 units at 34%
    26 units at 33.5%
    27 units at 33%
    and so forth.

    For 41 dryers, it is...

    5,000kVA * 41 units * (35% - (41-23)*0.5%) =
    5,000kVA * 41 units * (35% - 9%) =
    5,000kVA * 41 units * 26% = 53.3kVA
    I'll never get there. No matter where I go, I'm always here.

  7. #7
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    Quote Originally Posted by HeyItsMe View Post
    ...
    "2 or more single phase dryers are supplied by a 3-phase, 4-wire feeder or service, the total load shall be calculated on the basis of twice the max number connected between any 2 phases"
    ...
    If your service is 208/120V 3Ø, and each apartment is supplied 120/208 1Ø... and divided equal as possible...

    41 ÷ 3 = 14 rounded up

    5,000kVA * 14×2 * (35% - (14×2-23)*0.5%) =
    5,000kVA * 28 * (35% - 2.5%)
    5,000kVA * 28 * 32.5% = 45.5kVA
    I'll never get there. No matter where I go, I'm always here.

  8. #8
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    Quote Originally Posted by HeyItsMe View Post

    Then what ever number I come up with I multiply times 70% for the neutral, right?
    ...
    If the supply is split phase, such as 120/240, you can.

    If it is 120/208V 1Ø to each apartment, you cannot apply the 70% demand factor for the individual apartment feeder neutral [ref: 220.61(C)(1)]. On the supply side of the 208/120V 3Ø to 120/208V 1Ø distribution panel you can.

    Don't forget the 70% demand factor only applies to the ranges, ovens, counter-mounted cooking units, and electric clothes dryer portion of the maximum unbalanced load... not the entire load, and not the entire maximum unbalanced load.
    I'll never get there. No matter where I go, I'm always here.

  9. #9
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    Smart $ and everyone else, thanks! I now remember that one of the answers were 53.3, and I can see what I was doing wrong with my math. Thank you very much everyone here for helping with this question! Maybe one of these days I will be able to answers questions on here too.


    long time reader, first time posting......



    edit: spelling

  10. #10
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    Quote Originally Posted by Smart $ View Post
    You are applying the demand factor incorrectly.

    For each unit over 23, the demand factor decreases by 0.5%...

    24 units at 34.5%
    25 units at 34%
    26 units at 33.5%
    27 units at 33%
    and so forth.

    For 41 dryers, it is...

    5,000kVA * 41 units * (35% - (41-23)*0.5%) =
    5,000kVA * 41 units * (35% - 9%) =
    5,000kVA * 41 units * 26% = 53.3kVA
    Wait a minute ... so, if there were 93 dryers ...

    5,000kVA * 93 units * (35% - (93-23)*0.5%) =
    5,000kVA * 93 units * (35% - 35%) =
    5,000kVA * 93 units * 0% = 0kVA

    If there are 93 dryers, we don't have to count the load from any of them?

    And if there are more than 93 dryers we get a load credit, as it were?

    I think my head just exploded.

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