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Thread: Basic KWH question from exam prep course seems to be broken or I'm a total moron.....

  1. #1
    Join Date
    Jul 2017
    Location
    Berkeley California USA
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    20

    Basic KWH question from exam prep course seems to be broken or I'm a total moron.....

    First, let me say that the exam prep school i'm working with is not good and I have found another.
    While Ive gotten some value from their materials, the material is rife with typos, formatting errors and is based on 2005/2008 NEC. Boo. I also have extreme strengths in some areas and weakness' in others due to the unorthodox method I entered and worked in the field. Note that the CSLB has accepted my application irregardless. I'm weak in safety regs, big jobsite protocols and employment labor laws, etc.

    However, I'm very strong technically, so I'm mystifed by this question:

    Question 10:
    In a school, twenty 75-watt incandescent lights were replaced with twenty 20-watt fluorescent bulbs. If they pay $0.10/KWH, how much will they save in a year?

    $100
    $275
    $375
    $475

    By my calculations, the existing light array is 1.5kwh so it costed $.15 per hr so yearly costs were $.15 x24x365 assuming lights on 24x7. = $1314 per year

    The New array is .4kw so costs .4x $.10 per hr or $.04 so yearly costs are $.04 x24x365=$350 a year

    ~ Approx 1/4 power, 1/4 the cost, so the savings should be greater then any of those values unless I'm making a mistake. Any help would be appreciated. Thx, Humble Hank.

  2. #2
    Join Date
    Dec 2012
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    Placerville, CA, USA
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    You are assuming that classroom and other lights are on 24 hours per day.
    See what happens when you revise that assumption.

  3. #3
    Join Date
    Jul 2017
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    Berkeley California USA
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    I ran it for 12 hrs a day

  4. #4
    Join Date
    Jul 2017
    Location
    Berkeley California USA
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    thx

    Oh you are in Placerville! Cool! I ve been spending lots of time up that way-ish

    I'll re run it for 8 hrs a day

    Yah it's really hard to figure out what the baseline contexts are for some of these questions

    thx

  5. #5
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    Jul 2017
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    Berkeley California USA
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    @!

    I ran it for 8 and 12 hrs and none of the answers appear as a choice. They were close tho, close enough to have me feel sane again

  6. #6
    Join Date
    Dec 2012
    Location
    Placerville, CA, USA
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    If eight hours by seven days does not work, try five days.

    Sent from my XT1585 using Tapatalk

  7. #7
    Join Date
    Feb 2003
    Location
    Seattle, WA
    Posts
    18,672
    The person who wrote the test question failed in his or her responsibility to provide sufficient information to allow the student to obtain an answer. I would not concern myself over finding how they came up with one of the four possible answers. You can't get it right because the question's author got it wrong. You should find another school. Oh wait, you already did that. Good decision.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  8. #8
    Join Date
    Mar 2008
    Location
    Michigan. It's a beautiful penninsula, I've looked around.
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    Quote Originally Posted by GoldDigger View Post
    You are assuming that classroom and other lights are on 24 hours per day.
    See what happens when you revise that assumption.
    And for 12 months a year.
    Cheers and Stay Safe,

    Marky the Sparky

  9. #9
    Join Date
    Oct 2009
    Location
    Austin, TX, USA
    Posts
    8,483
    Quote Originally Posted by docj67 View Post
    First, let me say that the exam prep school i'm working with is not good and I have found another.
    While Ive gotten some value from their materials, the material is rife with typos, formatting errors and is based on 2005/2008 NEC. Boo. I also have extreme strengths in some areas and weakness' in others due to the unorthodox method I entered and worked in the field. Note that the CSLB has accepted my application irregardless. I'm weak in safety regs, big jobsite protocols and employment labor laws, etc.

    However, I'm very strong technically, so I'm mystifed by this question:

    Question 10:
    In a school, twenty 75-watt incandescent lights were replaced with twenty 20-watt fluorescent bulbs. If they pay $0.10/KWH, how much will they save in a year?

    $100
    $275
    $375
    $475

    By my calculations, the existing light array is 1.5kwh so it costed $.15 per hr so yearly costs were $.15 x24x365 assuming lights on 24x7. = $1314 per year

    The New array is .4kw so costs .4x $.10 per hr or $.04 so yearly costs are $.04 x24x365=$350 a year

    ~ Approx 1/4 power, 1/4 the cost, so the savings should be greater then any of those values unless I'm making a mistake. Any help would be appreciated. Thx, Humble Hank.
    Without usage data this question is nonsensical. Since it's a school it might be 8 hours/day, 5 days/week, 9 months/year, but who knows? We know the differential in steady state load is 55W/light = (55W)(20) = 1.1kW = 1.1kWh per hour, but for how many hours per year?
    Last edited by ggunn; 08-06-17 at 12:05 PM.

  10. #10
    Join Date
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    Location
    UK
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    10,228
    Quote Originally Posted by charlie b View Post
    The person who wrote the test question failed in his or her responsibility to provide sufficient information to allow the student to obtain an answer. I would not concern myself over finding how they came up with one of the four possible answers. You can't get it right because the question's author got it wrong. You should find another school. Oh wait, you already did that. Good decision.
    Totally agree.
    Si hoc legere scis nimium eruditionis habes.

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