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Thread: Test Questions

  1. #1
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    Test Questions

    I have a lot of questions as I prepare for journeyman test. All help is greatly appreciated!

    What is the minimum size copper conductor needed for a 208 V three-phase machine located 140 feet from the power source? ( Machine draws 27 amperes, 3% voltage drop, use 12.9 K)

  2. #2
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    Quote Originally Posted by sclement View Post
    I have a lot of questions as I prepare for journeyman test. All help is greatly appreciated!

    What is the minimum size copper conductor needed for a 208 V three-phase machine located 140 feet from the power source? ( Machine draws 27 amperes, 3% voltage drop, use 12.9 K)
    I have taken several journeyman tests and I do not think that I have ever seen a voltage drop calculation on any of them. It has been a while though. As is the practice, show your method and the folks here will chime in to help.

    #PrudentPrePlannigPreventsPissPoorPerformance

  3. #3
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    Quote Originally Posted by luckylerado View Post
    I have taken several journeyman tests and I do not think that I have ever seen a voltage drop calculation on any of them. It has been a while though. As is the practice, show your method and the folks here will chime in to help.
    I've heard the same thing, not sure if Utah has a couple voltage drop questions or not? But anyway I'm not sure exactly how to do this problem? That's kinda why I posted it here for some help with it?

  4. #4
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    Quote Originally Posted by sclement View Post
    I have a lot of questions as I prepare for journeyman test. All help is greatly appreciated!

    What is the minimum size copper conductor needed for a 208 V three-phase machine located 140 feet from the power source? ( Machine draws 27 amperes, 3% voltage drop, use 12.9 K)
    #10 THWN Solid= 10380 CM
    Using Vd= 1.73*12.9*27*420 / 10380
    You get 2.72 V
    Way less than 3% of 208V which is 6.24V

    12.9 = K
    27= Load Current
    420= Length of conductors (3X140)
    1.73=Sq. Root of 3

    Ampacity per 430.22 must be 125% of the FLC = 33.75 A

    #10 THW Ampacity @ 75 Degrees = 35 A
    Last edited by Unbridled; 08-13-17 at 12:47 AM.

  5. #5
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    Test question

    Quote Originally Posted by sclement View Post
    I've heard the same thing, not sure if Utah has a couple voltage drop questions or not? But anyway I'm not sure exactly how to do this problem? That's kinda why I posted it here for some help with it?
    Have taken 3 journeymen test in the last yr for diffrent states. Haven't seen any vd questions like it neither. ..I agree with unbridles' formula. But would uses 6.24 as my voltage drop in the formula since 208 is the applied voltage. 1.732*12.9×140*27/6.24=13,534cma which is larger than a number 10awg so we must go with #8..
    430.22 wouldn't apply in this case because its not stated that the motor is continous. I would take inconsideration to use what the question gives you when taken a test.They are tricky in the way the questions are asked.I'm open for corrections and learning. Thanks... formula...single phase. Cma=2×k×i×l /vd....3phase ...cma=1.732×k×i×l/vd..
    2 is the lenth back and forth,k is the costant,i is applied amperage,l is the actual length.,vd is percentage of the applied voltage. We switch (2) to(1.732) in the single to 3phase. When using this formula.

  6. #6
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    My Southwire VD calculator app says #8.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  7. #7
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    agree with post #5 and #6
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  8. #8
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    Quote Originally Posted by sclement View Post
    I have a lot of questions as I prepare for journeyman test. All help is greatly appreciated!

    What is the minimum size copper conductor needed for a 208 V three-phase machine located 140 feet from the power source? ( Machine draws 27 amperes, 3% voltage drop, use 12.9 K)
    Post #5 and 6 are correct. The way you had your question worded through me of.
    I used the voltage drop of #10 CU.(2.32 V)
    Your question states to use 3% Vd(6.24 V)

    3% Vd is the max. per NEC. In practical use, however, I don't calculate per wire size based on the possibility of a 3% Vd, I size it per the actual Vd of the conductor I'm contemplating on using.
    If you remove the wording "3%" from your question, wouldn't you calculate you conductor size just not to exceed 3%, such as #10??

  9. #9
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    Quote Originally Posted by luckylerado View Post
    I have taken several journeyman tests and I do not think that I have ever seen a voltage drop calculation on any of them. It has been a while though. As is the practice, show your method and the folks here will chime in to help.
    FYI- The Utah journeyman exam is actually 3 separate exams- code examination (80 questions), theory examination (50 questions), and a practical examination (7 practical exercises). There are 10 voltage drop calculation questions on the theory exam.

  10. #10
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    Quote Originally Posted by Unbridled View Post
    3% Vd is the max. per NEC.
    The NEC does not dictate a 3% max Vd.

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