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Thread: Can someone please explain this to me?

  1. #1
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    Can someone please explain this to me?

    I am still studying for my WV Master Electricians Exam. I have been going along pretty good but I just hit a brick wall. Here is the question from my practice test.
    A 30 amp 230 volt load is located 100 feet from the source. What is the minimum size branch circuit conductor required to operate within the limits for voltage drop that is recommended be the NEC?
    a. # 10 conductor
    b. # 8 conductor
    c. # 6 conductor
    d. #4 conductor

    The book says the correct answer is B. I have no idea how to come to this conclusion. Can anyone help me?

  2. #2
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    You need to do a voltage-drop calculation based on the voltage & load supplied, spread across the choices of conductors, to see what's the smallest size that will keep the voltage drop to the NEC-suggested minimum.

  3. #3
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    Quote Originally Posted by kefox81 View Post
    I am still studying for my WV Master Electricians Exam. I have been going along pretty good but I just hit a brick wall. Here is the question from my practice test.
    A 30 amp 230 volt load is located 100 feet from the source. What is the minimum size branch circuit conductor required to operate within the limits for voltage drop that is recommended be the NEC?
    a. # 10 conductor
    b. # 8 conductor
    c. # 6 conductor
    d. #4 conductor

    The book says the correct answer is B. I have no idea how to come to this conclusion. Can anyone help me?

    I used a voltage drop calculator app by "Southwire" and it says #10

    There are others online, try some and see what they show.

    Sometimes answer keys in study material are wrong too!

    Side note: as a rule of thumb, I've found that 100' doesn't change the ampacity much so just figure what's needed for the load amps if 100' or less.
    If you aim at nothing, you will hit it every time!

  4. #4
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    Can you show me please?

    Quote Originally Posted by 480sparky View Post
    You need to do a voltage-drop calculation based on the voltage & load supplied, spread across the choices of conductors, to see what's the smallest size that will keep the voltage drop to the NEC-suggested minimum.
    Can you show me please?

  5. #5
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    Quote Originally Posted by kefox81 View Post
    Can you show me please?
    Go to Mike Holt's site and check his formula
    http://www.mikeholt.com/technnical-v...s-part-one.php
    If you aim at nothing, you will hit it every time!

  6. #6
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    Thanks

    I appreciate that.

  7. #7
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    Quote Originally Posted by Little Bill View Post
    I used a voltage drop calculator app by "Southwire" and it says #10
    ...........
    Did you enter 100 or 200 feet of conductor?

  8. #8
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    Quote Originally Posted by kefox81 View Post
    I am still studying for my WV Master Electricians Exam. I have been going along pretty good but I just hit a brick wall. Here is the question from my practice test.
    A 30 amp 230 volt load is located 100 feet from the source. What is the minimum size branch circuit conductor required to operate within the limits for voltage drop that is recommended be the NEC?
    a. # 10 conductor
    b. # 8 conductor
    c. # 6 conductor
    d. #4 conductor

    The book says the correct answer is B. I have no idea how to come to this conclusion. Can anyone help me?

    Bear in mind that when you get questions like this, the voltage drop information could be a red herring. You could go through your voltage drop calculation and determine a size of conductor, that is insufficient for the circuit amps. Sometimes the voltage drop might be the factor that governs the answer, other times ampacity may govern it instead. In this case, all sizes indicated are sufficient for 30A noncontinuous, so voltage drop is the only calculation that will matter.

    For lack of additional information, we assume it is a single phase circuit, and we assume copper wires. Which means the total round trip length of the circuit, is twice the distance given. Call the given length L. The round trip length is 2*L.

    The wire has a resistance per unit length. Call it r. The total resistance (R) can be calculated with R = 2*L*r. This is a value in the lookup tables in units of thousand feet (kft) and kilometers, so you will often divide by 1000 to take care of this.

    Call the operating current, in units of amps, I.

    Construct Ohm's Law for voltage drop (dV):
    dV = I*R
    dV = I*2*r*L

    Solve for r:
    r = dV/(2*I*L)

    The NEC recommends (not requires, but recommends) not exceeding 3% voltage drop. For dV, plug in 3% of 230V. Plug in 30A for I, and plug in 100 ft for L. Our result is 0.0015 Ohms/ft, which translates to 1.15 Ohms/kft.

    We need a wire resistance per unit length that is does not exceed 1.15 Ohms/kft, which is #8 at 0.778 Ohms/kft.

  9. #9
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    Quote Originally Posted by 480sparky View Post
    Did you enter 100 or 200 feet of conductor?
    Well since the question said 100', that's what I put in.
    If you aim at nothing, you will hit it every time!

  10. #10
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    I get #10 as well. There are a few ways I can think of that they got #8 tho:

    1) Total recommended feeder and branch circuit drop is 5%. If you split that equally amongst the 2, you would need #8 to be below 2.5% VD for the branch circuit; #10 gets you to 2.63% per the Southwire calculator:

    1 conductors per phase utilizing a #10 Copper conductor will limit the voltage drop to 2.63% or less when supplying 30.0 amps for 100 feet on a 240 volt system.

    2) The load is a continuous load, so you would be limited to 80% of the breaker's capacity. You cant put #10 on a 40A breaker per 240.4(D) unless it's a motor, thus the need for #8.

    3) The answer is wrong.

    Changing the input variables from 230 or 240V, buried or conduit, even 1 or 3 phase, dont appreciable change the numbers you get. Making the run length 200' jumps you to #6 conductor. The only way you get #8 with the values given is if you use Al wire.

    eta: given the typo in the OPs question ("be" vs "by"), it isnt directly copy/pasted, thus there may also be a key piece of information missing.
    Electricians do it until it Hertz!

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