Can anyone help me????

Discussion in 'General Electronics Chat' started by Dave Benischek, Dec 4, 2014.

  1. Dave Benischek

    Thread Starter New Member

    Dec 4, 2014
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    Hi all, I am new here and am having some problems. I am wondering how I can find the resistance of a wire of a certain AWG at a certain temp. For instance: What is the resistance of 100 meters of #12 AWG copper wire?
    Also I am stuck on how to get the answer to questions like: What is the resistance of #10 AWG at zero degrees Celcius. Any help or information would be greatly appreciated. Thanks in advance.

    Dave
     
  2. JohnInTX

    Moderator

    Jun 26, 2012
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    Here you go.
    One of many calculators on line. Click on ADVANCED CALCULATOR
    Have fun.
     
  3. Dave Benischek

    Thread Starter New Member

    Dec 4, 2014
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    Thanks for the reply. I am trying to figure it out without online help as the question is from a practice test I have. I am trying to study and the ITA will not let me bring my cell phone or tables into the exam.

    Dave
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    Well, what information WILL you have available to you?
     
  5. Dave Benischek

    Thread Starter New Member

    Dec 4, 2014
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    Anything that I can memorize.

    Dave
     
  6. JohnInTX

    Moderator

    Jun 26, 2012
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    A quick paste:
    So.. resistivity depends on the conductor, temperature, purity of the material etc etc.
     
  7. WBahn

    Moderator

    Mar 31, 2012
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    Well then that's just stupid (the exam conditions, not you) and I have a hard time believing that this is the case. What sets the bounds of the list of things that you have to have memorized when you walk in there?

    Are you expected to memorize the resistivity of copper and steel and aluminum and iron and umpteen other materials?

    Are you expected to memorize the temperature dependence of the resistivities of all of these materials?

    Are you expected to memorize the mass density of all of these materials?

    Are you expected to memorize the thermal conductivity of all of these materials?

    Are you expected to memorize the cost per pound of all of these materials?

    Are you expected to memorize the table of wire gauge sizes?

    Are you expected to memorize the tap and drill sizes for all of the various machine screw sizes, both metric and imperial?

    Are you expected to memorize the frequency allocations for broadcast radio in the FM band?
     
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  8. WBahn

    Moderator

    Mar 31, 2012
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    After about three minutes of Googling, and not having the faintest idea what ITA is, it appears to be that you are talking about the Industry Training Authority in British Columbia, Canada. Is that correct?

    If so, and guessing that you are taking one of the electrical related exams, then you will either have access to the Canadian Electrical Code Book or will be expected to bring your copy with you.

    If that's the case, then get a copy of the code book and get real familiar with what is in it and how to locate information in it and use it when answering these kinds of questions.
     
  9. wayneh

    Expert

    Sep 9, 2010
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    You'd also have to remember the effect of temperature on resistivity, for each material.

    It wouldn't be too hard to remember some of the needed values for copper wire along with the temperature formula, but what a waste. I have a spreadsheet to do all that.
     
  10. Dave Benischek

    Thread Starter New Member

    Dec 4, 2014
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    I have asked the ITA those questions and they keep telling me that I can not bring any tables into the exam or formulas. So I am trying to memorize the stats of #10 AWG copper and how I can change the stats if the question is for a different size conductor. The ITA seems not to care.

    Dave
     
  11. WBahn

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    Mar 31, 2012
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  12. Dave Benischek

    Thread Starter New Member

    Dec 4, 2014
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    I can only bring in my Canadian Electrical Code Book 2009 edition. I am not allowed to bring in any other tables or formulas. Unless these are in the code book somewhere, I can not find them.
     
  13. WBahn

    Moderator

    Mar 31, 2012
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    Since I don't have a copy of the Canadian Electrical Code Book, I can't tell you if or where the information is located. But I imagine that between the wire size tables and wire ampacity tables and other information in there that the information can be gleaned.
     
  14. Dave Benischek

    Thread Starter New Member

    Dec 4, 2014
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    Thanks all for the information everyone. Now can anyone explain to me how to get the answer to my original post???? Any help is greatly appreciated. Thanks in advance.
     
  15. wayneh

    Expert

    Sep 9, 2010
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    Here's how my Excel spreadsheet does it.
    What is the resistance of 100 meters of #12 AWG copper wire?
    1. Wire gauge is converted to copper diameter in mm =1.1229322^(36-gauge)*5/1000*25.4 12 gauge is 2.053 mm
    2. The resistance per length is computed, in Ω per 1000m =1000*$P$24/(PI()*(diameter/2)^2) where P24 contains the constant 0.017241 Ω-mm^2/meter
    3. Divide by 10 to get Ω per 100m
    4. Correct for temperature in °C =(result in #3) *(1+(T-20°)*0.00393) There is no correction at 20°C
     
  16. WBahn

    Moderator

    Mar 31, 2012
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    What is the resistance of 100 meters of #12 AWG copper wire?

    I can bet that this information is in your code book. You are looking for a table such as the one found here:

    http://en.wikipedia.org/wiki/American_wire_gauge#Tables_of_AWG_wire_sizes

    Probably somewhere in your code book is the temperature coefficient of resistivity for copper and possibly other metals used for wiring. It may be in a footnote to one of the tables. Over the temperatures you are likely to be asked about, you can probably assume a linear relationship.
     
  17. Dave Benischek

    Thread Starter New Member

    Dec 4, 2014
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    Thanks for the info, but what formula do I use? As I can not use any compute aide.

    do
     
  18. wayneh

    Expert

    Sep 9, 2010
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    You can't use paper and pencil? The spreadsheet only speeds it up. The formulae I gave you are not that complicated although there are details you'd need to memorize.

    I find it hard to believe you are expected to memorize any of this. That would teach people to rely on memory for critical work and that is just the opposite of good practice. Learning where to find, and how to use, the tables is a far better exercise.
     
  19. Dave Benischek

    Thread Starter New Member

    Dec 4, 2014
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    Thank you everyone for your comments. I would just like to know the correct formula and what numbers go where in it.

    Dave
     
  20. Dave Benischek

    Thread Starter New Member

    Dec 4, 2014
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    I am also having a hard time with another practice test question. It is asking: What is the resistance of 100 meters of #12 AWG copper wire? (#12 AWG copper conductor has an area of 3.31 mm squared). The answer from the sheet is 0.405 ohms. I have tried all formulas and numbers I can think of but I still can not get the right answer. According to a variation in the equation I made CMA=(KxL)/r to find the CMA that the wire should have. The numbers I use are as follows: R answer = 0.405 ohms, K = 10.4, L = 328.1 feet. I multiply (KxL)/r so it is (10.4 x 328.1)/0.405 and that gives me 8425.3. My problem stems from the fact that from the Table I have the Area in cm for #12 AWG is 6530. When I use that I don't get any of the answers that are on the page. Can anyone tell me what I am doing wrong? Any help would be greatly appreciated. Thanks in advance.
     
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