I get 0.521Ω for 100m of #12 AWG at 20°C. Bringing the temperature down to ~ -35°C is the only way I see to get the ohms down to 0.405.
#10 AWG has to be over 70°C to get up to 0.405Ω for 100m.

I'm not sure where the problem is.

 

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

Dave

It is becoming very clear that you are no where close to being ready to take this exam. If, given a table of resistance per unit length, you can't figure out how to find the resistance of a particular length without someone telling you a formula and what numbers to put into it, then don't waste your money taking an exam for which you aren't even close to being prepared for.

 

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.

Does it make sense for an area to be in cm? Even if that made sense, does it make sense for the area of a wire that size to have a value of 6530 cm?

 

wayneh thanks for the help as I am starting to think that the answer sheet is wrong. The answers on the practice test are A) 50.1 ohms, B) 5.55 ohms, C) .501 Ohms and D) .405 ohms. I don't get any of those answers when I use the formula R=(KxL)/CML. I am wondering if I am using the wrong formula? As for WBahn, I am studying to take a test. On the practice test I have the questions that I have posted, I can not get the answers that are on the answer sheet (let alone listed under the questions). I am doing a self study and have no instructor I can ask for help or anyone else for that reason so I posted here hoping that I can get help and steer me in the right direction. Also I know that I am not allowed to take any tables or formulas into the exam that I have not memorized, I am looking for help in that department also. I do appreciate the help you have given me.

 

Does it make sense for an area to be in cm? Even if that made sense, does it make sense for the area of a wire that size to have a value of 6530 cm?

The CM stands for circular mills and not centimetres.

 

The CM stands for circular mills and not centimetres.

Ah. Okay. Thanks for the clarification.

 

WBahn, no problems. That is also why I posted my questions. I am confused as to why I am not getting the answers that the answer sheets have after I tried all the formulas and numbers. I am looking for clarification.

Dave

 

I only have worked with magnet wire. Is it possible that some other copper wire has different properties at a stated gauge? You gave the same cross-sectional area that I used, so maybe not. Reaching for straws. Could there be some nuance in the question that might give a clue? Temperature, application, that sort of thing.

 

Okay, so let's see what the table I posted would yield.

"What is the resistance of 100 meters of #12 AWG copper wire?"

You didn't say what the temperature was for this question, so we'll start with the temperature that the table is good for.

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

#12 AWG wire has a resistance of 5.211 Ω/km at 25°C. So that would be 0.521 Ω. Now, the table on Wikipedia does not state that the entire table is for 25°C, only the fusing current. So I am making a bit of an assumption there.

Based on that, the best answer is clearly (C), 0.501 Ω. If I were taking the exam, and got my result I would check it a second time to make sure I didn't goof and then answer (C) and move on.

Look at your copy of the Code Book for that table and see what entry they have and what temperature the table is for.

The difference between the two answers is (0.501 Ω - 0.521 Ω)/(0.521 Ω) = -3.84%.

From: http://hyperphysics.phy-astr.gsu.edu/hbase/tables/rstiv.html

The temperature coefficient of resistivity for copper at 20°C is 0.386%/°C. For relatively small temperature ranges, the temperature dependence of most metals is pretty linear. So that would mean that to get to 0.501 Ω the temperature would have to drop by 10°C making it 15°C. That is actually the temperature that is used for STP (standard temperature and pressure) so maybe that is what they are doing, but STP is primarily used for working with gases and such.

 

Just like with all "standards" there is considerable difference of opinion on what "STP" actually means.
The one I was familiar with was 20° C and 760 mmHg.

http://en.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressure

 

Yeah, I didn't mean to imply that there was a single STP, but I can sure see how it might appear that way. Thanks.

 

WBahn there was no temperature stated in the question. So I guess it would be 20 degrees Celsius. I don't have the table http://en.wikipedia.org/wiki/American_wire_gauge#Tables_of_AWG_wire_sizes in my 2009 Canadian Electrical Code Book.

 

I wrote the question as it appears on the practice test. I know there is not much to go on.

 

WBahn there was no temperature stated in the question. So I guess it would be 20 degrees Celsius. I don't have the table http://en.wikipedia.org/wiki/American_wire_gauge#Tables_of_AWG_wire_sizes in my 2009 Canadian Electrical Code Book.

I would be surprised that you don't have a table of wire ampacities in an electrical Code Book. Does anyone else here have access to a Canadian Electrical Code Book who can see what the table is called and where it is located, even if in a different year since the organization of these books tends to remain pretty stable?

 

Maybe the info is in here. http://members.rennlist.org/warren/elec-hbk.pdf

 

I wrote the question as it appears on the practice test. I know there is not much to go on.

There are many different "standard temperature and pressure" STP.
According to Wikipedia - Canadian electrical and gas are based on 60F. see footnote below the table of various STP used in different organizations. I'm not saying this is the answer but, if the OP is having trouble getting the exact answer, it could be the reason.

• EGIA: Electricity and Gas Inspection Act (of Canada)

http://en.m.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressure