# Is there a Resistivity of Conductors (K) chart in the NEC?



## eric7379 (Jan 5, 2010)

Chapter 9, table 8. 2008 NEC.


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## Golden Arc (Apr 28, 2008)

eric7379 said:


> Chapter 9, table 8. 2008 NEC.



Is that the closest they have in the NEC? That one didn't have a 20 degrees Celsius rating.

The one i am looking at had one number for each material and no actual wire sizes. For example Copper was 17 and then it had other materials listed like Aluminum, Silver, etc.


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## drsparky (Nov 13, 2008)

Golden Arc said:


> Is that the closest they have in the NEC? That one didn't have a 20 degrees Celsius rating.
> 
> The one i am looking at had one number for each material and no actual wire sizes. For example Copper was 17 and then it had other materials listed like Aluminum, Silver, etc.


NEC is not an engineering book. Read the often forgotten article 90.


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## Golden Arc (Apr 28, 2008)

So they don't have any listing under 75 degrees in the NEC? I am wanting to know the CM's resistivity per foot at 20 degrees for copper or aluminum.


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## Bob Badger (Apr 19, 2009)

drsparky said:


> NEC is not an engineering book. Read the often forgotten article 90.



Exactly



> *90.1 Purpose.
> 
> (A) Practical Safeguarding.* The purpose of this Code is
> the practical safeguarding of persons and property from
> ...




If you need engineering data you have to look elsewhere than the NEC.


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## Golden Arc (Apr 28, 2008)

So what table do i need to use to find the value of K, in finding what size CM of wire to use?

K = Ohms per mil foot


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## Golden Arc (Apr 28, 2008)

eric7379 said:


> Chapter 9, table 8. 2008 NEC.


Is there another formula for finding out what size wire to use that doesn't use K?


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## Greg (Aug 1, 2007)

Golden Arc said:


> Is there another formula for finding out what size wire to use that doesn't use K?


 Everything you need is in Table 8, it contains the Kcmil and the resistance of uncoated copper. Here is a scan of one my study aids, it shows formulas and when to use them. This stuff is easy do not over think it.


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## RePhase277 (Feb 5, 2008)

The K factor isn't specifically stated in the NEC, but wire sizes and their circular mils and resistance are. Using these, you can derive it from the tables in the NEC.

For example, Chapter 9, Table 8 says that 1000 kcmil uncoated copper at 75 degrees has a resistance of 0.0129 ohms/kft, suggesting that a conductor of 1 circular mil 1000 ft. long would have 1,000,000 times the resistance = 12900 ohms. Therefore a wire 1 circular mil in cross section 1 ft. long would have a resistance of 12.9 ohms.

K = 12.9 at 75 degrees C. This can be corrected for temperature with the formula R2 = 12.9 x [1+ 0.00323(T2-75)]

At 20 degrees C, this gives a K of 10.6.


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## Golden Arc (Apr 28, 2008)

InPhase277 said:


> The K factor isn't specifically stated in the NEC, but wire sizes and their circular mils and resistance are. Using these, you can derive it from the tables in the NEC.
> 
> For example, Chapter 9, Table 8 says that 1000 kcmil uncoated copper at 75 degrees has a resistance of 0.0129 ohms/kft, suggesting that a conductor of 1 circular mil 1000 ft. long would have 1,000,000 times the resistance = 12900 ohms. Therefore a wire 1 circular mil in cross section 1 ft. long would have a resistance of 12.9 ohms.
> 
> ...




Ok i think i have it now thanks.


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## gschultens (Oct 24, 2010)

K is the resistance of one circular mil foot, or a conductor with a cross-sectional area of 1 mil and a length of 1 foot. To find the resistance of a length of conductor, you have to divide the k factor by the conductor's circular mills (look it up on an AWG chart for conductor sizes) and multiply by the length.

Think of the k factor as a single resistor with the value of k. 1 foot of conductor is the the number of those resistors in parallel given by the cmils of the conductor. Then, the length in feet is that many resistances in series. The formula is, therefore, Rc = (K x L)/cmil.

The K factor is problematic, in as much as it depends on conductor material (aluminum) and temperature. Typical values assumed in the electrical industry are copper: [email protected], [email protected], [email protected] and aluminum: [email protected], [email protected] and [email protected]

To add to the K factor being problematic is that the intervals of resistance per circular mil foot at the given temperatures does not agree with what is derived using values from the field of physics. In physics, the use the forumlation or R = Rref x (1 + alpha(T - Tref). Alpha is the temperature coefficient for resistance of a material. The values for alpha (can't do the Greek lower case alpha here) are given as 0.004014 for copper and 0.004308 for aluminum. Using these values and the formula just given results in lower values for conductor resistance at increased temperatures than those given by the standard electrical tables for 25, 50 and 75C.

To get the same values as given in the standard electrical tables for K at different temperatures, you have to assume an alpha of 0..00390 for copper and 0.00495 for aluminum. I haven't found a resolution for the difference in alpha values between the electrical industry and what physics tells us. I tend to agree with the values from physics, but since I teach in electrical, and the electrical industry values will give a greater margin of safety, I've resigned myself to using the electrical industry values.


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## Publikmizfit (Apr 18, 2021)

Golden Arc said:


> Is that the closest they have in the NEC? That one didn't have a 20 degrees Celsius rating.
> 
> The one i am looking at had one number for each material and no actual wire sizes. For example Copper was 17 and then it had other materials listed like Aluminum, Silver, etc.


You are looking for the table shown in the dc theory text book


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## Wirenuting (Sep 12, 2010)

Publikmizfit said:


> You are looking for the table shown in the dc theory text book
> View attachment 155539


This is an 11 year old Zombie thread.


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## Viggmundir (Sep 13, 2019)

Wirenuting said:


> This is an 11 year old Zombie thread.


It took him that long to find the table.... So long he forgot his sign-in info and had to make a new account to post it!


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