Materials constants-where do I find them?

[john4]

I am a new tech ed teacher teaching electronics from the Goodheart-Wilcox text Electricity and Electronics. I am in the chapter (3) on conductors and the voltage drop section there is a formula for circular mils area of the conductor. One of the variables used is the constant K which is the materials constant. The book does not define these, or at least, I could not locate them. I know I should probably call the publisher but these should be available elsewhere and the NEC should know about them. And I want to familiarize myself with the folks in the biz.

That is my question, i.e. where are these constants located?

thanks,

John4

 

[bphgravity]

Re: Materials constants-where do I find them?

"K" is simply the resistance of a conductor at one mil-foot and at X?. For instance, Table 8 of the NEC provides the ohm/kFT for copper and aluminum conductors at a DC resistance at 75?C.

To figure K, K = R x cm / 1000

Ex. What is the "K" of #10 solid copper?

Answer: K = 1.21 x 10,380 / 1000 = 12.5598-ohms.

Typically, "K" for copper is considered around 12.9 and aluminum is 21.2. These are probably a little conservative.

 

[haskindm]

Re: Materials constants-where do I find them?

Bryan has provided a perfect explanation of the voltage drop constant. I have seen the constant referred to as "the circular mil foot resistance".

 

[bob]

Re: Materials constants-where do I find them?

K varies with the temperature. K at 20C is 10.5
Some of the cable mfg have this information.

[ March 02, 2006, 02:45 PM: Message edited by: bob ]

 

[bdarnell]

A good way to remember these values is to look at the resistance for 1000 kCMIL copper in Table 8. You'll see it's 0.0129 per Mft. Therefore, the resistance for a 1 CMIL wire would be 12.9. Aluminum works out the same way.

(Thanks Mike Holt for this handy tip for those of use who are old and can't remember the "K" in VD=2KID/CM.)