Ok, so we have two equations that should produce the same output for some related input. There you go, a formula to find K from ohms, circular mils and length. Notice these values of K are pretty damn close to 12.9, in fact if you try the values in table 8 of the National Electrical Code for any size copper wire, you will get an answer close to 12.9, thus we call it a constant. It is simply a shortcut to use as opposed to having to compute an exact resistance value based on the length of a wire. Now we can find ohms if we have length, Cm, and K. Ex.
Lets check it out to make sure.
So now we know where K comes from and what it is, at least I hope this helps to unmask the mystery behind it. Don't forget, Aluminum wire is done the same way, but you have to remember to use the ohm value for Aluminum from table 8 .