The K factor has changed through the years because the Ohms per 1000ft figure in Table 8 of Ch 9 depends on the assumed temperature. In the '87 code they stopped assuming 25C and started assuming 75C.
K is just shorthand for: (dc resistance value) X (circular mil area) divided by 1000.
You have to watch out for the assumed temperature if you are going to use the formula with a K factor...not to be confused with K factors of transformers!
[This message has been edited by Elzappr (edited 01-11-2002).]
