Re: voltage drop
Let?s take this back to basic principles. The resistance of any isotropic material (fancy word for ?properties are the same in any direction? ? we can include our common wires in this category) is found from
R = (rho) times ( l / A ),
where
?rho? is resistivity of the material (Not the same rho that shows up in Table B.310.7. It has units of resistance times length, e.g., ohms-ft),
?l? is the length of the material (i.e., wire), and
?A? is the cross sectional area of the material (i.e., wire).
(Reference: My 30+ year old High School physics book that I still keep at my desk.)
If you change the area from 1 cm to 1000 cm, you cut the resistance by a factor of 1000. If you change the length from 1 foot to 1000 feet, you increase the resistance by a factor of 1000. If you do both, you do not change the total resistance. In this respect, awwt was correct. In light of this truth, the formulas given by dereckbc and justin were both right. Therefore, justin was incorrect, when he said that dereckbc?s formula was incorrect.
As to the use of ?K? (versus ?k?): Within the scientific, mathematical, and engineering communities, we have made use of all 26 English letters (in both upper and lower cases), all 24 Greek letters (in both upper and lower cases), and a host of letters whose origins elude me. We?ve run out of letters, and we have to keep re-using them (witness the two uses of the Greek letter rho, as discussed above). The letter ?K? and its lower case version have both been used to denote ?kilo,? a prefix meaning 1000. In the context of the original question, the letter ?K? was probably chosen for this specific constant for no better reason that that it is one of the more popular letters used to denote constants.
As to dereckbc?s reference to the values shown in Table 8, the column head clearly shows the units of ohms/1000 ft. But that Table does not address the ?K? issue. If you know the formula, you can calculate an ?exact K? from the tabulated resistance values. Or you can just use the ?approximate K? values given by several contributors above.