But Op already mentioned he had made adjustments for more then 3 current carrying conductors in raceway. Though such adjustments do not always equate to an increase in conductor size, if they do voltage drop is changed. Voltage drop is independent of conductor ampacity. There are times when the run is short you can run a smaller conductor then required minimum ampacity and still satisfy any voltage drop requirements you may have. So one must determine minimum ampacity needed, then check to see if that conductor allows desired/required voltage drop allowances.
The confusion IMO is back in the OP where the term resistance derating and adjusting resistance are used. Nobody seems to know what that means, and the NEC doesn't use such terms. NEC does use terms like voltage drop and ampacity adjustment. Otherwise what is adjusting resistance? To me it is turning the knob on a potentiometer.:happyyes:
In this case, adjusting resistance means calculating a representative value for the resistance of the conductor at its operating temperature, from a known resistance value at a standard temperature and from the temperature coefficient of resistivity. It isn't an adjustment in the sense that you have physical control over it, like a potentiometer.
It is similar to the concept of temperature-adjustments in open circuit voltage for photovoltaic systems.
As I mentioned, this calculation is overkill in most applications. But if one wants to anticipate the physical difference between a 90 C operating conductor and a 75 C operating conductor, there is nothing wrong with accomodating this in a design.
