I have an instructor telling me that to do a VD calc, you use the following formula:
VD=(Amps x 1 way Length x R)/1000'

Where R = ohm/kFt from NEC Ch 9, Table 8

It seems to me like you would have to double the length since we are only counting the resistance in one direction...is this correct? I have always used the online calculators, and I'm trying to learn how to do it on paper.

Thanks, Mike

Yup. One way or the other you need to incorporate the round trip distance.

Some equations do this by fudging the resistance values (essentially using the resistance of 2000 feet of conductor, and reporting it as the resistance for 1000 feet 'there and back again')

Others look at the voltage drop on one side only because there is assumed to be no voltage drop in the neutral.

But in all cases there needs to be some consideration of the full circuit.

-Jon

VD=(Amps x 1 way Length x R)/1000'
This would be the correct formula if you are
using a 240 volt 2 wire circuit. There is no return distance required.
If you have a 2 wire 120 volt circuit then you need to count the return distance in the caculation.

Also, you need to use a return distance of the square of three (1.73) for a three phase circuit.