Just use NEC, Chapter 9, Table 8 and my handy formula (based on Ohm's Law):
3 phase VD = 1.73 x I x R x L / 1,000
where VD is the voltage drop
I is the current in amperes
R is the resistance per 1000 feet found in Table 8 for your wire size and type
L is the length of one conductor
I like this one because you don't have to worry about remembering "k"
If you need single phase, then substitute "2" for "1.73". If you want an even closer estimate of VD, then use Table 9, which also considers the inductive reactance of the conductors coupled with the type conduit.
