VD=2xKxDxI/Cm
I= Current
2= round trip loss (you'd use 1x if it was a balanced wye circuit)
KxD/Cm represents the resistance of this wire.
Voltage Drop = 2x Current x Resistance
D=length of wire
K/Cm=resistance per unit length.
K is the unit resistance of copper per circular mil
Cm is circular mills of the conductor. (roughly 2.8 Ohms/kcmil/100 feet DC) This equation is only accurate for DC. You can approximate AC (K is roughly 1.3 for AC), but it varies by cable size due to skin resistance and isn't terribly accurate.
This entire value (the resistance per unit length) can simply be looked up in NEC Tables 8 & 9, though. Really, you should also be adjusting it forpower factor and the type of conduit it's in and wire temperature, too, but usually if you have to go into that much detail, you should be upsizing anyhow.
