You have to separate X and R. X=SQRT(Z^2-R^2).The more suitable formula for three phases system it is:
VD=SQRT(3)*( X*SINFI+R*COSFI) x L x I /1000
COSFI=PF; SINFI=SQRT(1-COSFI^2)
The conductor resistance depends on temperature. For cross section more than 1/0 skin effect and proximity effect has to be taken into consideration.
If the cable is three-core cable then the rated voltage is involved for reactance. If the cables are single-core cable then the distance between conductors center line is necessary in order to calculate the reactance.
X=2*pi()*f*(0.1404*log10(S/r)+0.0153)/10^3 ohm/1000 ft
S= distance between conductors center line [inches]
r=conductor dia/2 [inches]
