R = Resistance per km per phase
X = Reactance per km per phase
I = Ampere or current
l = length in meter
100 = is because of percentage
1000 = to convert length from km to meter
380 = is line to line voltage
The above formula is the standard formula for voltage drop. If you need more formula about the voltage drop and in different condition they are used please see the attached file.
Single phase or 3 phase? Vd = I x R, single phase Vd = K x l x 2 / CSA, 3 phase Vd = K x I x 1.732 / CSA 3% recommendation not code requirement. Thats what I use.