For up to 600 V rated NEC 2014 art. (A) Feeders Not More Than 600 Volts. (1) General. Information note no.2:
“the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent”
For service no specifically voltage drop is recommended.
If you know the type of conductor employed you may use Table 9 from ch.9
The resistance is calculated for 3 phase 60 Hz 75oC in conduit and reactance in the same way in 3 conductors in cradle.
If you intend to calculate resistance per unit [kft or km] you have to calculate skin and proximity effect and to increase these effect for conduit as per Neher and McGrath Calculation of Temperature Rise and Load Capability of Cable Systems ch. Calculation of losses.
You may use BS7671 Tables 4D2B-4D4B or to follow IEC 60287-1-1 ch.2 Calculation of losses.
