How is the voltage drop calculated?

Branch circuits are the conductors located between the last over current device and the utilization equipment.  Branch circuits may be located in the service equipment or in a panel supplied by a feeder.  When we look at voltage drop, remember the total of 5% allowed is between the Feeder and branch circuit.  If we compensate or size the wire for 3% volt drop on the feeder, then we must compensate for only 2% on the branch circuit, the total being 5%.  If we compensate or size the wire for 2% volt drop on the feeder, then we must compensate for only 3% on the branch circuit, the total Vd being 5%. Vd = I x Rw or voltage drop = Load x the Resistance of the wires. Rw can be determined by Chapter 9 Table 8 conductor properties using the column marked DC resistance for uncoated wires where the resistance values for the size wire being used in the ohms for every kilofeet or 1000 feet. Therefore for single phase Vd = I x 2 wires x Rw of 1 wire. For 3 phase Vd = 1.73 x I x Rw X 1 wire. Rw of one wire = circuit length/1000feet x resistance value given in chapter 9 table 8.