Voltage drop

Single phase voltage drop:
Vd = 2K x L x I / Cm or Cm = 2K x L x I / Vd
For both examples: Vd = voltage drop, K= 12.9 for resistance for a copper conductor, L = length of circuit in ft., I = current or amps of the load, Cm = area of the conductor in Circular Mills.
Three phase voltage drop:
Vd = 1.73K x L x I / Cm or Cm = 1.73K x L x I / Vd
The first examples give you the voltage drop if you have the length of the conductor, the current draw of the load in amps and the size of the wire in Circular Mils.
The "or" examples give you the size of of the wire in Circular Mils if you have the length of the conductor, the current draw of the load in amps and the voltage drop you are considering.
I think that the NEC allows a 3% drop for feeders and a 5% drop for branch circuits, but I would have to confirm this in the handbook.
Find the voltage drop on a # 6 THWN copper, 3 wire, 120/240 Volt, single phase feeder circuit of 150 foot length having a 60 A load.
Use the formula above, and substitute the given values.
VD = (2 * 150' * .491 * 60) / 1000' = 8.838 Volts
The voltage drop is 8.838 Volts, we now have to check the % from the overall voltage.
% = (VD / V) * 100
Substituting values gives us: % = 8.838 Volts / 240 Volts = 0.0368 %
This falls above the 3 % max. allowed, so you would check to see if you can use a # 4 THWN copper. (check NEC Chapter 9 Table 8 - Conductor Properties - column- ohm / kft = 0.308)
Do the same as the above example, substituting the resistivity value for # 4 copper, and if the VD falls under the permitted 3 % range then use it!

Example (2) using VD = (2 * L * K * I) / CM
Find the voltage drop on a # 4 THWN copper, 3 wire, 120/240 Volt, single phase feeder circuit of 200 foot length having a 50 A load.
Use the formula above, and substitute the given values.
Use 12 for the resistivity constant for copper
VD = (2 * 200' * 12 * 50) / 41740 = 5.75 Volts
The voltage drop is 5.75 Volts, we now have to check the % from the overall voltage.
% = (VD / V) * 100
Substituting values gives us: % = 5.75 Volts / 240 Volts = 0.0239 %
This falls under the 3 % max. allowed, so you can use it!
Calculating Voltage Drop - 3 Phase
When calculating the voltage drop in 3 phase circuits, do the same as in the single phase but the voltage drop must be multiplied by 0.866. Basically the voltage drop between any two phases conductors is multiplied by (1.732 / 2) = 0.866.
Example
Find the voltage drop and size THWN copper, 120/208 Volt, 3 phase feeder circuit of 200 foot length having a 150 Amp load.
Try 3/O copper with resistivity of 0.0766
Step (1)
VD = (2 * L * R * I ) / 1000 ft = (2 * 200 * 0.0766 * 150) / 1000 ft = 4.596 VD
Step (2) as per 215-2(b)
VD = 4.596 * .866 = 3.98
Step (3)
208 Volts * 3 % = 6.24 Volts (max. allowed)
So, the VD = 3.98 Volts which is below the 6.24 Volts allowed.
Even though this size wire would work, try the next lower size wire to see if that would also work.
The reason is that if you can do the job with a smaller size wire, it will cost you less in supplies and leave you with more profit. (Check the 2/0 copper!)