Voltage drop formula [Step by Step Calculations and tables]
Voltage drop formula is used to calculate the voltage dropped across the conductor. Three equations are often used for doing calculations.
Voltage drop calculation formulas  

Ohm's law (1 ϕ)  V = IR 
Single phase  V_{d} =
2K * L * I
/
CM

Three phase  V_{d} =
1.73K * L * I
/
CM

where
I = Current
R = Resistance
K = Resistance in ohms in one CM of conductors
L = Length of the circuit in feet
CM = Circular mills
K = 12.9 for copper conductors
K = 21.2 for Aluminum Conductors
————————————
For other temperatures use the equation
R_{2} = R_{1} [1 + α (T_{2} − 75)]
Where
α_{cu} = 0.00323
α_{AL} = 0.00330
Contents
Circular Mills table
The CM (circular mills) are calculated from the solid copper wire American wire gauge table:
Size
(AWG) 
Diameter
(inches) 
Area (CM) 

4/0  0.46  211600 
3/0  0.4096  167800 
2/0  0.3648  133100 
1/0  0.3249  105600 
1  0.2893  83690 
2  0.2576  66360 
3  0.2294  52620 
4  0.2043  41740 
5  0.1819  33100 
6  0.162  26240 
7  0.1443  20820 
8  0.1285  16510 
9  0.1144  13090 
10  0.1019  10380 
11  0.09074  8234 
12  0.08081  6530 
13  0.07196  5178 
14  0.06408  4110 
15  0.05707  3257 
16  0.05082  2583 
17  0.04526  2048 
18  0.0403  1624 
19  0.03589  1288 
20  0.03196  1022 
21  0.02846  810.1 
22  0.02535  642.5 
23  0.02257  509.5 
24  0.0201  404 
25  0.0179  320.4 
26  0.01594  254.1 
27  0.0142  201.5 
28  0.01264  159.8 
29  0.01126  126.7 
30  0.01003  100.5 
31  0.008928  79.7 
32  0.00795  63.21 
33  0.00708  50.13 
34  0.006305  39.75 
35  0.005615  31.52 
36  0.005  25 
37  0.004453  19.83 
38  0.003965  15.72 
39  0.003531  12.47 
40  0.003145  9.888 
41  0.0028  7.842 
42  0.002494  6.219 
43  0.002221  4.932 
44  0.001978  3.911 
Single phase voltage drop formula calculations
The singlephase voltage drop is defined as the product of 2K into current into L divided by circular mills of wire.
Example: Find the voltage drop of a 180volt single phase # 6 copper conductor which provides 20 amp current to load. The length of wire is 200 feet. Take K = 12.9 as for 75° temp.
Solution: V_{d(single phase)} = [2K * L * I]/CM
= [2 * 12.9 * 200 * 20 ]/26240
= 3.93 V
Percentage voltage drop = 3.93/200 = 1.96%
As per standards the recomend drop is less than 3% .
Three phase voltage drop calculations
The threephase voltage drop calculations are similar to single phase with the difference of factor 1.73. Here the multiplication factor is 1.73 instead of 2. While all other calculations are same.
Example: Find the voltage drop of 1 kV threephase system which uses #5 copper wire and delivers 30 amp load at a distance of 800 feet. Take K = 12.9 as for 75° temp.
Solution: V_{d(three phase)} = [1.73K * L * I]/CM
= [1.73 * 12.9 * 1000 * 200]/33100 = 16.18 V
%age V_{d(three phase)} = 16.18/1000 = 1.6%.
Ohm’s law
Ohm’s law can also be used to calculate the voltage drop. Remember that Ohm’s law is only valid for single phase calculations.
Example: What is a voltage drop of two no 6 conductors when a current of 20 Amps flows through them. Use uncoated conductor at 75° for calculations.
Solution: R = 0.510/1000 feet. In our case:
R = 0.510/1000 * 400 = 0.204
V = IR
= 20 Amp * 0.102 = 4.08