Electric Circuits Voltage Drop
Ohm's law and voltage drop in electric circuits.
Voltage drop in an electric circuit can be calculated using Ohm's law as
U = R I (1)
where
U = voltage drop (volts, V)
R = electrical resistance in the electric circuit (ohms, Ω)
I = current (amps, A)
Example  Voltage Drop
Voltage drop in a 100 ft power line:
 2 x 100 ft
 #10 copper
 electrical resistance 1.02 Ω/1000 ft
 current 10 amps
The electric resistance in the circuit can be calculated
R = (1.02 Ω/1000 ft) (100 ft) 2
= 0.204 Ω
The voltage drop in the circuit can be calculated with (1)
U = (0.204 Ω) (10 amps)
= 2.04 volts
Circular mils and Voltage drop
Voltage drop can also be calculated using mils like
U = K P L I / A (2)
where
K = specific resistivity (Ω  circular mils/foot)
P = phase constant = 2 (for single phase) = 1.732 (for three phase)
L = wire length (ft)
A = wire area (circular mils)
Specific electric resistivity for different types of wire materials
 Solid Copper, K = 11 (temp 77^{o}F  121^{o}F), K = 12 (temp 122^{o}F  167^{o}F)
 Solid Aluminum, K = 18 (temp 77^{o}F  121^{o}F), K = 20 (temp 122^{o}F  167^{o}F)
 Stranded Copper, K = 11 (temp 77^{o}F  121^{o}F), K = 12 (temp 122^{o}F  167^{o}F)
 Stranded Aluminum, K = 19 (temp 77^{o}F  121^{o}F), K = 20 (temp 122^{o}F  167^{o}F)
Example  Specific resistivity and Voltage Drop
With values from the example above the voltage drop can be calculated as
U = (11 ohm  circular mils/foot) 2 (100 ft) (10 A) / (10400 mils)
= 2.11 V
Copper Conductor  Voltage Drop Table
The voltage drop in a copper conductor can be estimated with
U = f I L (3)
where
f = factor from table below
I = current (amps)
L = conductor length (ft)
Size  Factor  f   

AWG  Metric mm^{2}  
Singlephase  3phase  
14  2.08  0.476  0.42 
12  3.31  0.313  0.26 
10  5.26  0.196  0.17 
8  8.37  0.125  0.11 
6  13.3  0.0833  0.071 
4  21.2  0.0538  0.046 
3  0.0431  0.038  
2  33.6  0.0323  0.028 
1  42.4  0.0323  0.028 
1/0  53.5  0.0269  0.023 
2/0  67.4  0.0222  0.020 
3/0  85.0  0.019  0.016 
4/0  107.2  0.0161  0.014 
250  0.0147  0.013  
300  0.0131  0.011  
350  0.0121  0.011  
400  0.0115  0.009  
500  0.0101  0.009 
Related Topics

Electrical
Electrical units, amps and electrical wiring, wire gauge and AWG, electrical formulas and motors.
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