Electric Circuits Voltage Drop
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 oF - 121 oF), K = 12 (temp 122 oF - 167 oF)
- Solid Aluminum, K = 18 (temp 77 oF - 121 oF), K = 20 (temp 122 oF - 167 oF)
- Stranded Copper, K = 11 (temp 77 oF - 121 oF), K = 12 (temp 122 oF - 167 oF)
- Stranded Aluminum, K = 19 (temp 77 oF - 121 oF), K = 20 (temp 122 oF - 167 oF)
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 mm2 | ||
| Single-phase | 3-phase | ||
| 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 | |
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