Voltage Drop

Ohm's law and calculating voltage drop

Voltage drop can be calculated using Ohm's law like

E = R I         (1)

where

E = voltage drop (volts, V)

R = electrical resistance (ohms, Ω)

I = current (amps, A)

Example - Voltage Drop

Voltage drop in a 100 ft power line:

can be calculated as

 E = 2 (0.00102 Ω/foot) (100 ft) (10 A) 

    = 2.04 V

Circular mils and Voltage drop

Voltage drop can also be calculated using mils like

E = 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 resistivity various wires

  • Solid Copper, K = 11 (temp 77oF - 121oF), K = 12 (temp 122oF - 167oF)
  • Solid Aluminum, K = 18 (temp 77oF - 121oF), K = 20 (temp 122oF - 167oF)
  • Stranded Copper, K = 11 (temp 77oF - 121oF), K = 12 (temp 122oF - 167oF)
  • Stranded Aluminum, K = 19 (temp 77oF - 121oF), K = 20 (temp 122oF - 167oF)

Example - Specific resistivity and Voltage drop

With values from the example above the voltage drop can be calculated as

E = (11 ohm - circular mils/foot) 2 (100 ft) (10 A) / (10400 mils)

    = 2.11 V

Copper Conductor - Voltage Drop Table

The voltage drop in copper conductors can be estimated with

E = f I L (3)

where

f = factor from table below

I = current (amps)

L = conductor length (ft)

AWG Factor
Single-phase 3-phase
14 0.476 0.42
12 0.313 0.26
10 0.196 0.17
8 0.125 0.11
6 0.0833 0.071
4 0.0538 0.046
3 0.0431 0.038
2 0.0323 0.028
1 0.0323 0.028
1/0 0.0269 0.023
2/0 0.0222 0.020
3/0 0.019 0.016
4/0 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 - Amps and electrical wiring, AWG - wire gauge, electrical formulas, motors and units

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