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)

Voltage drop in a 100 ft power line:

The electric resistance in the circuit can be calculated

R = (1.02 Ω/1000 ft) (2×100 ft)

= 0.204 Ω

The voltage drop in the circuit can be calculated with (1)

U = (0.204 Ω) (10 amps)

= 2.04 volts

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)

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

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)