Electrical Engineering is a free introductory textbook to the basics of electrical engineering. See the editorial for more information....

Voltage Drop of Two-Wire and Three-Wire Systems

Author: E.E. Kimberly

In the twowire line in Example 3-4, there is a voltage drop equal to


In order that 230 volts may be available at the load, the generator voltage must be raised to 230 + 20 = 250 volts. The difference between the sending-end (generator) voltage and the receiving-end (load) voltage is called the line voltage drop.

If, in a three-wire circuit, the load is not equally divided between the two parts of the circuit, the voltages across the two parts at the load will not be equal. This inequality is caused by resistance in the middle wire.

Example 3-7. - A constant-voltage double generator set supplies 'power over a three-wire line with a resistance of 0.5 ohm per wire to unbalanced-load resistance units of 20 ohms and 30 ohms, as in Fig. 3-7. Calculate the currents in all three lines and the voltage appearing at each section of load.

Solution. - The load resistor of lesser value will have a greater current. Hence, Im will be in the direction shown, and


The sum of all voltage drops in a series circuit is equal to the applied voltage. In this case, the voltages applied to the top and bottom parts of the circuit are 125 volts. Therefore, by Kirchhoff's voltage laws, the equations are:


It having been established that Im is flowing toward the left, the ImRm drop is from right to left as indicated. Hence, in traversing the lower circuit in the direction of Ib, the established ImRm drop is taken as negative. Since Im = Ia - Ib,


By simultaneous solution Ib = 4.12 amp and Ia = 6.05 amp.

Then, Im = Ia - Ib = 1.93 amp.


Last Update: 2010-10-06