Deck 9: Voltage Drop Across Conductors

Question 1

The accompanying figure shows a motor operating from a 220-volt DC source and drawing 75 amperes at full load. If a 3 percent drop in line voltage is permitted, find the smallest size of conductor to use in a line 100 feet in length. Express your answer in circular mils. (Note: K=12.9 ohms/mil-foot)

Answer 1

Vd = 0.3 * 220 Volts = 6.6 Volts
CM = KIL/Vd
CM = [(12.9 ohms/mil-foot)*(75 amperes)*(200 feet)]/6.6 Volts = 28043.48 circular mils

Question 2

Voltage drop is not affected by the insulation on a conductor.

Answer 2

True

Question 3

If a motor operates from a 220-volt DC source and a 5 percent drop in line voltage is permitted, the maximum Vd is ____________________.

Answer 3

11 volts

Question 4

Resistance in a conductor is a factor of its ____________________ and area in circular mils or its diameter.

Question 5

Because of conductor resistance, the voltage applied to a motor is always greater than the voltage provided by the generator.

Question 6

Any conductor resistance causes a voltage drop that is determined by E = IR.

Question 7

The formula: CM=KIL/Vd calculates the maximum acceptable conductor size for a given circuit.

Question 8

Voltage drop is the loss of electrical potential in a conductor due to its ____________________.

Question 9

The accompanying figure shows a DC generator supplying 75 amperes to a motor located 250 feet away. The conductors are No. 4 copper wire. Find the voltage applied to the motor if the generator operates at 300 volts. (Note: K=12.9 ohms per mil-foot; CM=41,740 circular mils)

Question 10

The selection of a conductor is usually a compromise between cost and the permissible ____________________.