Revision 2 Unit 5

Motor branch circuits

For continuous-duty motors, the load on the branch circuit feeding a single motor is taken as …………….. of the full-load rated current of the motor.

• 200 %

• 125%

• 110%

• 100%

Motor branch circuits

What is the full load current for a 50-hp three-phase Synchronous motor if the armature voltage rating is 460 V?

• 41 A

• 52 A

• 61 A

• 101 A

Motor feeders and mains

The maximum-demand running current can be used in ……………………………….

• a.       determining the size of the wire required to carry the current safely and computing the voltage drop of the circuit.

• a.       determining the size of wire required to carry the current safely

• a.       determining the proper size of short-circuit and ground-fault protective devices for the circuit,

• No answer is correct

Motor feeders and mains

What is the percentage of full-load current which can be used to determine the starting current for single phase motor and the protection device is the inverse time breaker

• 800 %

• 250 %

• 175 %

• 300 %

Motor feeders and mains

Determine the maximum-demand starting current and maximum-demand running current for a 120-V DC feeder supplying five 3-hp motors, four 5-hp motors, and two 7.5-hp motors. The protection device is an inverse time breaker

• a.       Maximum-demand starting current = 347.7 A, Maximum-demand running current= 366.7 A

• a.       Maximum-demand starting current = 327.1 A, Maximum-demand running current= 312.6 A

• a.       Maximum-demand starting current = 114 A, Maximum-demand running current= 252.7 A

• Maximum-demand starting current = 252.7 A, Maximum-demand running current= 95 A

Voltage drop

Which of the following formulas is used for computing the voltage drop in single-phase circuits when the effect of inductance can NOT BE NEGLECTED

• VD=2K*L*I

• VD=2R*L*I

• VD=(2K*L*I)/cmil

• VD= Drop factor* dc voltage drop

Voltage drop: when the effect of inductance CAN BE NEGLECTED

What will be the voltage drop of the motor feeder if the length of the circuit is 250 ft and a 4/0 AWG Type RH copper wire is employed and maximum-demand running current of 200 A and the carrying capacity of 230 A?

“Use R not K”

• 3666.2 V

• 3.6662 V

• 6.26 V

• 6260 V

Voltage drop: when the effect of inductance CAN BE NEGLECTED

What will be the voltage drop of the motor feeder if the length of the circuit is 1000 ft and a 0 AWG Type RH copper wire is employed and maximum-demand running current of 300 A and the carrying capacity of 350 A?

“Use the chart”

• 66 V

• 60 V

• 90 V

• 6 V

Power factors of feeders or mains

Determine the power factor for a feeder supplying five 3-hp motors, two 35-hp motors, and one 60-hp motor.

• 90 %

• 0.94 Lead

• 0.8 Lag

• 0.94 Lag

Which of the following loads has the highest power factor ?

• a.       Incandescent lamp

• a.       Induction motors

• a.       Fluorescent lamps

• Arc lamps