Example Problems
1. A 20 horsepower, DC motor is connected to a 500 volt DC line. What is the full load running current of this motor?
2. What rating is used to find the full load running current of a torque motor?
3. A 3⁄4 horsepower, single-phase squirrel cage motor is connected to a 240 volt AC line. What is the full load current rating of this motor and what is the minimum size NEMA and IEC starters that should be used?
4. A 30 horsepower, two-phase motor is connected to a 230 volt AC line. What is the rated current of the phase conductors and the rated current of the neutral?
5. A 125 horsepower, synchronous motor is con- nected to a 230 volt three-phase AC line. The motor is intended to operate at 80% power factor. What is the full load running current of this mo- tor? What is the minimum size NEMA and IEC starters that should be used to connect this motor to the line?
6. What is the full load running current of a three- phase, 50 horsepower motor connected to a 560 volt line? What minimum size NEMA and IEC starters should be used to connect this motor to the line?
7. A 125 horsepower, three-phase squirrel cage induction motor is connected to 560 volts. The nameplate current is 115 amperes. It has a marked temperature rise of 40°C and a code letter J. The conductors are to be type THHN copper and they are run in conduit. The short-circuit protective device is dual-element time delay fuses. Find the conductor size, overload size, fuse size, minimum NEMA and IEC starter sizes, and the upper and lower range of starting current for this motor.
8. A 7.5 horsepower, single-phase squirrel cage induction motor is connected to 120 volts AC. The motor has a code letter of H. The nameplate current is 76 amperes. The conductors are copper with type TW insulation. The short-circuit protection device is a non-time delay fuse. Find the conductor size, overload size, fuse size, minimum NEMA and IEC starter sizes, and upper and lower starting currents.
9. A 75 horsepower, three-phase, synchronous motor is connected to a 230 volt line. The motor is to be operated at 80% power factor. The motor name- plate lists a full load current of 185 amperes, a temperature rise of 40°C, and a code letter A. The conductors are to be made of copper and have type THHN insulation. The short-circuit protective device is to be an inverse time circuit breaker. Determine the conductor size, overload size, circuit breaker size, minimum size NEMA and IEC starters, and the upper and lower starting current.
10. Three motors are connected to a single branch circuit. The motors are connected to a 480 volt three-phase line. Motor #1 is a 50 horsepower induction motor with a NEMA code B. Motor #2 is 40 horsepower with a code letter of H, and motor #3 is 50 horsepower with a NEMA code C. Deter- mine the conductor size needed for the branch circuit supplying these three motors. The conductors are copper with type THWN-2 insulation.
11. The short-circuit protective device supplying the motors in question #10 is an inverse time circuit breaker. What size circuit breaker should be used?
12. Five 5 horsepower, three-phase motors with NEMA code B are connected to a 240 volt line. The conductors are copper with type THWN insulation. What size conductor should be used to supply all of these motors?
13. If dual-element time delay fuses are to be used as the short-circuit protective device, what size fuses should be used to protect the circuit in question #12?
14. A 75 horsepower, three-phase squirrel cage induc- tion motor is connected to 480 volts. The motor has a NEMA code D. What is the starting current for this motor?
15. A 20 horsepower, three-phase squirrel cage induction motor has a NEMA code B. The motor is connected to 208 volts. What is the starting current for this motor?