Chapter 10 Motor Installation, Review Questions
What is the minimum conductor ampacity, overload protection, and dual-element fuse rating for a circuit supplying a 560-volt, 3-phase, code J, 40°C, 125-horsepower motor with a nameplate full-load current of 115 amperes?
Minimum Conductor Ampacity- 156 amperes Overload Protection- 144 amperes Dual Element Fuse Rating- 219 amperes
What is the minimum conductor size, overload protection, and inverse-time circuit breaker rating for a circuit supplying a 230-volt, 3-phase, code A, 40°C, 75-horsepower synchronous motor operating at an 80% power factor with a nameplate full-load current of 185 amperes?
Minimum Conductor Size- 250 kcmil Minimum Overload Protection- 250 amperes Inverse-time Circuit Breaker Rating- 250 amperes
Five, 5 HP, 3- phase motors with NEMA code B are connected to a 240-volt line. The conductors are copper with type THWN insulation. What size conductors should be used to supply all of these motors?
#6 AWG
The short-circuit protective device supplying the motors in question #12 is an inverse time circuit breaker. What size circuit breaker should be used?
250 amp
Three motors are connected to a single branch circuit. The motors are connected to a 480-volt, 3- phase line. Motor #1 is a 50 HP induction motor with a NEMA code B. Motor #2 is 40 HP with a code letter of H, and motor #3 is 50 HP with a NEMA code C. Determine the conductor size needed for the branch circuit supplying these three motors. The conductors are copper with type THWN-2 insulation.
3/0 AWG
What size conductor would be required to supply three motors connected to a 440- volt, 3-phase branch circuit? Motor #1 is a 50-horsepower, code B induction motor; motor #2 is a 40-horsepower, code H induction motor; and motor #3 is a 50-horsepower, code J induction motor.
3/0 AWG
What is the full-load current of a 230-volt, 3-phase, 125-horsepower synchronous motor when it is operated at an 80% power factor?
316 amperes
What is the full-load current of a 500-volt, DC, 20-horsepower motor?
34 amperes 27 amperes
What is the full-load current of a 600-volt, 3-phase, 50-horsepower induction motor?
52 amperes
What is the rated full-load current of a ¾-horsepower, 208-volt single-phase motor?
7.6 amperes
A 75 HP, 3-phase, synchronous motor is connected to a 230-volt line. The motor is to be operated at 80% power factor. The motor nameplate 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 THWN insulation. The short-circuit protective device is to be an inverse time circuit breaker. Determine the conductor size, overload heater size, circuit breaker size, and the minimum size NEMA and IEC starters. The starter contains current transformers with a ratio of 300:5 to reduce current to the overload heaters.
Conductor Size- 250 kcmil Overload Heater Size- X40 Circuit Breaker Size- 250 amp Minimum NEMA Starter Size- NEMA - 5 Minimum IEC Starter Size- IEC- S
Which NEC table is used to determine the full-load current of a torque motor?
NEC Table 430.7(B)
What are the minimum allowable ampacities of the phase conductors and the neutral conductor that are to supply a 2-phase, 3-wire, 230-volt, 30-horsepower motor?
Phase = 69 Amperes; Neutral = 97.29
