Lesson 5
As the motor accelerates, ? starters provide very smooth starting due to increasing voltage across the motor terminals.
Primary-resistor
One 15-horsepower, 440-volt, 3-phase, continuous-duty, induction type Design B motor and one 25-horsepower, 440-volt, 3-phase, continuous-duty, induction type Design B motor are supplied by a single motor branch circuit. Calculate the minimum ampacity for the single motor branch-circuit conductors supplying both motors.
63.5 A
What is the maximum rating of the motor branch-circuit short-circuit and ground-fault protective device for a 7 1/2-horsepower, 208-volt, 3-phase squirrel-cage induction motor using time-delay fuses?
45A
Determine the maximum overload protection, using overload relays, when starting current is a problem, for a 50-horsepower, 208-volt, continuous-duty motor with a motor nameplate full-load current rating of 148 amperes and a service factor of 1.15.
207.2A
What is the minimum current rating of the motor disconnecting means for a 7.5-horsepower, 115-volt, single-phase motor?
92A
In order to prevent motor damage, reduced-voltage starting must be applied to DC motors that are ? or larger.
1 hp
Match the motor conductor ampacity to the correct THWN (75°C) copper wire size using Table 310.16.
90A -
103A -
120A -
211A -
90A - 3awg
103A - 2awg
120A - 1awg
211A - 4/0awg
An adjustable, instantaneous trip circuit breaker that is a part of a listed combination motor controller with coordinated protection in each conductor is used for branch-circuit short-circuit and ground-fault protection for a 15-horsepower, 440-volt, 3-phase squirrel-cage motor. Calculate the maximum setting for the instantaneous trip rating of this circuit breaker.
168A
Overload relays are used for the overload protection of a 7.5-horsepower, 208-volt, single-phase, continuous-duty motor with a motor nameplate full-load current rating of 46 amperes, a service factor of 1.10, and a temperature rise of 50°C. Calculate the maximum overload protection.
52.9A
What is the average locked-rotor current for a 20-horsepower, 240-volt, single-phase motor with code letter H?
557.9A
? starting devices are small, durable, and have no contacts.
Solid State
A single 100-horsepower, 440-volt, 3-phase, continuous-duty, induction type Design B motor is supplied by a motor branch circuit. Calculate the minimum ampacity for the motor branch-circuit conductors.
155 A
A 60-horsepower, 3-phase, 208-volt squirrel-cage motor is installed with a motor nameplate full-load current rating of 165 amperes. The minimum ampacity of the branch-circuit conductors is calculated to be 211 amperes. The branch-circuit conductors are required to be not less than 4/0 AWG THWN copper, and they are installed in a two-inch rigid metal conduit. The maximum rating of the motor branch-circuit short-circuit and ground-fault protective device using time-delay fuses has been calculated to be not greater than a 350-ampere time-delay fuse when starting current is a problem. Therefore, this motor with 4/0 AWG THWN copper conductors and a full-load current of 169 amperes can be protected by a 350-ampere time-delay fuse.
True
A 1/2-horsepower, 120-volt, continuous-duty motor is not permanently installed, is not automatically started, is within sight of the controller, and utilizes a 20-ampere branch-circuit short-circuit and ground-fault protective device as the overload protection. According to the NEC, this is an acceptable installation.
True
What is the minimum horsepower rating for a motor disconnecting means for a 50-horsepower, 440-volt, 3-phase motor marked with a code letter B using the maximum locked-rotor rating?
40A
A ? is often used to short out the soft starter of a running motor.
bypass contactor
One branch circuit supplies a piece of fixed electric space heating equipment (motor-operated) with one 5-horsepower, 230-volt, single-phase motor and one unit of electric heat rated at 12,500 watts at 230 volts. Calculate the minimum branch-circuit ampacity to supply the combined load.
103 A
What is the maximum standard rating or setting of the motor branch-circuit short-circuit and ground-fault protective device for a five-horsepower, 240-volt, single-phase capacitor motor using non–time-delay fuses when the starting current is a problem?
110A
Overload relays are used for the overload protection of a five-horsepower, 240-volt, single-phase motor with a 1.15 service factor and a motor nameplate full-load current rating of 31.5 amperes. Calculate the maximum overload protection when starting current is a problem.
44.1A
What is the minimum horsepower rating of the motor circuit switch for a single-phase, 240-volt 5-horsepower motor marked with code letter M using maximum locked-rotor kVA (kilovolt-ampere)?
7.5
In a(n) ? starting circuit, the various windings of the transformer are added to and taken away from the motor circuit to provide reduced voltage when starting.
Autotransformer
A 230-volt, 3-phase, 25-horsepower squirrel-cage motor and a 230-volt, 30-horsepower synchronous motor are installed on the same motor branch circuit. What is the minimum ampacity of the conductors for this single branch circuit?
148A
Match the following motor circuit conductors to the correct size of the raceway specified.
Three 4 AWG THWN in a PVC Schedule 80 Conduit
Three 4 AWG THWN in a Rigid Metal Conduit
Three 6 AWG THWN in a PVC Schedule 80 Conduit
Three 6 AWG THWN in a Rigid Metal Conduit
Three 4 AWG THWN in a PVC Schedule 80 Conduit
Answer - 1"
Three 4 AWG THWN in a Rigid Metal Conduit
Answer - 1"
Three 6 AWG THWN in a PVC Schedule 80 Conduit
Answer - 3/4"
Three 6 AWG THWN in a Rigid Metal Conduit
Answer - 3/4"
A 3-phase, 75-horsepower, 240-volt, continuous-duty motor is installed with a motor nameplate full-load current rating of 188 amperes, a temperature rise of 40°C, and a marked service factor of 1.15. Starting current is not an issue.
Determine the ampacity of the branch-circuit conductors for this 75-horsepower motor.
240A
A 12-kilowatt, 240-volt, single-phase heater and a 5-horsepower, 240-volt, single-phase motor are to be controlled by the same motor circuit switch. The motors can be started simultaneously.
Calculate the minimum current rating of the switch.
89.7A