Licensed Electrical Contractor Motors and Motor Controls Questions and Answers 1 — Questions and Answers
Question 1: A contractor is installing a single continuous-duty, 3-phase, 460V, 25 HP squirrel-cage induction motor. According to NEC Table 430.250, the motor has a Full Load Current (FLC) of 34A. What is the minimum required ampacity for the branch-circuit conductors?
- 34A
- 42.5A (Correct answer)
- 38A
- 50A
Correct answer: 42.5A
According to NEC 430.22, branch-circuit conductors supplying a single continuous-duty motor must have an ampacity of not less than 125% of the motor's Full Load Current (FLC). The FLC is determined from NEC tables, in this case, 34A. Therefore, the calculation is 34A x 1.25 = 42.5A.
Question 2: Which of the following is the primary value used to determine the size of branch-circuit short-circuit and ground-fault protection for a motor, according to NEC 430.6(A)(1)?
- The motor's nameplate Full Load Amperes (FLA)
- The motor's locked-rotor current
- The motor's Full Load Current (FLC) from NEC Tables 430.247-430.250 (Correct answer)
- The service factor adjusted amperage
Correct answer: The motor's Full Load Current (FLC) from NEC Tables 430.247-430.250
NEC 430.6(A)(1) explicitly states that the Full Load Current (FLC) values from Tables 430.247 through 430.250 are to be used for determining the size of motor branch-circuit short-circuit and ground-fault protective devices, not the nameplate FLA. The nameplate FLA is used for sizing the separate overload protection.
Question 3: A motor controller for a 480V motor is located in an electrical room. According to NEC 430.102(B), the disconnecting means for the motor must be located in sight from the motor and the driven machinery. What is the maximum distance the disconnect can be from the motor to be considered 'in sight'?
- 25 feet
- 100 feet
- 50 feet (Correct answer)
- 75 feet
Correct answer: 50 feet
As defined in NEC Article 100, the term 'In Sight From' (or 'Within Sight From') means that the specified equipment must be visible and not more than 50 feet distant from the other. This rule ensures that a person working on the motor can see the disconnecting means.
Question 4: A motor with a nameplate service factor of 1.15 and a nameplate current rating of 20A is installed for continuous duty. What is the maximum rating for a separate overload protection device for this motor?
- 20A
- 23A
- 30A
- 25A (Correct answer)
Correct answer: 25A
According to NEC 430.32(A)(1), for a motor with a marked service factor of 1.15 or greater, the separate overload device shall be selected to trip at not more than 125% of the motor's nameplate full-load current rating. The calculation is 20A x 1.25 = 25A.
Question 5: When sizing a feeder that supplies two continuous-duty motors, how is the overcurrent protective device (OCPD) for the feeder determined according to NEC 430.62(A)?
- Sum the FLC of both motors and multiply by 125%.
- Sum the ratings of the branch-circuit OCPDs for both motors.
- Take the rating of the largest motor's branch-circuit short-circuit and ground-fault protective device and add the FLC of the other motor. (Correct answer)
- Take 125% of the largest motor's FLC and add 100% of the other motor's FLC.
Correct answer: Take the rating of the largest motor's branch-circuit short-circuit and ground-fault protective device and add the FLC of the other motor.
NEC 430.62(A) specifies that the rating of the feeder short-circuit and ground-fault protective device shall not be greater than the largest rating of the branch-circuit short-circuit and ground-fault protective device for any motor in the group, plus the sum of the full-load currents of the other motors in the group.
Question 6: Which of the following components in a motor circuit is specifically designed to protect the motor and conductors from excessive heating due to motor overloads and failure to start?
- Inverse-time circuit breaker
- Time-delay fuse
- Overload relay (heater) (Correct answer)
- Main bonding jumper
Correct answer: Overload relay (heater)
Overload devices, such as overload relays with heater elements, are designed to protect motors and conductors from overcurrent conditions resulting from overloads or failure to start. In contrast, branch-circuit devices like fuses and circuit breakers provide short-circuit and ground-fault protection, which are different types of overcurrent events.
Licensed Electrical Contractor Motors and Motor Controls Questions and Answers 1
A contractor is installing a single continuous-duty, 3-phase, 460V, 25 HP squirrel-cage induction motor.
According to NEC Table 430.250, the motor has a Full Load Current (FLC) of 34A.
What is the minimum required ampacity for the branch-circuit conductors?