Free Journeyman Electrician’s Motors and Motor Controls Questions and Answers — Questions and Answers
Question 1: An electrician is installing a branch circuit for a 10 HP, 230V, 3-phase, continuous-duty induction motor. The motor will be supplied with 75°C rated THWN copper conductors. According to the NEC, what is the minimum size for these branch-circuit conductors?
- 12 AWG
- 8 AWG
- 6 AWG
- 10 AWG (Correct answer)
Correct answer: 10 AWG
According to NEC Table 430.250, the Full-Load Current (FLC) for a 10 HP, 230V, 3-phase motor is 28A. NEC 430.22 requires that branch-circuit conductors for a single continuous-duty motor have an ampacity of not less than 125% of the motor's FLC. The calculation is 28A x 1.25 = 35A. Referring to NEC Table 310.16, in the 75°C column for copper conductors, a 10 AWG conductor is rated for 35A, making it the minimum required size.
Question 2: A 460V, three-phase motor has a nameplate full-load current (FLA) of 52A, a service factor of 1.15, and a temperature rise not over 40°C. What is the maximum permitted rating for a separate overload protection device?
- 52A
- 59.8A
- 70A
- 65A (Correct answer)
Correct answer: 65A
As per NEC 430.32(A)(1), for a motor marked with a service factor of 1.15 or greater, or a temperature rise of 40°C or less, the separate overload device shall be selected to trip at no more than 125% of the motor's nameplate Full-Load Ampere (FLA) rating. The calculation is 52A x 1.25 = 65A.
Question 3: Which of the following is the maximum standard size non-time-delay fuse that can be used for branch-circuit, short-circuit, and ground-fault protection for a 25 HP, 460V, 3-phase, squirrel-cage motor?
- 110A (Correct answer)
- 100A
- 90A
- 125A
Correct answer: 110A
First, determine the motor's FLC from NEC Table 430.250, which is 34A for a 25 HP, 460V, 3-phase motor. Next, use NEC Table 430.52, which allows non-time-delay fuses to be sized up to 300% of the FLC for a squirrel-cage motor. The calculation is 34A x 300% = 102A. Since 102A is not a standard fuse size listed in NEC 240.6(A), NEC 430.52(C)(1) Exception No. 1 permits using the next higher standard size. The next standard size above 102A is 110A.
Question 4: An electrician is installing a motor on a rooftop air-conditioning unit. The motor controller is located inside a mechanical room on the floor below. According to the NEC, what is required for the disconnecting means?
- A circuit breaker in a panelboard on the same floor is sufficient.
- The disconnect at the controller must simply be lockable.
- A disconnecting means must be located in sight from the motor. (Correct answer)
- A horsepower-rated switch must be installed within 25 feet of the motor.
Correct answer: A disconnecting means must be located in sight from the motor.
NEC 430.102(B)(1) requires that a disconnecting means for a motor be located 'in sight from' the motor location and the driven machinery location. Article 100 defines 'In Sight From' as being visible and not more than 50 feet away from the equipment. While an exception exists if the controller's disconnect is lockable, the primary and most direct requirement is to have a disconnect within sight.
Question 5: When a motor control circuit is tapped from the load side of the motor's branch-circuit protective device, what is the primary factor used to determine if the control circuit conductors require their own supplementary overcurrent protection?
- The horsepower rating of the motor.
- The length of the control circuit conductors.
- The rating of the motor's branch-circuit protective device. (Correct answer)
- The voltage of the control circuit.
Correct answer: The rating of the motor's branch-circuit protective device.
According to NEC 430.72(B), the need for supplementary overcurrent protection for motor control circuit conductors is determined by comparing the ampacity of the control conductors to the rating of the motor's branch-circuit short-circuit and ground-fault protective device. Table 430.72(B) provides maximum ratings of the branch-circuit device for given control conductor sizes, beyond which supplementary protection is required.
Question 6: A feeder supplies a group of three-phase, 460-volt continuous-duty motors with the following full-load currents (FLC) taken from NEC tables: 52A, 21A, and 14A. What is the minimum required ampacity for the feeder conductors?
- 87A
- 100A (Correct answer)
- 110A
- 93.5A
Correct answer: 100A
In accordance with NEC 430.24, the ampacity of feeder conductors supplying a group of motors must be calculated as 125% of the full-load current (FLC) of the highest-rated motor, plus the sum of the FLCs of all other motors in the group. The calculation is (1.25 x 52A) + 21A + 14A = 65A + 35A = 100A.
