FREE Journeyman Electrician’s Motors and Motor Controls 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?

Accepted Answer

10 AWG

Answer

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?

Accepted Answer

65A

Answer

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?

Accepted Answer

110A

Answer

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?

Accepted Answer

A disconnecting means must be located in sight from the motor.

Answer

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?

Accepted Answer

The rating of the motor's branch-circuit protective device.

Answer

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.

Journeyman Electrician’s Exam Practice Test

Journeyman Electrician’s Exam Practice Test

FREE Journeyman Electrician’s Motors and Motor Controls Questions and Answers