Licensed Electrical Contractor Motors and Motor Controls 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?

Accepted Answer

42.5A

Answer

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)?

Accepted Answer

The motor's Full Load Current (FLC) from NEC Tables 430.247-430.250

Answer

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'?

Accepted Answer

50 feet

Answer

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?

Accepted Answer

25A

Answer

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)?

Accepted 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.

Answer

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.

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Licensed Electrical Contractor Motors and Motor Controls Questions and Answers