IEC - Independent Electrical Contractors Motors and Motor Controls Questions and Answers

Question 1

An electrician is installing a single continuous-duty 10 HP, 230V, 3-phase motor. According to the NEC tables, its Full Load Current (FLC) is 28A. The motor has a service factor of 1.15 and its nameplate FLA is 26A. What is the maximum standard size for the separate overload protection device?

Accepted Answer

32.5A

Answer

According to NEC 430.32(A)(1), separate overload protection is sized based on the motor's nameplate Full Load Amperes (FLA), not the FLC from the tables. For a motor with a service factor of 1.15 or greater, the overload device shall be selected at not more than 125% of the nameplate FLA. Calculation: 26A (FLA) x 1.25 = 32.5A.

Question 2

According to NEC 430.102(B), a disconnecting means for a motor must be located in sight from the motor and the driven machinery. Which of the following meets the NEC definition of 'in sight from'?

Accepted Answer

Visible and not more than 50 feet distant from the other.

Answer

NEC Article 100 defines 'In Sight From (Within Sight From, Within Sight)' as being visible and not more than 50 feet (15 m) distant from the other equipment. This ensures that a worker servicing the motor can keep the disconnecting means in view to prevent accidental energization.

Question 3

A motor control circuit is tapped from the load side of a motor's branch-circuit short-circuit and ground-fault protective device. According to NEC 430.72(B) and Table 430.72(B)(2), what is the primary factor in determining the maximum rating of the overcurrent device for the control circuit conductors?

Accepted Answer

The conductor size (AWG) and whether it extends beyond the enclosure.

Answer

NEC 430.72(B) and its associated table specify the maximum rating for the overcurrent protection of control circuit conductors based on the conductor's AWG size. The table has different columns for conductors that remain within the motor control enclosure versus those that extend beyond it, reflecting different protection needs.

Question 4

An electrician needs to connect two 1/2 HP, 120V motors on a single 20A branch circuit in a workshop. The FLC for each motor is 9.8A, but the nameplate FLA is 5.8A. Under what condition is this installation permitted by NEC 430.53(A)?

Accepted Answer

The nameplate FLA of each motor does not exceed 6 amperes, and each motor has individual overload protection.

Answer

NEC 430.53(A) allows two or more motors on a single 120V, 20A branch circuit if several conditions are met. Key among these are that each motor's full-load current rating does not exceed 6 amperes (based on nameplate), and each motor has its own individual overload protection as per 430.32.

Question 5

Which of the following is a primary function of a motor controller, as defined by the NEC?

Accepted Answer

To start and stop the motor, and interrupt the locked-rotor current.

Answer

According to NEC 430.81 and 430.82, a motor controller's function is to start and stop the motor. A key capability requirement is that it must be able to interrupt the locked-rotor current of the motor, which is the high current drawn by the motor during startup.

IEC - Independent Electrical Contractors Practice Test

IEC - Independent Electrical Contractors Practice Test

IEC - Independent Electrical Contractors Motors and Motor Controls Questions and Answers