IEC - Independent Electrical Contractors Motors and Motor Controls Questions and Answers — 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?
- 30A
- 32.5A (Correct answer)
- 35A
- 28.75A
Correct answer: 32.5A
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'?
- On the opposite side of a permanent wall, but directly behind the motor.
- Visible and not more than 100 feet distant from the other.
- Unobstructed, but located on the floor above the motor.
- Visible and not more than 50 feet distant from the other. (Correct answer)
Correct answer: Visible and not more than 50 feet distant from the other.
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?
- The horsepower rating of the motor.
- The voltage of the control circuit.
- The conductor size (AWG) and whether it extends beyond the enclosure. (Correct answer)
- The type of motor controller (e.g., manual or magnetic).
Correct answer: The conductor size (AWG) and whether it extends beyond the enclosure.
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)?
- The branch-circuit protective device rating is not exceeded, and the FLC of each motor is less than 10A.
- The nameplate FLA of each motor does not exceed 6 amperes, and each motor has individual overload protection. (Correct answer)
- As long as the combined FLC (11.6A) is less than 80% of the breaker rating.
- This installation is never permitted; each motor requires its own branch circuit.
Correct answer: The nameplate FLA of each motor does not exceed 6 amperes, and each motor has individual overload protection.
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?
- To protect the motor from short-circuits and ground-faults.
- To start and stop the motor, and interrupt the locked-rotor current. (Correct answer)
- To provide thermal overload protection based on the motor's service factor.
- To serve as the primary disconnecting means for the branch circuit.
Correct answer: To start and stop the motor, and interrupt the locked-rotor current.
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.
Question 6: The current a motor draws at the instant of starting when full voltage is applied, before the rotor begins to turn, is known as what?
- Full-Load Current (FLC)
- Service Factor Current (SFC)
- Locked-Rotor Current (LRC) (Correct answer)
- In-service Amperage (ISA)
Correct answer: Locked-Rotor Current (LRC)
Locked-Rotor Current (LRC), also known as Locked-Rotor Amps (LRA), is the inrush current drawn by a motor at startup when the rotor is stationary ('locked'). This current is typically 6 to 8 times the motor's normal full-load current and is a critical value for sizing short-circuit protection and motor controllers.
IEC - Independent Electrical Contractors Motors and Motor Controls Questions and Answers
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?