CME CME Motors, Controls & Industrial Wiring

Question 1

Per NEC 430.52, a NEMA Type D motor with a full-load current of 30A is protected by an inverse-time circuit breaker. What is the maximum allowable overcurrent device rating?

Accepted Answer

250% of FLA (75A)

Answer

NEC Table 430.52 permits inverse-time breakers for Design D motors at up to 250% of FLA, yielding 75A for a 30A motor.

Question 2

What is the primary function of a motor overload relay in a motor branch circuit?

Accepted Answer

Protect the motor windings from sustained overcurrent due to overloading or locked rotor

Answer

Overload relays sense running overcurrent (typically 105–125% FLA) and trip the contactor before the motor's thermal limits are exceeded.

Question 3

Which reduced-voltage starting method is most appropriate for a large centrifugal pump motor where torque requirements are low at startup?

Accepted Answer

Autotransformer starting

Answer

Autotransformer starters reduce both voltage and inrush current while providing sufficient starting torque for centrifugal pump loads with low breakaway torque.

Question 4

Per NEC 430.24, conductors supplying several motors must have an ampacity of at least the sum of all motor FLAs plus what additional percentage of the largest motor?

Accepted Answer

25%

Answer

NEC 430.24 requires conductors serving multiple motors to carry 125% of the largest motor's FLA plus 100% of all other motor FLAs.

Question 5

What does NEMA enclosure type 4X provide that a standard NEMA 4 enclosure does not?

Accepted Answer

Corrosion resistance in addition to watertight and dust-tight protection

Answer

NEMA 4X adds corrosion resistance (typically stainless steel or fiberglass construction) to the watertight and dust-tight protection of NEMA 4 enclosures.

(CME) Certified Master Electrician Practice Test

(CME) Certified Master Electrician Practice Test

CME CME Motors, Controls & Industrial Wiring