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CME Motors, Controls & Industrial Wiring Flashcards

6 cards from real CME practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

Read the first 6 CME Motors, Controls & Industrial Wiring flashcards as text

  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?

    Answer: 250% of FLA (75A)

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

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

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

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

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

    Answer: Autotransformer starting

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

  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?

    Answer: 25%

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

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

    Answer: Corrosion resistance in addition to watertight and dust-tight protection

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

  6. When a variable frequency drive (VFD) is used to control a motor, why must the motor nameplate current rating be derated?

    Answer: VFDs produce harmonic currents that cause additional heating in motor windings beyond what sinusoidal supply would cause

    Non-sinusoidal PWM output from VFDs introduces harmonic heating in stator windings, requiring the motor to be derated or an inverter-duty motor to be used.