Motor disconnecting means questions are crucial for electrical professionals as they directly impact safety and code compliance. These questions typically focus on selecting the appropriate disconnecting means based on motor specifications and operating conditions. This knowledge is essential for protecting both equipment and personnel, as improper disconnecting means can lead to hazardous situations during maintenance or emergencies.

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Example Motor Disconnecting Means Questions on NEC Electrical Exams

A general-use switch rated at ________ amperes is the minimum permitted to serve as the disconnecting means for a 2 hp, 208-volt stationary motor.
For a 75 hp motor with an autotransformer-type controller, a general-use switch can be used as the disconnecting means if the motor drives a generator with ________ protection.
An installation consists of one 5-hp, one 3-hp, and two ½-hp motors, all rated 240 V, 3-phase. All motors are Design B motors. Determine the size of the safety switch required for this combination load.

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How to Identify a Motor Disconnecting Means Question on NEC Electrical Exams

Key phrases to look out for:

  • "Disconnecting means"
  • "General-use switch"
  • "Controller disconnect"
  • "Motor hp rating"
  • "Full-load current"
  • "Locked-rotor current"
  • "Combination load"
  • "Motor disconnect sizing"

When you spot these elements:

  • Confirm it's a motor disconnect question, not a motor circuit protector or overload protection question
  • Refer to NEC Article 430, Part IX (430.109-430.113)
  • Check if it involves single motor or combination loads
  • Verify if locked-rotor current considerations are needed

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Motor Disconnecting Means Articles: NEC 430 Part IX

To correctly apply NEC Article 430 Part IX, focus on these main elements:

  1. 430.109: Required ratings and types of disconnecting means
    • Types of acceptable disconnecting means
    • Specific requirements for different motor applications
    • Conditions for using general-use switches
  2. 430.110: Ampere rating and interrupting capacity
    • Minimum ampere ratings for various applications
    • Requirements for combination loads
    • Special provisions for specific motor types
  3. 430.111: Switch or circuit breaker as disconnecting means
    • Requirements for indicating switch position
    • Accessibility requirements
    • Installation considerations
  4. 430.112: Motors served by single disconnecting means
    • Group motor installation requirements
    • Location requirements
    • Capacity considerations
  5. 430.113: Location requirements
    • Accessibility standards
    • Line-of-sight requirements
    • Emergency considerations

Key points to remember:

  • Must consider both full-load current and locked-rotor current
  • Both current rating and horsepower rating must be evaluated
  • Different types of motors and controllers have specific disconnect requirements
  • Location and accessibility of disconnecting means is crucial

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Walkthrough for a NEC Electrical Exam Motor Disconnecting Means Question

Question:

An installation consists of one 5-hp, one 3-hp, and two ½-hp motors, all rated 240 V, 3-phase. All motors are Design B motors. Determine the size of the safety switch required for this combination load.

Step 1: Identify Full-Load and Locked-Rotor Currents

Using Table 430.250 for FLC and Table 430.251(B) for LRC:

  • 5-hp motor:
    • FLC = 15.2A
    • LRC = 92A
  • 3-hp motor:
    • FLC = 9.6A
    • LRC = 64A
  • Two ½-hp motors:
    • FLC = 2.2A each
    • LRC = 20A each

Step 2: Calculate Combined Load Currents

Calculate total full-load current (FLC):

  • Total FLC = 15.2A + 9.6A + (2 × 2.2A) = 29.2A

Calculate total locked-rotor current (LRC):

  • Total LRC = 92A + 64A + (2 × 20A) = 196A

Step 3: Determine Minimum Ampere Rating

Per 430.110:

  • Minimum ampere rating = FLC × 115%
  • Minimum ampere rating = 29.2A × 1.15 = 33.58A

Step 4: Determine Required Horsepower Rating

Using Table 430.251(B):

  • Combined LRC of 196A corresponds to a 15 hp motor
  • Table 430.250: A 15 hp, 230V motor has FLC of 42A
  • 42A > 33.58A (calculated minimum ampere rating)
  • Therefore, 15 hp rating is sufficient

Step 5: Final Safety Switch Requirements

The safety switch must be:

  • Rated minimum 15 hp
  • Have ampere rating not less than 33.58A
  • Listed for use as motor disconnect
  • Rated for 240V, 3-phase service