CME CME Motors, Controls & Industrial Wiring 2 — Questions and Answers
Question 1: Per NEC 430.102(B), what is required at the location where a motor disconnect is not within sight of the motor controller?
- The disconnect must be lockable in the open position (Correct answer)
- An additional fused disconnect at the motor must be installed
- A warning sign must be posted at the motor indicating remote disconnect location
- A ground fault relay must be installed in the motor circuit
Correct answer: The disconnect must be lockable in the open position
NEC 430.102(B) allows a motor disconnecting means that is not within sight of the motor if it can be locked in the open (off) position.
Question 2: Which control circuit wiring method is required by NEC 725.136 when control conductors are installed in the same raceway as power conductors?
- Control conductors must be rated for the voltage of the power conductors (Correct answer)
- Control conductors must use a separate raceway exclusively
- Control conductors must be shielded twisted pair
- Control conductors must use metal conduit regardless of installation location
Correct answer: Control conductors must be rated for the voltage of the power conductors
NEC 725.136(A) requires that Class 1 circuit conductors installed with power conductors be rated for the maximum voltage present in the raceway.
Question 3: What is the purpose of a contactor's auxiliary contact in a motor control circuit?
- Provide a holding circuit to maintain the contactor energized after the start button is released (Correct answer)
- Protect the motor against phase loss conditions
- Limit inrush current during initial motor energization
- Monitor motor temperature and trip on overtemperature
Correct answer: Provide a holding circuit to maintain the contactor energized after the start button is released
The normally-open auxiliary (sealing) contact creates a parallel path around the momentary start pushbutton, latching the coil circuit closed after the button is released.
Question 4: For a three-phase induction motor, which condition is indicated when all three phase currents are balanced but slightly above FLA?
- Motor is overloaded beyond its rated mechanical output (Correct answer)
- Open phase on the supply causing current redistribution
- Short circuit between motor windings
- Bearing failure increasing mechanical friction
Correct answer: Motor is overloaded beyond its rated mechanical output
Balanced currents above FLA across all three phases indicate the mechanical load exceeds the motor's rated horsepower output, not an electrical fault.
Question 5: What is the minimum required short-circuit current rating (SCCR) for industrial control panels per NEC 409.22?
- Must be equal to or greater than the available fault current at the panel's supply terminals (Correct answer)
- 5,000A minimum regardless of available fault current
- 10,000A for panels served from 480V systems only
- Determined solely by the branch circuit overcurrent device rating
Correct answer: Must be equal to or greater than the available fault current at the panel's supply terminals
NEC 409.22 requires the marked SCCR of an industrial control panel to be not less than the available short-circuit current at its supply terminals.
Question 6: A motor nameplate shows: 460V, 3Ø, 60Hz, 25HP, 1.15 SF. What is the maximum continuous current the motor can draw without damaging the insulation?
- FLA multiplied by the 1.15 service factor (Correct answer)
- FLA only, service factor does not affect current
- FLA multiplied by 125% per NEC overload sizing rules
- FLA minus 15% to account for the service factor margin
Correct answer: FLA multiplied by the 1.15 service factor
The service factor indicates the motor can operate continuously at 1.15 × FLA (nameplate current × SF) without exceeding its thermal limits.
CME CME Motors, Controls & Industrial Wiring 2
Per NEC 430.102(B), what is required at the location where a motor disconnect is not within sight of the motor controller?