Journeyman Electrician’s Exam Journeyman Electrician's Motors and Motor Controls 2 — Questions and Answers
Question 1: According to NEC Article 430, the branch-circuit short-circuit and ground-fault protection for a motor must not exceed what percentage of the motor's full-load current for an inverse time breaker?
- 150%
- 175%
- 250% (Correct answer)
- 400%
Correct answer: 250%
NEC 430.52 allows inverse time breakers to be sized up to 250% of the motor's full-load current for branch-circuit protection.
Question 2: A motor nameplate reads 'SF 1.15'. What does this service factor indicate?
- The motor can run at 115% of rated load continuously without damage (Correct answer)
- The motor efficiency is 115%
- The motor has a 15% safety margin for voltage fluctuations only
- The motor starting current is 115% of full-load current
Correct answer: The motor can run at 115% of rated load continuously without damage
A service factor of 1.15 means the motor can operate at 115% of its nameplate horsepower rating continuously under rated conditions.
Question 3: When sizing conductors for a motor with a 1.15 service factor and 40°C ambient temperature rating, what percentage of full-load current is used?
- 100%
- 115%
- 125% (Correct answer)
- 150%
Correct answer: 125%
Per NEC 430.22, motor branch-circuit conductors must have an ampacity of not less than 125% of the motor full-load current.
Question 4: A NEMA MG 1 designation of 'Design B' motor is characterized by:
- High starting torque and low starting current
- Normal starting torque, normal starting current, and low slip (Correct answer)
- High starting torque and high starting current
- Low starting torque and very high slip
Correct answer: Normal starting torque, normal starting current, and low slip
NEMA Design B motors have normal starting torque, normal starting current, and low slip at full load — the most common general-purpose motor design.
Question 5: What is the purpose of a motor control center (MCC) compared to individual motor starters?
- An MCC provides higher voltage capacity only
- An MCC centralizes multiple motor starters in one enclosure for easier management and wiring (Correct answer)
- An MCC eliminates the need for overload protection
- An MCC is required only for motors above 100 HP
Correct answer: An MCC centralizes multiple motor starters in one enclosure for easier management and wiring
A motor control center consolidates starters, disconnects, and controls into one assembly, reducing wiring costs and improving maintenance access.
Question 6: A 3-phase motor is connected in a wye (Y) configuration. If the line voltage is 480V, what is the voltage across each motor winding?
- 480V
- 277V (Correct answer)
- 208V
- 240V
Correct answer: 277V
In a wye connection, phase voltage equals line voltage divided by √3, so 480V ÷ 1.732 ≈ 277V per winding.
Question 7: Under NEC 430.109, which device is NOT acceptable as a motor disconnecting means?
- A motor circuit switch rated in horsepower
- A molded-case circuit breaker
- A standard lighting switch rated 15A (Correct answer)
- An inverse time circuit breaker
Correct answer: A standard lighting switch rated 15A
A standard lighting switch is not rated for motor loads and is not listed as an acceptable disconnecting means under NEC 430.109.
