PC Motors and Motor Controls 1 — Questions and Answers
Question 1: Which NEC article covers motors, motor circuits, and controllers?
- Article 430 (Correct answer)
- Article 440
- Article 450
- Article 460
Correct answer: Article 430
NEC Article 430 covers motors, motor circuits, and controllers including disconnecting means and overcurrent protection.
Question 2: What is the minimum ampacity required for branch circuit conductors supplying a single continuous-duty motor?
- 100% of motor FLA
- 115% of motor FLA
- 125% of motor FLA (Correct answer)
- 150% of motor FLA
Correct answer: 125% of motor FLA
Per NEC 430.22, branch circuit conductors for a single motor must be sized at no less than 125% of the motor's full-load current.
Question 3: Which device provides both overload protection and short-circuit protection for a motor in a single assembly?
- Thermal overload relay
- Fused disconnect switch
- Combination motor controller (Correct answer)
- Manual motor starter
Correct answer: Combination motor controller
A combination motor controller integrates a disconnect means, overcurrent protection, and overload protection in one unit.
Question 4: Per NEC Table 430.52, what is the maximum inverse time circuit breaker rating for a Design B motor with an FLA of 40A?
- 50A
- 75A
- 100A (Correct answer)
- 150A
Correct answer: 100A
For Design B motors, NEC Table 430.52 permits up to 250% of FLA for inverse time breakers: 40A × 2.5 = 100A.
Question 5: What is the primary purpose of a motor overload relay?
- To protect against short circuits in the branch circuit
- To protect motor windings from damage due to overheating (Correct answer)
- To provide ground fault protection for personnel
- To protect against voltage surges from the utility
Correct answer: To protect motor windings from damage due to overheating
Motor overload relays monitor current over time and trip to prevent motor windings from overheating under sustained overload conditions.
Question 6: A motor has a nameplate FLA of 20A and a service factor of 1.15. What is the maximum overload protection rating per NEC 430.32?
- 23A (115%)
- 25A (125%)
- 26A (130%) (Correct answer)
- 28A (140%)
Correct answer: 26A (130%)
Per NEC 430.32(A)(1), motors with a service factor of 1.15 or higher may use overload protection sized up to 130% of FLA: 20A × 1.30 = 26A.
PC Motors and Motor Controls 1
Which NEC article covers motors, motor circuits, and controllers?