E2 Motors and Transformers 3 — Questions and Answers
Question 1: A 3-phase, 460V motor draws 28 A at full load. What is the minimum ampacity of the disconnecting means per NEC 430.110?
- 28 A
- 35 A (Correct answer)
- 42 A
- 56 A
Correct answer: 35 A
NEC 430.110 requires the motor disconnecting means to have an ampere rating of at least 115% of the motor's full-load current.
Question 2: What is the purpose of a motor's locked-rotor code letter on the nameplate?
- It indicates the motor's thermal class
- It identifies the kilovolt-ampere input per horsepower at locked rotor (Correct answer)
- It specifies the maximum ambient temperature
- It identifies the NEMA enclosure type
Correct answer: It identifies the kilovolt-ampere input per horsepower at locked rotor
NEC Table 430.7(B) lists locked-rotor code letters that represent kVA/HP ranges used to size motor branch-circuit protection.
Question 3: Per NEC 450.9, the minimum ventilation clearance around a dry-type transformer rated over 112.5 kVA installed indoors must allow for adequate:
- Access to tap changers
- Cooling air circulation (Correct answer)
- Conduit entry on all sides
- Grounding conductor routing
Correct answer: Cooling air circulation
NEC 450.9 requires transformers to be installed so that cooling is not blocked, ensuring thermal dissipation from the windings.
Question 4: A 100 HP, 460V, 3-phase motor has a FLC of 124 A per Table 430.250. Using dual-element time-delay fuses, what is the maximum fuse rating per NEC Table 430.52?
- 175 A
- 200 A
- 225 A (Correct answer)
- 175% of 124 A
Correct answer: 225 A
NEC Table 430.52 allows dual-element time-delay fuses up to 175% of FLC, and 175% × 124 A = 217 A, rounded up to the next standard size of 225 A.
Question 5: What transformer connection produces a 30-degree phase shift between primary and secondary voltages?
- Delta–delta
- Wye–wye
- Delta–wye (Correct answer)
- Zigzag–wye
Correct answer: Delta–wye
A delta–wye (or wye–delta) transformer connection inherently produces a 30-degree phase displacement between primary and secondary line voltages.
Question 6: Under NEC 430.87, when two or more motors are served from the same controller, each motor must have:
- Its own branch-circuit fuse
- Individual overload protection (Correct answer)
- A common disconnect within sight
- Separate grounding electrode conductors
Correct answer: Individual overload protection
NEC 430.87 states that each motor must be protected by individual overload devices even when served from a common controller.
Question 7: What is the maximum distance allowed between a motor disconnect and the motor it serves, per NEC 430.102(B)?
- The disconnect must be within 10 feet
- The disconnect must be within sight of the motor (Correct answer)
- No distance limit if a lockable disconnect is provided
- The disconnect must be in the same room
Correct answer: The disconnect must be within sight of the motor
NEC 430.102(B) requires the disconnecting means to be within sight of the motor location and driven machinery, unless it is lockable in the open position.
E2 Motors and Transformers 3
A 3-phase, 460V motor draws 28 A at full load.
What is the minimum ampacity of the disconnecting means per NEC 430.110?