E2 Motors and Transformers 2 — Questions and Answers
Question 1: According to the NEC, what is the minimum branch-circuit conductor ampacity for a single motor with a full-load current of 40 A?
- 40 A
- 50 A (Correct answer)
- 44 A
- 48 A
Correct answer: 50 A
NEC 430.22 requires motor branch-circuit conductors to have an ampacity of not less than 125% of the motor's full-load current rating.
Question 2: A transformer nameplate lists an impedance of 5.75%. This value is primarily used to calculate:
- No-load losses
- Available fault current (Correct answer)
- Efficiency at full load
- Magnetizing inrush current
Correct answer: Available fault current
Transformer impedance (%Z) is the key parameter for calculating available short-circuit current on the secondary side.
Question 3: What NEC article governs the installation of air-conditioning and refrigeration equipment motors?
- Article 430
- Article 440 (Correct answer)
- Article 445
- Article 460
Correct answer: Article 440
Article 440 covers motor-driven air-conditioning and refrigerating equipment, supplementing Article 430.
Question 4: A 480V delta–wye transformer secondary supplies a 208Y/120V panelboard. The neutral conductor on the secondary is derived from:
- The delta primary winding center tap
- The wye-connected secondary center point (Correct answer)
- A separately derived grounding electrode
- The primary overcurrent device
Correct answer: The wye-connected secondary center point
In a wye-configured secondary, the neutral is the center (star) point of the three wye windings connected to ground.
Question 5: Per NEC 430.52, the maximum rating of an inverse-time circuit breaker used as motor branch-circuit protection for a Design B squirrel-cage motor is:
- 150% of FLC
- 200% of FLC
- 250% of FLC (Correct answer)
- 400% of FLC
Correct answer: 250% of FLC
NEC Table 430.52 allows inverse-time breakers to be rated up to 250% of the motor FLC for Design B squirrel-cage motors.
Question 6: Which motor nameplate data item is used to determine the correct overload protection size per NEC 430.32?
- Service factor and nameplate current (Correct answer)
- Horsepower and locked-rotor current
- NEMA design letter and frame size
- Torque class and efficiency rating
Correct answer: Service factor and nameplate current
NEC 430.32 bases overload sizing on the motor's nameplate full-load amperes and service factor (or temperature rise).
Question 7: A 75 kVA, 480–208Y/120V transformer is considered a separately derived system. What grounding electrode conductor size is required for a 4/0 AWG secondary conductor per NEC Table 250.66?
- 4 AWG copper
- 2 AWG copper (Correct answer)
- 1/0 AWG copper
- 3/0 AWG copper
Correct answer: 2 AWG copper
NEC Table 250.66 specifies a 2 AWG copper grounding electrode conductor for secondary conductors sized 2/0–3/0 AWG (or 4/0 in some brackets).
E2 Motors and Transformers 2
According to the NEC, what is the minimum branch-circuit conductor ampacity for a single motor with a full-load current of 40 A?