RSES Electrical Systems & Controls 2 — Questions and Answers
Question 1: A hermetic compressor motor draws locked-rotor amperage (LRA) of 45A. What is the minimum wire gauge required for a 240V single-phase circuit using copper conductors at 75°C?
- 14 AWG
- 12 AWG
- 10 AWG (Correct answer)
- 8 AWG
Correct answer: 10 AWG
LRA × 1.25 = 56.25A; 10 AWG copper at 75°C is rated for 35A continuous, but NEC 440.52 sizing for motors requires selecting wire rated for at least 125% of FLA, and 10 AWG handles the circuit with proper OCPD.
Question 2: Which type of overload relay automatically resets after cooling down without requiring manual intervention?
- Manual-reset bimetallic relay
- Auto-reset bimetallic relay (Correct answer)
- Fuse-type overload
- Magnetic overload relay
Correct answer: Auto-reset bimetallic relay
An auto-reset bimetallic overload relay restores the circuit automatically once the bimetal strip cools and returns to its original position.
Question 3: A capacitor-start, capacitor-run (CSCR) motor fails to start but hums loudly. Which component is most likely faulty?
- Run capacitor
- Start capacitor or centrifugal switch (Correct answer)
- Main winding
- Overload protector
Correct answer: Start capacitor or centrifugal switch
Humming without starting indicates the start winding circuit is open, typically caused by a failed start capacitor or a defective centrifugal switch that won't close.
Question 4: What does a megohmmeter (megger) primarily measure in refrigeration system electrical diagnostics?
- Voltage drop across windings
- Insulation resistance of motor windings (Correct answer)
- Capacitance of run capacitors
- Continuity of control circuit fuses
Correct answer: Insulation resistance of motor windings
A megohmmeter applies high DC voltage (typically 500–1000V) to measure insulation resistance in megohms, revealing winding degradation or moisture ingress.
Question 5: In a refrigeration control circuit, a normally open (NO) pressure switch closes at 250 psig and opens at 300 psig. This switch is functioning as a:
- Low-pressure safety cutout
- High-pressure safety cutout
- Low-pressure operating control
- High-pressure operating control (Correct answer)
Correct answer: High-pressure operating control
A switch that closes on pressure rise and opens at a high setpoint is a high-pressure operating control, cycling the system to maintain discharge pressure within a range.
Question 6: A three-phase compressor motor is running but drawing unequal current on the three legs. The most likely cause is:
- Overcharged refrigerant system
- Voltage imbalance or open motor winding (Correct answer)
- Shorted run capacitor
- Defective crankcase heater
Correct answer: Voltage imbalance or open motor winding
Current imbalance in a three-phase motor typically indicates either a supply voltage imbalance or a partially shorted/open winding, both of which cause unequal current draw per phase.
Question 7: Which NEC article specifically governs the installation of air-conditioning and refrigerating equipment?
- Article 430
- Article 440 (Correct answer)
- Article 450
- Article 460
Correct answer: Article 440
NEC Article 440 covers air-conditioning and refrigerating equipment, including hermetic refrigerant motor-compressors and their associated electrical circuits.
RSES Electrical Systems & Controls 2
A hermetic compressor motor draws locked-rotor amperage (LRA) of 45A.
What is the minimum wire gauge required for a 240V single-phase circuit using copper conductors at 75°C?