Air Conditioning Repair Electrical Systems and Controls 4 — Questions and Answers
Question 1: A dual-run capacitor on a condenser unit is labeled 45+5 MFD. What components does it serve?
- Compressor start winding and outdoor fan motor
- Compressor run winding and outdoor fan motor (Correct answer)
- Indoor blower motor and compressor start winding
- Two separate compressor windings
Correct answer: Compressor run winding and outdoor fan motor
A dual-run capacitor has two capacitance values in one can: the larger value (45 MFD) serves the compressor run winding and the smaller (5 MFD) serves the condenser fan motor.
Question 2: During an electrical inspection, a technician notices the disconnect box at the outdoor unit contains a fused disconnect with 40-amp fuses, but the unit nameplate specifies a maximum overcurrent protection of 30 amps. What action is required?
- No action needed; larger fuses provide more protection
- Replace the fuses with correctly rated 30-amp fuses (Correct answer)
- Replace the entire disconnect box
- Add a ground fault circuit interrupter upstream
Correct answer: Replace the fuses with correctly rated 30-amp fuses
Overcurrent protection must not exceed the manufacturer's maximum rating; 40-amp fuses must be replaced with 30-amp fuses to comply with NEC and nameplate requirements.
Question 3: A technician measures capacitor capacitance and finds it reads 38 MFD on a capacitor rated 45 MFD ±6%. Is the capacitor within tolerance?
- Yes, it is within the ±6% tolerance
- No, it is below the allowable tolerance range (Correct answer)
- Yes, any reading above 35 MFD is acceptable
- No, only readings above 42 MFD are acceptable
Correct answer: No, it is below the allowable tolerance range
±6% of 45 MFD gives an acceptable range of 42.3–47.7 MFD; 38 MFD is below this range and the capacitor must be replaced.
Question 4: What does the term 'minimum circuit ampacity' (MCA) on an equipment nameplate indicate?
- The maximum fuse size allowed for the unit
- The minimum wire ampacity required for the supply conductors (Correct answer)
- The locked rotor amperage of the compressor
- The total harmonic distortion rating of the unit
Correct answer: The minimum wire ampacity required for the supply conductors
MCA specifies the minimum current-carrying capacity the supply wiring must have to safely power the unit under operating conditions.
Question 5: A technician finds the compressor starts but the outdoor condenser fan does not run. Voltage at the fan motor terminals reads 240V. What is the most likely cause?
- Failed contactor
- Open run capacitor for the fan motor (Correct answer)
- Tripped low-pressure switch
- Shorted transformer secondary
Correct answer: Open run capacitor for the fan motor
Voltage present at the motor with no rotation points to an open run capacitor, which prevents the motor from developing the torque needed to start.
Question 6: Which NEC article specifically covers the installation of air conditioning and refrigerating equipment?
- Article 210
- Article 430
- Article 440 (Correct answer)
- Article 250
Correct answer: Article 440
NEC Article 440 specifically addresses air conditioning and refrigerating equipment, including hermetic refrigerant motor-compressors.
Question 7: A technician is troubleshooting an air handler that blows warm air in cooling mode. The compressor and condenser fan are running. What electrical component failure could cause this symptom?
- Open circuit in the reversing valve solenoid (heat pump) (Correct answer)
- Failed thermostat R-terminal fuse
- Shorted contactor holding coil
- Open circuit in the indoor blower capacitor
Correct answer: Open circuit in the reversing valve solenoid (heat pump)
On a heat pump, an open reversing valve solenoid coil prevents the valve from switching to cooling mode, causing the system to deliver warm air.
Air Conditioning Repair Electrical Systems and Controls 4
A dual-run capacitor on a condenser unit is labeled 45+5 MFD.
What components does it serve?