On a hot summer day, the air conditioner is the hardest-working machine in an American home, and one of the biggest electrical loads. Behind the humming outdoor unit is a set of electrical parts, a disconnect, a contactor and a capacitor, that every technician must understand. This post explains what each part does and how the compressor circuit is protected.
The Parts of a Home AC System
The compressor. The pump in the outdoor unit that compresses refrigerant gas and moves heat out of the house.
The condenser fan motor. Blows air across the outdoor coil to dump heat outside.
The contactor. A heavy-duty relay that switches 240 volts to the compressor and fan motor when the thermostat calls for cooling.
The run capacitor. Gives the motors the phase shift they need to run with full torque.
The AC Disconnect
Under NEC Article 440.14, the disconnect for an air conditioner must be within sight of the unit and readily accessible. In practice this is a grey pull-out disconnect box mounted on the wall beside the outdoor unit. Pulling the handle out cuts power to the unit so a technician can work on it safely.
The Contactor: A Relay for Heavy Loads
The thermostat works on 24 volts, but the compressor runs on 240 volts, so something has to connect the two. That is the contactor. When the thermostat calls for cooling, it sends 24 volts to the contactor coil. The coil is an electromagnet: it pulls in a set of heavy contacts that connect 240 volts to the compressor and the fan motor. When the thermostat is satisfied, the coil drops out and the contacts open.
Contactor contacts carry high current and wear out over time. Pitted contacts can stick closed, keeping the compressor running non-stop, or fail to close, leaving the unit dead. A common home unit is a two-pole, 30-amp contactor with a 24-volt coil.
Diagram: The AC control circuit: the thermostat sends 24 V to the contactor coil, which closes the contacts and feeds 240 V to the compressor and fan motor.
The Run Capacitor
A single-phase motor cannot start or run efficiently on its own, because a single-phase supply has no rotating magnetic field. The run capacitor solves this by shifting the current in the start winding, creating the phase difference the motor needs.
Most home AC units use a dual run capacitor, one can with three terminals: C for common, FAN for the fan motor, and HERM for the hermetically sealed compressor. A common rating is 35/5 µF, meaning 35 microfarads for the compressor and 5 for the fan, at 370 or 440 volts. A weak capacitor is the most common cause of a humming compressor that will not start.
Diagram: A dual run capacitor: C is common, FAN serves the fan motor, and HERM serves the compressor. A typical rating is 35/5 µF at 370 or 440 volts.
Compressor Circuits and NEC Article 440
The compressor nameplate lists two currents: RLA, the rated-load amps it draws while running, and LRA, the locked-rotor amps it draws at start-up, which can be five or six times higher. The minimum circuit ampacity (MCA) sets the wire size, and the maximum overcurrent protection (MOCP) sets the largest breaker allowed. NEC Article 440 allows the breaker to be larger than the wire ampacity, because the motor's own overload protection guards the motor, while the breaker guards against short circuits and ground faults. A typical 3-ton unit has an MCA of 24 amps and an MOCP of 40 amps, wired with 10 AWG copper on a 40-amp breaker.
Key Takeaways
The outdoor AC unit is switched by a contactor, a 24-volt coil relay that connects 240 volts to the compressor and fan. A pull-out disconnect within sight of the unit lets a technician kill the power safely, as NEC Article 440.14 requires. The dual run capacitor gives the motors their phase shift, and a weak one is the most common cause of a humming unit that will not start. Compressor circuits use the MCA for wire size and the MOCP for the largest breaker.
Objective Questions
1. Under NEC Article 440.14, where must the air conditioner disconnect be?
a) Inside the main panel only
b) Within sight of the unit and readily accessible
c) At least 50 feet from the unit
d) Inside the house next to the thermostat
Answer: (b) Within sight of the unit and readily accessible
Explanation: Article 440.14 requires the disconnect to be within sight of the air conditioning equipment and readily accessible.
2. What does the contactor do in an AC system?
a) It cools the refrigerant
b) It lets a 24-volt thermostat signal switch 240 volts to the compressor and fan motor
c) It stores electric charge for starting the motor
d) It measures the temperature of the room
Answer: (b) It lets a 24-volt thermostat signal switch 240 volts to the compressor and fan motor
Explanation: The contactor coil is energised by 24 V from the thermostat, and its heavy contacts then connect 240 V to the compressor and fan motor.
3. On a dual run capacitor, which terminal serves the compressor?
a) C
b) FAN
c) HERM
d) START
Answer: (c) HERM
Explanation: HERM stands for the hermetically sealed compressor, FAN serves the fan motor, and C is the common terminal.
4. What is the most common cause of a humming compressor that will not start?
a) A weak run capacitor
b) A dirty air filter
c) A thermostat set too low
d) A loose disconnect handle
Answer: (a) A weak run capacitor
Explanation: A weak or failed run capacitor cannot give the motor the phase shift it needs, so the compressor hums but will not start.
5. In NEC Article 440, what does the MCA (minimum circuit ampacity) on the nameplate set?
a) The largest breaker allowed
b) The wire size for the circuit
c) The size of the disconnect box
d) The voltage of the contactor coil
Answer: (b) The wire size for the circuit
Explanation: The MCA sets the minimum wire size, while the MOCP (maximum overcurrent protection) sets the largest breaker allowed.
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