how to wire contactor on ac unit



When it comes to an air conditioning unit, the contactor plays a crucial role in controlling the flow of electricity to various components of the system. It acts as a switch that enables or disables the power supply to the compressor and condenser fan motor. Understanding how to wire a contactor correctly is essential for ensuring the smooth operation of your AC unit. In this article, we will guide you through the process of wiring a contactor on an AC unit, providing you with step-by-step instructions and essential tips to follow.

The Role of a Contactor in an AC Unit

In order to understand why wiring a contactor is necessary, it is important to grasp the role it plays in an AC unit. A contactor is an electrical switch that controls the power supply to the compressor and condenser fan motor. When the thermostat signals the AC unit to start cooling, the contactor engages, allowing the electrical supply to flow through to the compressor and fan motor. Conversely, when the desired temperature is reached or when the AC unit is turned off, the contactor disengages, breaking the electrical circuit and stopping the operation of the compressor and fan motor.

On the surface, a contactor might seem like a simple component, but its proper functioning is crucial for the efficient and safe operation of an AC system. Therefore, it is essential to know how to wire a contactor correctly to avoid potential issues such as erratic cooling, overheating, or even electrical hazards.

Gather the Necessary Tools and Equipment

Before diving into the process of wiring a contactor, it is important to gather the necessary tools and equipment to ensure a smooth installation process. Here are the essential items you need:

1. Screwdriver: A flat-head or Phillips screwdriver will be required to remove and install screws throughout the wiring process.

2. Wire Strippers: These tools help you strip the insulation from the wires, enabling proper connections.

3. Wire Nuts: Wire nuts are small plastic caps that twist onto the ends of wires to connect them securely.

4. Electrical Tape: Electrical tape is used to insulate wire connections, preventing any electrical shorts or hazards.

5. Voltage Tester: A voltage tester is an indispensable tool for checking the presence of electrical power and verifying if the circuit is energized or de-energized.

6. Contactor: Of course, you will need the contactor itself, which can be purchased from an HVAC supply store or online. Make sure to select the right contactor for your AC unit model and electrical specifications.

Once you have gathered these tools and equipment, you are ready to proceed with the installation process.

Step-by-Step Guide to Wiring a Contactor on an AC Unit

Wiring a contactor on an AC unit might sound daunting, but by carefully following these step-by-step instructions, you can tackle the task with confidence:

Step 1: Safety First

Before starting any electrical work, it is crucial to ensure your safety. Always turn off the electrical power to the AC unit at the main circuit breaker or disconnect switch. Additionally, use a voltage tester to double-check and ensure the absence of any electrical power to the unit. Safety should always be your top priority when working with electrical components.

Step 2: Access the Contactor Compartment

To access the contactor compartment, you will need to remove the service panel of the outdoor AC unit. This panel is typically located on the side or top of the unit and is held in place by screws. Once the screws are removed, carefully lift off the panel to expose the internal components, including the contactor.

Step 3: Identify the Wiring Terminals

Inside the contactor compartment, you will find a series of electrical terminals where the wires will be connected. These terminals are usually labeled with letters such as L1, L2, T1, T2, etc. It is crucial to identify these terminals correctly, as miswiring can lead to improper operation or potential damage to the AC unit.

Step 4: Cut and Strip the Wires

Before connecting the wires to the contactor terminals, measure and cut them to the appropriate lengths, leaving some slack for ease of installation. Using wire strippers, carefully strip the insulation from the ends of the wires, exposing the bare conductor for connection. It is essential to strip just enough insulation to make a secure connection without exposing too much of the conductor.

Step 5: Connect the Wires to the Contactor Terminals

Now comes the crucial step of connecting the wires to the appropriate contactor terminals. Remember to double-check the correct terminal identification for your specific AC unit model and refer to the unit's wiring diagram if needed. Here are the typical wire connections:

1. Power Supply Wires: Connect the L1 and L2 wires from the power supply to the corresponding L1 and L2 terminals on the contactor. These wires deliver the electrical power to the contactor.

2. Compressor Wires: Connect the wires from the compressor to the T1 and T2 terminals. Typically, one wire is connected to T1 and the other to T2. These wires are responsible for providing power to the compressor.

3. Condenser Fan Motor Wires: Connect the wires from the condenser fan motor to the appropriate terminals. Similar to the compressor wires, these are typically labeled as T1 and T2.

4. Low Voltage Control Wires: If your AC unit has low voltage control wiring, which is often the case for systems with advanced features like intelligent thermostats, connect the low voltage wires to their designated terminals. These connections control the signal to the contactor, telling it when to engage or disengage.

Step 6: Secure the Wire Connections

After making the necessary wire connections, it is vital to secure them properly to prevent accidental disconnections or electrical hazards. Use wire nuts to twist and secure the wires together, ensuring a tight connection. Additionally, wrap electrical tape around the wire nuts to provide extra insulation and protection.

Step 7: Test the Contact

With all the wiring in place, it is time to test the contactor to ensure proper functioning. Turn on the electrical power to the AC unit and set the thermostat to activate the cooling mode. Observe the contactor to see if it engages, allowing the power to flow to the compressor and condenser fan motor. If the contactor operates smoothly and enables the AC unit to function as expected, your wiring is successful.


Wiring a contactor on an AC unit is a crucial task for ensuring the proper functioning and safety of the system. By following the step-by-step instructions outlined in this article, you can confidently tackle the installation process. Remember to prioritize safety, gather the necessary tools, and carefully connect the wires to the appropriate terminals. With the contactor correctly wired, you can enjoy the cool comfort provided by your air conditioning system without any worries or issues.


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