A Beginner’s Guide to Outdoor AC Unit Wiring at Home

Mini-splits are the talk of the town where I live in California, largely because of the likelihood that gas appliances may not be available for much longer here. A mini-split heat pump system is basically an air conditioning unit that runs in reverse, providing both cooling and heating. You wire one just like a dedicated air conditioner. We asked master electrician John Williamson to explain the process.

Whether for cooling or heating, the main working part is the condenser unit, installed just outside the house. The evaporator and blower fan unit go inside the house.

When you install a mini-split, you connect a factory assembly of pipes that circulate refrigerant from the outdoor unit through the wall to the indoor unit. You need to supply power to the outdoor condenser. And because the indoor unit receives its power from the outdoor one, no indoor wiring should be necessary.

The procedure for hooking up the power is straightforward. It’s similar to hard-wiring an in-wall AC or the outdoor unit for a ducted central air conditioner. Mini-split internal wiring is generally done at the factory. You simply supply power to the outdoor condenser unit from the home’s main electrical panel.

Beginner’s Guide to Outdoor AC Unit Wiring

You don’t have to be an electrician to wire an air conditioning unit, but a good understanding of home wiring practices helps.

All but the smallest window air conditioners operate at 240 volts. Every 240-volt appliance runs on a dedicated circuit, so you’ll need to install a new 240-volt circuit breaker in your main electrical panel and run the wires to an outdoor air-conditioning disconnect switch. That switch must be within sight of and accessible from the condenser unit.

In addition, a certain amount of working space is required in front of the air-conditioner disconnect switch—30 inches wide and at least 36 inches deep. This is so a service technician can safely turn off power to the condensing unit, with room to perform diagnostic or repair work on the switch and the condensing unit.

If you lack confidence in your wiring skills or your knowledge of electrical codes, wire gauge is important. Hire an electrician for this part of the job.

How to Prep for Air Conditioner Electrical Hook-Up?

Once you’ve installed the circuit breaker and run the wires to the disconnect switch, you’ll need to run wires from the switch to the air conditioning unit, then connect them to the appropriate lugs in the control panel.
Before doing your AC condenser wiring, check with your local building permit department, because you’ll probably need a permit before you can proceed. Once you sort all that out, assemble your tools:

• Phillips screwdriver
• Utility knife
• Wire stripper
• Pliers
• Screw-on wire connectors
• Non-contact voltage tester

You’ll also need a length of flexible nonmetallic conduit and cable long enough to reach from the disconnect switch to the air conditioner. The cable needs to be the same gauge as the cable that goes back to the panel of the wiring AC disconnect box.

Outdoor AC Unit Wiring Safety

The top safety advice for any electrical project: Don’t get electrocuted.

Turn off the circuit breaker before you work on a circuit and use your voltage tester to make sure wires are dead before you touch them. Remember to check all HVAC unit electrical connections first.

Prevent fires by:

• Following the code requirements for wire gauge and circuit breaker rating.
• Making your connections tight to prevent wires from slipping and causing arcing.
• Double-check the cable for damage before installing it.
• Ensuring no exposed wires peek out from wire cap splices. It’s a good idea to wrap electrical tape around these splices, just to be sure.

Basic Steps for Wiring an Air Conditioner

1. Run electrical cable from the service panel to an air conditioner disconnect switch near the A/C unit and connect it to a new circuit breaker in the panel. Turn the breaker off.
2. Prepare a length of cable to run from the disconnect switch to the A/C unit’s control panel. Insert it into a flexible nonmetallic conduit, then feed the ends through knockout holes in the disconnect switch and control panel. Make sure you have enough slack on both ends to make connections. Affix the conduit to the boxes. Here’s everything you need to know about disconnect switches.
3. Take a utility knife, cut about six inches of insulation from the ends of both cables in the electrical box, then separate the wires. If you’re wiring a 240-volt unit, you’ll find a red wire, a black one and a bare or green one. Test the red and black wires coming from the panel with a non-contact voltage tester to make sure they’re dead. Learn how to insulate A/C lines.
4. Using wire strippers, strip about an inch of insulation from the ends of all the wires. Splice wires of like colors by twisting the ends together clockwise with pliers, then screwing on a wire cap. Black goes to black, red to red and ground to ground.
5. Move over to the A/C unit control panel and locate the main terminals. Strip the ends of the wires, make clockwise hooks on the end of the black and red wires, hook them over the terminal screws and tighten down the screws. Either wire can go on either screw. For a more secure connection, crimp a ring connector onto each wire and attach the connector to the terminal screw.
6. Locate the ground screw, which should be somewhere on the housing close to the terminals. Attach the ground wire to that screw.

When Should You Hire a Pro?

If any internal wiring inside the air conditioning unit needs to be connected, it’s best to hire an electrician to reconnect it.

t’s possible to do this yourself using the manufacturer’s wiring diagram for reference, but a mistake can damage the equipment. It’s safer to leave it to a well-trained professional.

FAQ

What’s the difference between wiring a mini-split and a central AC unit?

Both of these units use 240-volt wiring and a disconnect switch to establish a healthy wiring system. Mini-split systems are simpler to wire than central air systems that involve more components. In fact, mini-splits consolidate power at the outdoor unit, so there is no need for indoor electrical wiring. This also makes the process less time-consuming or risky.

About the Expert:

• John Williamson, a master electrician and building official with over 45 years of experience, has worked extensively in heating and cooling systems, ensuring compliance with local electrical codes.

Sources:

• Lowe’s: “Eaton 30 Amp Single-phase (124V/240V) Fusible Outdoor General-duty Disconnect.”
• Home Improvement: “Need to identify wires coming from external AC units.”