How to Wire an AC Disconnect

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What You'll Need
AC Disconnect switch
Screwdriver
Drill
Drill bits
Screws
Heavy gauge cable
Wire Cutters
Wire Stripper
Utility knife
Electrical Voltage Meter

Sometimes you just need to shut it down. In electrical work and construction, a disconnect switch, also known as an isolator, can be used to separate an electrical circuit from the main power source. This can help ensure the equipment is completely de-energized while performing maintenance or servicing, eliminating the need to go back to the main electric panel every time it needs to be turned off and on.

Disconnect switches can be especially helpful for large appliances located far from your home's breaker, as is often the case with outdoor AC units. Electrical work like this is typically a job for a professional, and in some cases it's even illegal to do without certification, but if you're interested in learning about this process, these are the steps.

Considerations Before Starting

Adding a disconnect to an existing AC unit is essentially a matter of cutting the 220 volt feed to the AC unit and adding the disconnect in between, thus bridging the current flow through to the AC unit.

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You can also fit a disconnect switch while installing a new AC unit, which makes it easier to choose a practical location.

outdoor wall mounted AC unit

Safety First

When adding a disconnect to an already existing AC, the first thing to do is to go to the main electric panel and turn OFF the 220-Volts circuit breaker indicated in Figure 3. This is a crucial step to avoid serious accidents, like potentially fatal shocks.

circuit breaker switches

For a new installation, it is the main breaker in Figure 4 instead that will be turned off in Step 4. If you're not sure what switches govern which areas, play it safe and shut the power down completely.

Step 1 - Choose the Switch Location

The disconnect switch should be screwed into place on the wall next to the AC unit. Depending on where the cables will be inserted into the switch box, the proper knock-out plugs can be removed once you've mounted the switch, and cable connectors can be fastened into place.

If the disconnect is added to an existing AC Unit, it should be within reach of the already existing cable, so only a short cable is needed to bridge the switch to the AC unit.

Step 2 - Prepare to Wire the Disconnect Switch

With the breaker turned OFF, the wires can be disconnected and the cable removed from the electrical compartment of the AC Unit.

For a new AC installation, a cable must be installed from the disconnect to reach up to the main electric panel. A hole must be drilled through the wall to bring the cable inside. It’s a good practice to pre-drill a smaller hole with a long drill bit to make sure it’s where it should be and avoid unpleasant surprises. The hole can then be drilled to its full size to accommodate the cable. Passing the cable through can be done more easily from the inside when the panel is some distance away.

For either case, this cable, the feed line from the main electric panel, has to be prepared by stripping some of its external jacket to expose the wires. Remove about .75 inches (19 mm) of their insulation before inserting the cable into the cable connector and clamping it in place in the disconnect box. Then, cut another cable of the same rating with about 16-20 extra inches and prepare it at both ends in the same manner.

One end of this cable is then inserted and secured into the AC unit’s electric panel, while the other end is fixed in the same manner into the disconnect switch, where it is secured at both ends with cable connectors. The wires can now be connected to their respective terminals inside the AC’s electrical box by matching their colors with the colors of the wires already in place. With this done, the cover can be put back on the AC’s electric box.

Step 3 - Hook up the Disconnect

ac disconnect switch

The disconnect switch is shown in Figures 5, 6, and 7. Figure 5 shows a pullout switch with the cover lifted and. Remove the double-contact pullout inside (Figure 6) to open both contacts of the switch.

Figure 7 is the same switch with all covers removed, showing where to connect the wires so both red wires go to one set of contacts, while the black wires go to the other set of contacts, which can both be disconnected by pulling a handle.

The two ground wires go to the ground terminals, and, finally, the two white neutral wires are simply connected together inside with a wire nut. Remember to put the covers back once you're done.

AC disconnect switch

If this was an addition to an existing AC unit, the dedicated breaker can be turned back on—ready to go.

Step 4 - Wiring the AC to the Electric Panel

This is where previous experience working inside a 220V electrical panel is an asset.

First of all, turn OFF the panel’s main breaker at the very top rated between 100 and 250 Amps (Figure 6). This will switch off everything electrical in the house, so the work should be carefully planned ahead to minimize the extent of the interruption to an appliances like fridges and freezers. With this done, the front cover of the large panel can be removed, exposing a whole bunch of wires and breakers.

Once the spot is picked, a 220 Volts circuit breaker can be installed and secured in place in the OFF position. The most appropriate knockout plug for the wiring can then removed and the cable connector installed.

Measure the length of the wires so you know they'll reach from the breaker’s terminals to the knockout hole, accounting for all the bends and the .75 inch (19 mm) skinning to set them in their terminals.

Mark the cable’s jacket at that length, and cut the outer jacket across its perimeter, separating it from the rest of the cable. Use a utility knife, and take care to apply only light pressure while cutting, to avoid slicing into the insulation. Once this is done, cut the jacket through from that cut right to the end and remove the outer layer, exposing a black, a red, a white, and a ground wire.

Pass the cable through the cable connector until half an inch of the jacket shows inside to protect the wires from the clamp. After the .75 inches (19 mm) of insulation are removed from the end of each wire, the bare copper wire can be wired to the ground bus bar, the white wire to the neutral bus bar, and the black and red-colored wires to each one of the two terminals on the circuit breaker. It doesn't matter which of the two goes where.

Step 5 - Turn the Power Back On

To prevent a surge of power through the house, it's a good idea to switch all the breakers off inside the electric panel before switching on the main power breaker. Then, starting with the main breaker, turn every breaker back ON one at a time, leaving the newly added breaker for last.