Building codes normally require that an external disconnect box be installed near the condensing unit located outside of the house. This box will allow the power to be cut to the condensing unit without entering the residence. Installing one of these boxes is not difficult. Be extremely cautious when dealing with this wiring, since the condensing unit requires 220 volts of power. While not all boxes are made exactly alike, the wiring layout should be close enough not to cause a problem in following this procedure for installing the disconnect box.
Determine where the air conditioning condensing unit will be placed. These units need to be as close to the air handlers as possible to function best. This task will usually be decided by the company installing your air conditioning unit.
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Run wire from the circuit box to the wall where the condensing unit will be installed. Always make sure that the power is off and test the wires with a voltage checker to make sure they are dead before handling them.
Mount the box outside the house in a convenient location 2 feet or more from the ground. Examine the box to see where the wire will need to enter it. Many of these boxes will have punch outs on the back, side and bottom to give you a selection. If you can bring the house wire in through the back of the box, you will not need any conduit for that portion of the wiring.
Drill through the wall from the outside using the proper size bit for the required opening. Take care not to drill the hole through the interior wall unless you are having to run the wiring along the inside wall. Push or pull the wires through the opening. Cut the wire to the needed length to reach the terminals inside the disconnect box. If you were not able to bring the wire through the back of the box, thread the wire through the conduit to the disconnect box. If the wire is a tight fit, use a small piece of wire as a guide to pull it through the conduit.
Separate the three wires and strip the insulation about 1/2 inch from the ends of the coated wires. Slide the wires into the terminals. The uncoated neutral wire will usually fasten at a post away from the main hot and ground wires. The disconnect box should have four additional terminals inside. Slide the black wire into the post marked "line" or as a hot wire. The white wire will be ground. Both of these wires should be installed side by side in most boxes. If you are uncertain, it is always a good idea to consult the manufacturer's directions, if available, to be sure.
Install the wires to the condensing unit. Cut enough conduit to reach from the disconnect box to the wiring panel on the air conditioning condensing unit. Pull the wire through the conduit while leaving enough on each end to make the connections. Strip about 1/2 inch of insulation from the ends of the wires inside the disconnect box. Install these wires the same way as the house wires, but into the other half of the box. Make sure that all of the screw connections are secure and that the conduit is firmly attached to the disconnect box and condensing unit.
Install the actual disconnect now. Almost all disconnect boxes have a pull out that will disconnect the power from the condensing unit. The disconnect will have a handle to let you pull it out from the disconnect box. On the other end it will have two plates that stick into slots to connect and disconnect the circuit. The disconnect will be marked as "on" or "off" depending on which way it is inserted into the disconnect box. When it is upside down in the "off" position, the two plates will go into empty slots that do not allow the electricity to flow through the box. Until the air conditioner is operational, it is best to insert it in the "off" position.
Turn on the power in the house and test the box for proper voltage. With the unit disconnected, test the condensing unit to make sure that the power is off on the other side of the disconnect box.
Attach the box solidly to the building. If there are gaps around the box that will let in moisture and bugs, caulk it. It is best if the disconnect box is well sealed all the way around. Excess moisture inside the box can let the electricity jump across the box even if the disconnect is removed. Open and close the box a few times to make sure that the lid functions correctly.