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AC Installation Guide - 8

Prepare High-Voltage Connections

A central air conditioner runs on 220 volts of electricity.  A connection must be made between the main electrical circuit panel in your home and the condensing unit in order for the air conditioning to run.  You can prepare the wiring for hook-up, but only a skilled contractor should complete the high-voltage wiring.  Inexperience can cause electrical shock or damage to your equipment or property.  

You will need to select the correct thickness of wire and corresponding breaker based on your condensing unit.  Wires are rated by gauge.  The higher the gauge number, the thinner the wire.  Most residential AC units use 8 or 10-gauge wire.  See the manufacturer's spec sheets for more information.  

The first step to preparing your high-voltage connections is to install the disconnect.  Electrical codes require a disconnect/safety switch to be located near the condensing unit.  Check your local electrical codes for specifics.

The disconnect will have two electrical whips running in and out of it:  one runs from the condensing unit to the disconnect and the other runs from the home's electrical circuit panel to the disconnect. 

Back to Step 7                  Continue To Step 9

Install Disconnect Box and Electrical Whips

1.  Select a spot on wall to mount disconnect box. 

  • Locate it near the hole in the house and close to condensing unit. 
  • Place it high enough up to keep it out of snow or water on ground. 
  • See electrical codes for specifics on where to locate the disconnect.   
  2.  Pull cover off of disconnect box to see inside.  You will find spots to attach wires and small holes to mount the box with screws.
3.  Remove the On-Off switch. 
  4.  Punch out one pre-stamped hole on each side (or bottom) of disconnect box.  Punch out the holes that are properly-sized for the electrical whips.  One electrical whip will be inserted on each side. 
5. Mount the box to the wall with screws and drill. 
  6.  Slide whips through holes in box and tighten nuts.  (See Step 6 for more details.)
  7.  One whip has a 90-degree bend and one has a straight end.  Use the whip on each side that best fits your application. 
  Hook wires up inside the disconnect box.  The green wires are the ground wires. 

8.  Cut the two green wires to size and strip back 1/2" of insulation.
9.  Hook up both wires, each one under either lug in center of disconnect and tighten down with screwdriver.  Make a good solid connection. 

For the whip closest to condensing unit, hook up the red and black wires inside box at the "load" terminals (in center, near top of disconnect).   

10.  Cut the red and black wires to size and strip off 1/2" of insulation. 
11.  Hook up each wire under either lug. 

  12. Tighten lugs down with screwdriver.  High voltage needs a good, solid connection. 

13.  On the other whip, cut the red and black wires to size and strip back 1/2" of insulation. 
14.  Hook up the red wire to the terminal closest to the red wire just installed.  Tighten down with screwdriver. 

  15.  Hook up the remaining black wire to the terminal closest to the black wire just installed.  Tighten down with screwdriver.
  16.  Slide the electrical whip closest to hole in house, through the hole. 
  17. Replace On/Off Switch in disconnect box.
18.  Turn switch to Off until contractor starts unit.
Mount Whip To Ceiling, Connect Flexible Wire and Run to Home's Electrical Panel

Back inside home, the whip needs to be mounted to the ceiling, wires connected to it and the wires run to home's electrical circuit panel. 

19.  Find a spot on the ceiling to mount the whip and wires.  See local electrical and building codes for more allowable placement. 

  20.  Use a standard 4x4 junction box (available at any hardware store) to mount whip and wires.   Screw box into place. 
  21.  Punch out appropriately-sized holes on both sides of junction box.
  22. Run whip into junction box on side closest to hole in house. 
23.  Run flexible wire from junction box to home's electrical panel.  Use the correct wire gauge (based on your condensing unit).  Flexible wire such as Romex may be used if building/electrical codes permit.  Some building/electrical codes require the use of solid, conduit pipe to pass wires through. 
24.  Strip about one foot of insulation from wire. 
25.  Slide wires into box connector (available at any hardware store). 
26.  Slide box connector down the wires to where the insulation begins. 

27.  Tighten down top screw on box connector until it holds insulated wire securely. 

  • Don't pinch the wires by tightening the screws down too tightly. 
  • The connector itself is not screwed into anything, yet. 
28.  Tighten down bottom screw on box connector.
29.  Remove nut on end of box connector. 
30.  Slide wires through hole in side of junction box. 
31.  Replace nut.
32.  Hand-tighten nut to secure wires. 

33.  Match wires by color inside junction box.  Attach wires from whips to corresponding Romex wires.  

  • Green (ground) matches copper.
  • Red matches red or white.
  • Black matches black. 

34.  Trim wires and cut back 1/2" of insullation.  Use wire nuts to make connections. 


35.  Replace cover on junction box once you have matched all wires. 

36.  Secure flexible wire to ceiling with nails, wire staples and hammer. 
37.  Run flexible wire over to main electrical circuit panel in home.
38.  Spool wire up for contractor to complete high voltage wiring on panel. 
Hook Up High Voltage Wiring to Condensing Unit
  39.  Outside, remove screws from access panel on front of condensing unit. 
  40.  Remove access panel. 

41.  Remove nut from electrical whip hanging from disconnect box. 
42.  Run whip up through the high-voltage electrical hole that protrudes from corner of condensing unit. 

  43.  Slide nut over wires. 
  44.  Tighten nut down on washers. 

45.  Trim ground (green) wire and strip back 1/2". 
46.  Place ground wire under lug located on side. 
47.  Tighten down with screwdriver.    


The remaining black and red wire from whip will be connected to either side of contactor. 

  Use spade connectors to make an easy connection. 
  48.  Slip spade connectors over end of red and black wires. 
  49.  Crimp end of spade connectors with spade crimpers or pliers.   
  50.  Press red and black wires with spade connectors up into contactors, one on each side.  

Back to Step 7                  Continue To Step 9

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