A window air conditioner should be installed close enough to an outlet to allow the cord to reach, but when you need immediate relief from the heat, you can use a heavy-duty extension cord for your new appliance. The one you choose must be heavy enough to handle the load, which for many air conditioners is around 12 amps. While a 14-gauge cord can handle that much current without overheating, a 12-gauge cord is safer and a 10-gauge cord even more so.
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A Word of Caution
Keep in mind that extension cords are recommended only for temporary use with an air conditioner. The fire danger posed by melting plugs and receptacles only gets worse in hot weather. Extension cords can also get dangerously hot, especially when covered by carpeting or taped to the wall, but if you leave them uncovered, they become tripping hazards.
In addition, use of an extension cord may void the air conditioner manufacturer's warranty on the unit as well as affect your homeowner's insurance. Undersized extension cords have historically been a leading cause of fire and death. If the unit is permanent, you should replace the cord or install a new outlet.
Select the Right Cord
To properly size your extension cord, you need to know how much power your air conditioner consumes, which you'll find on a label affixed to the appliance. Typical wattage for a window air conditioner is between 900 and 1,500 watts. Extension cords are rated by the number of amps they can handle, however, so you have to convert the wattage to current. To do this, simply divide by the voltage displayed on the label, which should be 120 volts.
In the case of a 1,500-watt unit, the current is 12.5 amps. A 14-guage extension cord can handle that much current, but remember that an air conditioner, like most appliances, draws more current when it starts up. For this reason, and to prevent poor performance because of voltage drop, it's safer to use a 12-gauge cord.
You can find a 12-gauge heavy-duty extension cord in the electrical section at a hardware store, but probably not in a general-purpose store, such as your local pharmacy. The utility cords stocked in such stores are safe for lamps and electronic equipment, but not air conditioners. Most air conditioner cords have a three-prong connection, so be sure to select a three-prong extension cord. It's important to preserve the ground connection to prevent damage to the appliance in the event of a power surge or ground fault.
Secure the Cord Safely
To prevent the cord from being damaged by furniture legs and vacuum cleaners, and to keep it away from children and pets, you should attach it to the top of the baseboard rather than leaving it loose on the floor. Use a cord that's just long enough to make the connection, but no longer, and wrap the junction between the air conditioner cord and the extension cord with electrical tape to keep it from separating. Do not route the cord through doorways, windows or along heavily traveled parts of the floor.
Better Than an Extension Cord
If the outlet is less than 10 feet away from the air conditioner, you may be able to replace the built-in cord with a longer one. This is a fairly simple procedure as long as you do it before you install the unit. You can leave this cord permanently plugged into the outlet without any safety worries.
Another alternative is to install a new outlet. If you choose this option, connect the outlet to a dedicated circuit rated for 20 amps. That way, you'll avoid tripping the breaker every time the air conditioner switches on.
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker and Family Handyman.