Central AC units are normally hardwired into the electrical system. Home air conditioners are offered in two styles: a whole home central AC unit that is often combined with a central heating system, and window units designed to cool off individual rooms. The two types of appliances require different electrical voltages to work properly. Even a small air conditioner can put a strain on a home electrical system. Central Air Conditioners Central air conditioners require a 220-volt or 240-volt, dedicated circuit for operation. When a central air conditioner starts up, it may require up to 5,000 watts of electricity, making it one of the largest consumers of electric power in the home. An air conditioner's cooling capacity is measured either in British thermal units (BTUs) or in "tons." The greater the number of BTUs or tons, the greater the cooling power of the unit. A larger unit also requires more electrical power. Buying a unit too large for your home wastes electricity, and causes the unit to wear out faster.

Window Air Conditioners Window air conditioners are offered in either 110/120-volt or 220/240-volt models, depending upon their cooling capacity. A unit designed for cooling one small room such as a bedroom normally provides 15,000 BTUs and often works on a smaller, standard 110/120-volt wall outlet. Because of its large power consumption, you should not have any other appliance on the circuit to avoid an overload. A window unit larger than 15,000 BTUs may require a 220/240 dedicated circuit. cost of replacing ac unitAdding Circuits Many homes do not have 220/240-volt circuits as standard equipment in living spaces, and they must be added by a qualified electrician. american standard ac unit coversThis can cost between $500 to $1,500 as of the date of publication, depending on the type of home and its location and the work that must be done, according to Bill Ferrera of Ferrera Electric in San Francisco. ac unit sliding window

All window air conditioners provide the necessary information about the electrical requirements on the unit packaging. As with a central unit, buying a window unit that is too large wastes energy and money. Sizing a Unit The specifications listed for individual air conditioner units state the size of the area they cool. For example, an AC unit that cools a 120-square-foot area would work best in rooms that are 10 feet wide and 12 feet across. By matching your room or home size with the unit's specifications you can select the appropriate size air conditioner. Other factors you might consider are the number of windows and amount of insulation. If more heat comes through the windows, you will need a larger unit. A poorly insulated home will allow cool air to escape faster and might require a larger window unit. A trained technician can help you determine the proper central AC unit for your needs. References Sylvane: 5 Things To Consider When Buying A Window Air ConditionerDon Rowe: How Many Watts Do You Need?

Popular Mechanics: Buying A Central Air Conditioning System Photo Credits Jupiterimages/Comstock/Getty Images Suggest a Correction I have an outside A/C compressor and indoor furnace fan combo in the attic. The unit was off and the 3 amp fuse was blown. I replaced the fuse and used the thermostat to turn the fan on followed by the heater then the A/C. I let the unit run the A/C. After a while the A/C turned off. I checked and the furnace was off, fuse blown and thermostat was asking for cool. I do not know if the fuse blew during operation, shutdown, or when the next request for cool happened. I checked the wires leading to the outside unit. They show open when disconnected and 20.8 ohms when connected to the contactor. The wires leading from the thermostat to the furnace show open until fan, heat or A/C is selected and they show continuity as expected. The volts across the red and green terminals shows 27 volts. The transformer is showing 120 in and 27 out. My first reaction is to replace the contactor on the outside unit, but that 27 volts is interesting to me because the schematic and Xformer say 24 volts.

Performed the following tests. Disconnected the wires to the outside unit and ran the fan for a few hours no issues. There are 3 wires going from the furnace to the outside unit. White wire goes to 1 terminal of the contactor relay. Red and green wires were tied together and attached to the other terminal. On the furnace outside white is tied to yellow, outside green is tied to blue. Outside red was disconnected and loose on furnace. I disconnected red from green outside and put a wire nut on it. Measured the voltage on the relay of 27 volts. The resistance without wires is 16 ohms. Ran the A/C briefly without issue. Running the A/C today without 240 to see in the contactor heats up. The furnace is a Lennox gas heater model number G40UH-36A-070-15. The outside A/C is a Lennox 13ACD-036-230-02. I went ahead and ordered a new relay because it is pretty cheap. It should arrive by this weekend. What should I do next to troubleshoot this? Set the fan to 'on' and have someone monitor the furnace blower.

Go outside and hold the AC contacter down for as long as you're willing to. If the furnace blower never trips out, then it is that contacter getting hot and drawing an over current. that's my two cents. You could also try toggling the fan to 'on' a bunch of times and see if that does it (could be a hard start after all; old motor/bearings). I guess if you want to freeze your unit and check that contacter at the same time (without anyone having to stand there) pull the power feed to the blower motor (from its clips on the control board) and call for AC. If it never trips out, it was the blower motor. If it does blow out, then it's somewhere between the control board, that contacter, or the thermostat and its wires (have fun ;). Did you find this question interesting? Sign up for our newsletter and get our top new questions delivered to your inbox (). I concede that the answer to this problem depends on some specifics that are lacking. If the OP fuse is a glass cartridge style, then it is likely that a slow blow (MDL type or similar) is required in place of the fast acting automotive type.

If the fuse is the automotive blade type as Mazura pointed out, then fast acting is the only option for those and the problem lies elsewhere. Electrical contactors do draw relatively high current until the contacts seat into the closed position, so I also have to agree that a sticky A/C contactor could blow the fuse even if the contactor coil is actually good. To test if the contactor is sticking, I would turn off the outdoor disconnect to the compressor and open the condenser access panel for inspection. Turn the thermostat to "cool" and watch the contactor pull in. If the contactor action is sluggish, that could cause the problem. With power to the compressor turned off at the disconnect, this test could be performed repeatedly. Check all of the wires where they pass through metal. The metal may have worn through the insulation on the wire, causing an intermittent short. Also, inspect the wires anywhere the outer insulation is stripped back. It's pretty easy to damage the inner insulation when stripping off the outer, which could also lead to an intermittent short.

So the problem ended up being the contactor on the outside unit. How I went about troubleshooting the problem was to disconnect the outside from high voltage power. Then I turned on the thermostat and allowed it to cycle a few times over the course of 2 days. The fan would come on as expected and the fuse never blew. I ordered a new contactor and installed it. Luckily the temperature in Houston was in the 80's in December so I tested the A/C with the new unit in it. It is worth noting that the old contactor showed a little oxidation and the points on it were burnt somewhat. If you think that over-voltage is blowing the fuse (unlikely, but possible) you can always use a larger fuse. While this is not recommended, there is some wiggle room. If you were to put a 4 amp 50 volt time-delay fuse in it's place, you would still provide short-circuit protection while bypassing this slight(ish) problem. Don't quote me on this, but I believe the standard deviation for these types of fuses is like 20%.