The Basics of 240/250-volt Air Conditioner Circuits

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While most small plug-in appliances and electrical fixtures in your home operate on 120-volt circuits and are plugged into ordinary outlet receptacles, larger appliances—including electric stoves, electric dryers, electric water heaters, and larger air conditioners—are powered by 240/ 250-volt circuits. Sometimes these larger appliances are "hardwired"—meaning that the circuit wires are connected directly to the appliances—but other larger appliances are connected via plug-in cords.

When compared to a 120-volt outlet, a 240/250-volt outlet will have a noticeably different slot configuration and will vary depending on whether the circuit is 20 amps, 30 amps, or larger. A 50-amp 240-volt electric range outlet, for example, will look quite different than a 20-amp 240-volt air conditioner outlet. 

It's important to note that ALL 240/250-volt appliances require a dedicated circuit—one that feeds only the appliance and no other fixtures along the circuit. This is in contrast to many 120-volt circuits, which may feed a number of light fixtures or wall outlets along with their runs. 

Why the Variable Rating?

The voltage ratings on electrical fixtures and circuits can be a little confusing. You may see a circuit or large appliance rated at 208- 220-, 230-, 240- or 250-volts—or it may be labeled something like "240/ 250-volts." To all intents and purposes, these are the same ratings. The different numbers represent the fact that household circuit voltage is somewhat variable as it is delivered to your home from the utility company. At one time, double-pole circuits were routinely known as 240-volt circuits, but electrical service today can be somewhat higher than this, so circuits may carry the label "240/250 volts." When you see a rating with numbers such as 208- 220-, 230-, 240-, or 250-volts, they all refer to the same thing—a double-pole circuit that is fed by both hot bus bars in the service panel. 

Similarly, 120-volt single-pole circuits can vary between about 110 and 130 volts; they all refer to the same type of circuit—a single-pole circuit fed by a single hot bus bar at the main service panel. 

When an appliance such as an air conditioner carries a variable voltage rating, such as 208/230 volts, it indicates the high and low range of what the appliance will draw in current while in operation. Such ratings aren't particularly significant because all residential double-pole circuits easily handle that voltage range. 

Air Conditioner Circuits

Some portable air conditioners are 120-volt appliances that plug into standard wall outlets, but larger air conditioners—some window and portable models as well as permanent central air conditioning units—are powered by 240/250-volt circuits. The largest central units may require a 50-amp circuit, while the very smallest central conditioning units may call for 15-amp circuits. For most homes, 30-amp or 40-amp 240/250-volt circuits are typical for a central air conditioner. 

For portable 240/250-volt plug-in air conditioners like those that fit into windows, 20-amp circuits are typical. In either case, the circuit must be dedicated to the air conditioner unit. To be in compliance with Code, the circuit cannot serve any other appliances or fixtures in your home. 

Cable Wiring for Air Conditioner Circuits

Like any other electrical circuit, air conditioner circuits are generally wired with nonmetallic (NM) cable. The wire gauge must be appropriate for the amperage of the circuit. For a 20-amp window air conditioner, 10-gauge wire is typically used. This is a slightly heavier wire than would be normally used for a 120-volt 20-amp circuit, where a 12-gauge wire is standard. The reason for the heavier gauge is that air conditioners will sometimes trip a circuit breaker during the start-up surge; wiring with heavier-gauge wire prevents this. 

Outlet Wiring for Air Conditioner Circuits

In a 2-wire cable for a 120-volt circuit, the "hot" current is carried by the black wire, while the white wire is the neutral. However, in a 240/250-volt circuit, there is no traditional neutral, since both wires are each carrying 120-volts of "hot" current. Because both wires are hot, it's traditional for the white wire to be labeled with a tab of black or red tape near the screw terminal connection on the outlet receptacle. This will tell a service person that the connection is hot. 

The outlet receptacle used must match the amperage rating of the circuit and the air conditioner—a 20-amp receptacle has a different configuration than a 30-amp receptacle. The circuit ground wire should connect to the outlet and, if the electrical box is metal, also to the box.