Proper Voltage for a 120V Air Conditioner

Understanding AC

• Various manufacturers might describe the power needs of an air conditioner in slightly different terms even though the units run on the same type of power supply. Some companies rate units for 120 volts AC, while others refer to the required power as 110 or 115 volts AC. These numbers describe average values. Household wiring in the United States now uses a three-wire system for 120-volt power, but older homes might have either polarized two-wire systems or two-wire systems with no polarity. The unpolarized system used outlets with socket holes of equal size. Today, if your air conditioner's power plug fits the wall socket, it should work safely. Never adapt an appliance plug to fit an outdated outlet.

Evolving Safety

• In unpolarized systems, either hole in the socket might lead to the hot side of the power system. AC-powered appliances work whether the plug enters the socket correctly or reversed, because both methods complete the circuit. Older appliances sometimes used "hot chassis" systems, applying one side of the power circuit directly to the chassis, including the appliance cabinet and handles. If the plug connected in the wrong way, the hot wire electrified the chassis of the appliance. Turning the plug over in the outlet rendered the same appliance safe. Polarized outlets, those with differently sized prong holes, prevent potentially deadly mistakes.

Grounded Outlets

• Three-wire systems offer greater safety. The ground wire connects the cabinets of most appliances directly to ground, shielding the user from electric shock. The different sizes of hot and neutral terminals ensure that the hot wire connects to the proper side of the appliance's internal circuit. In older homes, upgrading to three-wire outlets meant running new ground wires throughout the house. Such houses might contain three-wire outlets with no ground connections. Untrained electricians often believed the two sides of the AC power system carried the same voltage and used them interchangeably. Three-wire outlets might appear to meet modern electrical codes even when they don't

Actual Voltage

• The "hot" and "neutral" supply wires in a household system both carry current when the appliance connected to them operates. Without an electrical load, only one wire can be considered "hot." An AC voltmeter reads line voltage as a "root mean square" value that represents average usable power on the line. An AC voltmeter measures about 120 volts from the hot wire to ground or from the hot wire to the neutral wire. Actual voltage changes from positive to negative 60 times each second, peaking at plus or minus 170 volts. The voltmeter measures a lower voltage from the neutral wire to ground, but that reading results from electromagnetic induction and can't power an appliance.