Comparison of 120 Volt & 220 Volt Systems

Basics

• North American 120 volt systems are actually three-wire systems with two wires that carry a voltage and a third neutral wire. The two live wires have a voltage of 110 to 120 volts when measured against the neutral wire, and 220 to 240 volts when measured from one live wire to the other. At the breaker panel, a single pole breaker can interrupt the single live wire for a 120 volt circuit, but the 220 volt circuits with two live wires require a two-pole breaker.

Currents

• For a given load, doubling the voltage means that the circuit only needs half the current. For small loads, this reduction in the current is not important because the cables carrying the small currents are flexible and inexpensive. For large loads, 120 volt circuits require very large cables that are expensive and hard to run. Using circuits with the higher voltage and lower current means that the cables can be smaller.

Power

• Changing the voltage of the circuit does not change the power used by the load. A five kilowatt load can run on 110 volts and 46 amps or 220 volts and 23 amps. Since the electric utility company charges for power, increasing the voltage does not reduce the electricity bill.

Cables

• The two voltages use wiring that looks similar, but many new installations have the second hot wire on a 220 volt system colored red. The 120 volt cables have one live wire, one neutral wire and a ground. The 220 volt cables have either two live wires and a ground or two live wires, a neutral and a ground.

Plugs

• The plugs used for the two systems are quite different because plugging a 110 volt appliance into a 220 volt socket is dangerous, and is likely to destroy the unit. Type A plugs for 110 volts have two prongs, with one slightly wider to make sure the plug is inserted the right way around. Type B plugs have two flat prongs and the neutral wire connected to a third, round prong. Plugs for 220 volts have either three or four rectangular prongs that will only fit into a 220 volt receptacle.