A 20-amp circuit is an electrical circuit designed to carry an electrical load of up to 20 amperes at a time. Electrical appliances draw current, measured in amperes. Measuring the current draw of an appliance is a matter of knowing its wattage and the voltage.
In the U.S. most circuits run on a 120-volt power source. The wattage rating, or the amount of electrical power used at a given time, of an appliance is listed on the appliance. Dividing the wattage by the voltage yields the amperage. For example, a 40-watt light bulb on a 120-volt circuit draws 0.3 amperes of current.