In electrical engineering, "power" is defined as "voltage times current." To function properly, every electrical appliance has a power rating, measured in "watts." Because standard electrical circuits in the United States supply constant voltage (either 120V for a standard circuit or 240V for a "double" circuit), the required amperage is directly proportional to the total watts of power consumed by all of the appliances connected to the circuit. Before you decide which breaker and wire to use for the circuit, you first need to calculate the maximum power it will require.
Determine the wattage of all appliances directly wired into the circuit, such as refrigerators, furnaces, light bulbs (via light fixtures), electric oven, microwave, air conditioners and water pumps. Note: the wattage will usually be printed on the back or underside of the appliance. If you're unsure, visit the manufacturer's website and look up the model's product specifications.
Determine the wattage of all appliances regularly plugged into an outlet, such as televisions, computers, lamps and aquariums.
Determine the wattage of appliances likely to be plugged into an outlet on the circuit at some point. For example, if you own a table saw, pneumatic air compressor or other power tools, you might include them in the wattage requirements for a circuit that services your garage. Likewise, if you're computing the wattage for a bathroom-based circuit, add the wattage of your curling iron.
Add up the wattage of all the appliances that either are or may be attached to the circuit, i.e. from Steps 1, 2 and 3.
Divide this total by the voltage in the circuit. This will give you the minimum necessary amperage for the entire circuit.
Multiply this number by "1.25" to calculate the amperage of the circuit breaker you'll need. It's a standard safety convention to have the circuit's amperage capacity be 25 per cent more than its theoretical maximum load.
Determine the gauge of wire required to carry the circuit breaker's amperage rating. Visit "http://www.powerstream.com/Wire_Size.htm" for a complete chart of wire gauges and their respective load capacities. Note: On the chart, the value you want is in the column titled "Maximum amps for power transmission."
The calculations in this article are based on the assumption that the circuit (and its components) are all wired in parallel. If you're not sure whether the circuit is wired in series or parallel, please swallow your pride and hire an electrician; they're expensive but the safety of you and your loved ones is priceless.