Circuit load refers to the amount of power that is drawn through a given electrical circuit. Typical home circuits are rated for 15, 20, or 30 ampere maximum load and should have a normal extended operating load no greater than 80% of their rated maximum. The maximum circuit load for any particular circuit is determined by the wiring used in that circuit. Different types of appliances will place different loads on a building’s electrical system, and circuits should be planned to support the appliances that will eventually draw power from them.
The maximum circuit load is determined by the rating of the circuit breaker or,in older buildings, the fuse on that circuit and by the thickness of wire used to install the circuit. Thicker wire can safely carry more current. Fourteen gauge wire, which has a diameter of 0.0641 inches (1.628 mm), can safely carry a 15 amp circuit load, 12 gauge, 0.080 inch (2.053 mm) diameter wire, can handle 20 amps, and 10 gauge, 0.1019 inch (2.588 mm) wire, is rated for 30 amps. Longer runs of wire require larger wire, to account for the gradual increase in resistance imposed by the longer run. Local building codes may modify these base requirements further, and should always be consulted.
Circuit load can be calculated by adding up the power that individual appliances on that circuit require. Some appliances will indicate the amount of power that they draw in amps. Others will be rated in watts and volts. Almost all household circuits will run at 110 volts, while certain appliances that require a great deal of power, such as electric ranges or dryers, will need to be placed on circuits with higher voltage. To determine an amp rating for an appliance, use the watt and volt ratings in the equation: watts = volts x amps. For example, an appliance that draws 330 watts at 110 volts would require 330 / 110, or 3, amps of power.
When calculating circuit load for a circuit that will include lighting, calculations should always be based on the highest power consumption that could be expected. If a light fixture can safely accept bulbs up to 100 watts, then calculations should be based on the assumption that 100 watt bulbs will be used. Compact fluorescent bulbs indicate the wattage that they draw, but some other types of fluorescent fixtures require power based on the needs of the ballasts within the lamp, rather than only on the bulbs used. This information should be listed on the exterior of the lamp, or on the ballast.