Calculating lighting load relies on a single equation: watts = volts x amps. This is mathematically written as W=VA. In most lighting situations, you will know two of the three elements. From there, you can use the formula to find out the third. Household circuits in the U.S. are 120 volts, although a few circuits may have 240-volt capacity, and most allow 15 to 20 amps. If you are planning a special event with lighting or wanting to add specialty lighting to your home, you will need to calculate the lighting load to ensure you do not draw too much power and trip a circuit breaker.
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Calculating Maximum Load
Verify circuit amperage by locating the breaker panel or circuit box. The breaker for each circuit should be labeled with the number of amps, often on the switch.
Multiply the amp number by the voltage. In the case of a 15-amp circuit at 120 volts, the formula would look like this: W = 120 x 15. The result is W = 1,800, so 1,800 watts is your maximum power load.
Subtract the wattage of any other loads on the circuit, such as appliances, portable heating units or office equipment. The number of watts left is your maximum load available for lighting.
Calculating Power Capacity
Calculate the load your lights will place on your circuit by adding together the wattage of each lamp. For example, if you are calculating the lighting load for a special event that uses four 500-watt lights, multiply 4 by 500 to get 2,000, which is your load.
Plug the load into your formula. In this case, you have 2,000 = VA. Since standard household power is 120 volts, you can add that to your formula too, resulting in 2,000 = 120A. In math terms, 120A is the same as 120 x A. Use 240 instead of 120 if you are on a 240-volt circuit.
Figure the amps by dividing both sides of the equal sign by the voltage. You do this because 120 divided by 120 equals 1, and 1A is the same as A. Divide the wattage by 120 to even out the equation. You should end up with a result that looks like this: 16.67 = A. This means you need a circuit with a 16.67 or higher amp capacity to place the load of all four lights on that circuit. If you find that your circuits are only 15 amps, you must find a separate circuit for your fourth light.