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Ask Eli - Calculating Wattage?

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How many lights can I have on one circuit? Can I add one more light? - Tim


You will need to calculate the wattage being used and the wattage available for the circuit. The formula for calculating wattage is Volts x Amps = Watts. Lighting circuits are often 120 volts with a 15 amp breaker. Plugging those numbers into the formula we get 15 amps X 120 volts = 1800 watts. With lighting circuits you are only supposed to use 80% of the watts available for the circuit so you need to subtract 20% from the maximum wattage for the circuit:  1800 watts - 20% = 1440 watts. Therefore, on a typical 15 amp lighting circuit you will have a maximum of 1440 watts available. The reason for the 80% rule is to give you a safety margin so you are not running your circuit at the full 100% capacity.


Add a light fixture

Each light fixture in the circuit is rated for a certain maximum wattage. Most basic light fixtures say 60 watts max and a can light might say 100 watts max. Even if you plan on using CFL bulbs, which use much less wattage, you still need to do your calculations using the max watts for the fixture. If you calculate using the CFL bulbs and later sell the house to someone that doesn't use CFL bulbs, then the circuit will end up overloaded.  So, if you have 10 light fixtures on the circuit and each has 2 bulbs at a maximum of 60 watts, you would have a total of 1200 watts.  10 fixtures X 2 bulbs each = 20 light bulbs total, 20 light bulbs X 60 watts each = 1200 watts total. If this is on a 15 amp breaker then you would be able to safely add 240 watts to the circuit. 1440 watts maximum - 1200 watts used = 240 watts available.

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