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Electronic Theatre Controls Inc

How Many Fixtures Can I Put on a Circuit

Explanation

To determine the number of fixtures, or load, you can safely put on a circuit, you will want to use the following mathematical formula:

Wattage = Voltage X Amperage (commonly referred to as W/VA or the "West Virginia" equation)

To make this calculation and see if your fixtures will overload the circuit, you will need to know:

  • The total wattage of the load (if you have 2 fixtures, each with 575w lamps, your total load wattage will be 1150)
  • The voltage of the circuit (most electrical installs in the United States use 120v or 277v)
  • The amperage of the circuit (most easily determined by looking at the circuit breaker, is it 15a, 20a, or something else?)

 

See the following examples, one that will work and one that will not:

 

You have a 120v, 20a circuit. You want to put 3 300w fixtures on the load

  • 120v x 20a = 2400w (so you can have a total of 2400w on the circuit)
  • 3 x 300w = 900w
  • 2400w > 900w, so this will work

 

You have a 120v, 15a circuit. You want to put 4 575w fixtures on the load

  • 120v x 15a = 1800w (so you can have a total of 1800w on the circuit)
  • 4 x 575w = 2300w
  • 1800w < 2300w, so this will NOT work
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