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Lighting Formula

written by: Shane Burley•edited by: Rhonda Callow•updated: 10/22/2009

Calculate before you light! Apply this simple formula so you won’t blow a fuse or trip the circuit breaker.

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    Lighting Formula

    The electricity formula for lighting a set is a basic algebraic equation that will surely prove useful before you light your set. Because studio lights have considerably high wattage, preparing ahead of time will not only help you decide the room’s outlets you’ll need to use, but help save you the embarrassment of the horrific episode of tripping the circuit breaker or blowing a fuse, or even worse, creating a fire!

    When you’re busy doing multiple things at once on a set, sometimes it may be easy to forget the importance of checking the lamp-voltages ratios. Electrical requirements vary depending on the location and your lighting equipment. By using this simple mathematical formula, you will save money and even perhaps time! Because it’s an algebraic formula, you can substitute each of the inputs so that the equation may be either a simple multiplication problem or a division one. As long as you have two of the inputs, you’ll be able to solve for the other input.

    What is the formula?

    W(watts) = V (volts) A (Amps)

    Other variations:

    V= W/A; A = W/V

    Let’s take a look at each of the elements found in the formula above.

    Wattage (aka Watts) = is a measurement of the amount of electricity power consumed by the light source; the rate the electrons travel in the wire.

    Voltage (aka Volt(s)) = a measurement of the electricity pressure

    Ampere (aka Amp(s)) = the actual amount of power that travels in a conductor per one coulomb per second

     

    How do I Know Which Formula to Use?

    The first thing to consider before plugging in any figures in this formula is to find out how many amps the facility has. Usually, in the US, the common circuit breaker holds between 15 to 20 amps. (Open the circuit breaker or fuse box; the amp rating should be listed inside, usually on the inside of the door panel.) Also, the standard amount of voltage in the US varies, usually from 120V to about 240V. The voltage usage should be listed inside the circuit breaker or fuse box as well. (If you’re planning to shoot in another country, find out this information beforehand.)

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    You’ll need to count the electrical outlets in the room at which you plan to shoot. Chances are, you’ll have at least two outlets; these may be on the same circuit; however, some outlets may be located on different circuits and you’ll need to figure this out, especially if you need to hook up your equipment on more than one circuit For every 1000 watts, it’s safe to use 10 amps. (although more precisely, it’s 9 amps per every 1000 watts) You may need to run extension cords if you’ll be needing additional circuits with outlets further away. Please ask someone if this is alright to do beforehand. Extension cords can be easily tripped over and can quickly crowd pathways, doorways, and rooms. (If it’s not absolutely necessary to use an additional light source,think about using bounce cards, mirrors, etc to acheive the lighting you want. You save power too!)

    Now, considering ALL the light sources you’ll be using, figure out the total amount of watts you’ll need to hook up. Remember a safe bet for each circuit: for every 1000 watts, it’s 10 amps. So, if you’re using a 20 amp circuit, you can only use two 1000 watt lights on that one circuit. Likewise, if you’re using a 15 amp circuit, you may only use one 1000 watt light and a 500 watt, or a light with less wattage.

    Example 1:

    You’re shooting in a room with a 20 amp circuit breaker and have four 500 watt lights. What is the voltage you’ll be using?

    Plug it in:

    V = W/A

    Total wattage: 2000W

    amps: 20

    Volts: 100

     

     

    Example 2:

    If you have a 500 watt light and a 650 watt light plugged into the same 20 amp circuit, can you hook up two 300 watt lights on the same circuit?

    figure it out:

    You have a total of 1150 watts and am wondering if you can plug in an additional 600 watts, right? This is a complete total of 1750 watts.

    remember: 1000 watts per 10 amps

    and you have: 20 amps, which is = 2000 watts (max. you may use on one breaker)

    therefore: 2000 watts - 1750 watts = 250 watts

    answer: sure! with an additional amount of 250 watts