Power Management on Set

Most film lights draw huge amounts of power, so understanding a few basic concepts of electricity will ensure you don’t ever blow a breaker while out on a shoot. Let’s get started with a refresher on electricity. [Header photo credit: Ricardo Diaz]

The Basics of Electricity

There are three things you need to be familiar with if you are going to handle power on set. Volts, amps and watts. So lets get the basics of electricity down before moving forward, to do this we are going to use the analogy of water pipes. (Reference the picture below.) In this scenario volts are equivalent to the water pressure in the pipes whereas amps represent the flow rate of the water. Circuits are sections of connected pipe. Each circuit can have a maximum of 20 amps being drawn out of the pipes, go higher and you will blow a circuit breaker. Back to the water analogy,  you can only draw a maximum of 20 gallons (amps) of water per second from a water pump (circuit breaker) that is connected to a circuit. If you draw more than the pump can keep up with, you will blow the circuit breaker. The number of outlets on the circuit does not affect the fact that you must stay under 20 amps. So in a scenario where you have 4 outlets on one circuit, you could either have 5 amps on 4 outlets or 10 amps on two.

This is the challenge of location shooting. Typically you have no idea which sockets are attached to what circuits. A good rule of thumb is to treat all the sockets on one wall as a circuit. Most modern commercial and residential buildings have 20 amp breakers on all circuits, older buildings might have 15 amp breakers.

Determining the Number of Amps a Light Draws

So how do you know how many amps your lights fixture pulls? Having a good understanding of the different bulbs and their power consumption characteristics is a good start, but luckily as long as you know two of the three variables there is a formula you can use to determine the amps a light will pull.

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Amps = Watts / Volts

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Most tungsten and HMI lights are named by their wattage so it is easy to plug in the numbers to determine the amps. For example, a 1000w ARRI ST1 Fresnel light operating in the US off of a 120V socket would pull 8.3 amps. (8.3a=1000w / 120v). This formula comes in handy for lights that don’t have their wattage in the title. For example a KinoFlo Divalite 401. It uses 4x 55w tubes on a 120v socket. So in this case our formula would tell us that it uses a mere 1.8 amps. (1.8a=[55w x 4]/120). For more on different types of lights and how their energy efficiency compares go check out XXXXXXXXX.

Generators

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So lets look at another situation, remote filming. If you are filming a sequence out in the Rocky Mountains, chances are there isn’t an outlet within reach. This is where generators come in handy. Most generators are gas powered and feature multiple 20 amp breakers depending on their wattage. Smaller generators are great because they fit on the grip truck and usually provide in the neighborhood of 10K watts with four 20 amp breakers.

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Another situation when generators come in is when you need to power very large light sources. Lets say you need the punch of a Alpha 4K HMI to provide illumination to an architectural element of a building. After using our formula we determine that this one light will draw nearly 34 amps, and as we know from earlier in the article modern buildings have 20 amp breakers. This is when you step into tow generator territory. Most towable generators start at 500 amps and can go as high as 2500 amps. That is some serious power, so make sure you know what your doing, and if you don’t hire a Gaffer to take care of it for you.

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Laying Cable

Now that you know how to get enough power to your lights without throwing a breaker, its time to look at how to get your lights plugged in with Stingers. In the film industry we call extension cords Stingers. For the most part it is as simple as plugging in a light and plugging the stinger into the wall but taking a second to plan makes a world of difference. Figure out where all the lights are going to be first, then figure out how many sockets you have at your disposal. If you are running short on outlets, consider using a tri-tap if it isn’t built into the stinger already. Next, think about how you want to lay main power paths throughout set. Its much easier to have one group of cables all traveling together and breaking off at the closest point to the lights. This way you have less to tape down and less chance of tripping on anything. I usually keep the cables floating while deciding where the lights need to go. Once 80% of the lighting is set, start taping down the stingers with cable tape. Be sure to leave about a foot or two of slack near the light so that it can be fine tuned if necessary.

Stay Safe

Seriously! All electricity is dangerous but the amount of power some of the larger lights use can be fatal if you aren’t careful. If you don’t know what you are doing grab someone who does. Be sure to wear grip gloves when dealing with lights and power and keep a tidy set with no trip hazards.

So now you are all set to start striking lights. Have any tricks you use when setting up lights? If so leave a comment below!

2 Comments

  1. I think you’ve got some interesting points, but I’m just not sold. If you really want to get the crowd behind you, you’ve got to entertain us, man.

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