OK, you are producing a shoot in an office where you are conducting an interview with a high-ranking official of an organization.  She has just exited the office for a bit to allow you and your crew time to set up.

She has left her computer on, and you actually want it to stay on and have it seen in the background of the shot.

As the crew sets up, you overhear that they want to plug in two 600 watt lamps and one 400 watt lamps.  And the question arises, ”Are we going to blow a circuit?”

Uh, a voice goes off in your head crying, “Danger, Danger, Will Robinson!,” as you  recognize, perhaps more than anyone else in the crew (since it was you who spent days setting this shoot up), that being responsible for a black-out in this interviewee’s office  would not be a good thing . . . AT ALL!

But hey, you can impress the crew (and amaze your friends!) by remembering the diagram above, which reflects Ohm’s Law (named for 19th-century German physicist, Georg Ohm http://tinyurl.com/q8k6e6 )

E = Pressure (measured in volts)

I = Movement [or the amount of electricity going by one fixed point at any one time] (measured in amps)

R = Resistance (ohms)

The formula works like this:  E =  I x R or I = P/R or R = E/I

You know that the voltage of wall plugs in the US are 120 volts, so you have the value of E.

And you remember the maintenance engineer saying during the scout (you did the location beforehand with a tech crew member, right?) saying that the office was on a standard 20amp circuit breaker.  So you know somehow you have to be sure you stay under that number.

“But wait a minute,” you say, “What’s about the “watts” of the lamps — I don’t see anything in Ohm’s law about that?”

Well, let’s digress a moment and look at an analogy.

Let’s take the water coming into your house.

The water pressure from the street would be analogous to E.

The movement of the water flow would relate to I.

And let’s say the pipes in their normal configuration create a resistance to the flow of the water by an arbitrary value of 1.

Now, if you were to replace one foot of the intake pipe with a thinner diameter of pipe, you’d in effect be increasing the resistance to the water flow right?  So the value of R would increase to let’s say 2.  And that would lessen the flow or movement (I) of water out the tap, right?  So see, the formula works . . . as the R increases, then I decreases.

“OK, but again, what about the ‘watts’?” you ask.  Stay with me.

We still don’t know how much actual water in gallons is coming out the tap, right?  Or another way of putting it, how much actual “work” or “power” is being generated out of the tap.

That’s where the next formula, Watt’s formula (named in honor of Scottish inventor James Watt http://tinyurl.com/2cszwe ), comes in:

Power (watts) = E (volts)  x I (amps)

Let’s go back to the water analogy to analyze this formula.

If you increased the water pressure from the street connection, more water would come out the tap, right?  Or if you could do something to increase the water flow, that too would increase the end result (aka ”work”), right?

The formula therefore makes sense.

So, using Watt’s Law, we can answer the question of the camera crew.

P = E x I

or said another way, P/E = I

Plug in the numbers:

1,600 watts / 120 volts = 13.3 amps, or well under the circuit breaker limit of 20 amps.  (Please note: commercial building standard circuit breakers are 20amps, most home setups are 15amp breakers, except in kitchens which are usually 20)

Now it pays to be cautious here.  For example, what about the computer and printer your interviewee left on?  And how about the camera and monitor?

Your crew should be able to tell you how many amps their gear pulls, and for anything else on in the room, you can look at the small print on those devices. 

With whatever info you find, either expressed in amps, ohms or watts, by applying the two formulas, Ohm’s Law & Watt’s Law, you should be able to figure out the total electrical “draw.”

Now if you read this far, ok, yes, there is an easy online calculator to help you figure this all out:  http://tinyurl.com/pua2yc  But hey, you wanted to know the logic behind it all, didn’t you?  That’s why you are a producer — you are naturally curious! (smile)

As always, I welcome your comments below.

If you enjoyed this article, you might also be interested in Chris Vazquez’ article explaining what “anamorphic” means:  http://tinyurl.com/59jtfd

Need captions for HD tapes? http://tinyurl.com/6bpxje

Contact David Ryan at dryan@videolabs.net or 301-217-0000 x104

Follow me on Twitter @ www.Twitter.com/DRMediaSolution