Mar 15, 2008

How many lights can I plug into this circuit?

You're on location, in a house built in the 1940s. But you're worried about blowing fuses or popping a circuit breaker with the bright and hot lights from the nice Arrilite kit you brought.

You need a way to tell in advance, how to know when the next light you plug in will blow the circuit. You're not an electrician, you heard there was a formula, by some Ohm guy, but you have no idea what it is.

No matter -- There is a simple way to figure this out, and keep out of hot water (and in the light) with the homeowner.

It seems confusing, because the electrical draw of your light fixtures is expressed in Watts, while circuit capacity is marked in Amps. Ohm's Law is an electrical formula that electricians use to make this conversion, but in practical application, you don't really need to use a calculator for this sort of simple situation.

Let's start by looking at the situation in the house. A circuit is a single line of electrical supply, that is usually shared by several outlets or lights in the home. Each outlet is NOT a circuit! Instead, multiple outlets are shared by one circuit. Also, normally, all the outlets or lights in a room are not dedicated to a single circuit. They are usually spread across several rooms. In a two story house, the lights will usually be on circuits shared between floors. This seems illogical, until you realize what happens if the circuits were divided by floor. If that circuit blows, all the lights on that floor go out, and no one can see, until they can find the a flashlight, and get to the circuit breaker.

Fortunately, discovering the capacity of a circuit in a home is easy. First, find the circuit breaker panel. (The following refers solely to 120 Volt electrical service as commonly found in North America.) It might be in a utility closet or it might be on the back porch. If the home has a garage, it is often found there. Upon opening the panel, you'll normally find two rows of black switches, the circuit breakers. These have two digit numbers printed on them, normally a a 15 or a 20. That number is the number of Amps the circuit can safe handle, before the breaker trips off, cutting off the power. If the circuit breaker didn't do this, the wiring in the walls would overheat, and start a fire. This is a special effect we normally want to discourage -- and there are better ways to meet firemen.

If the house is older, built before the 1960s, and never been re-wired, it may have fuses. These are very often only 10 amps. NEVER replace a 10 amp fuse with a higher number fuse, this is very unsafe! If you're blowing fuses, it's for a reason. More current is being drawn than the circuit can safely handle without overheating.

The next puzzle is to figure out which outlets in the house are assigned to which circuits. If you're very fortunate, sometime in the past, someone else already figured this out for you, and filled in the blanks in the circuit description on the circuit breaker panel door. Sadly, we often find that we're the first persons to bother filling in this card. If that's the case, you have little choice but to experiment, and turn breakers off and on to determine which plugs they control. Before you start, make sure there is nothing plugged in or turned on that might be harmed by the loss, or sudden resumption of power. Home electronics, computers etc. Now you could have another crew member walk around the house with a light, plugging it in to every outlet, yelling out when the light is extinguished, as you flip breakers in turn. A slightly more elegant solution is to use a cheap AC outlet powered radio instead of a light. That'll preserve your vocal cords. It also works well if you don't have an assistant--although with a lot more steps as you walk back each time to move the radio's plug. But you needed some exercise anyways, right?

You'll notice too that in addition to the breakers marked 15 and 20, there are other breakers, with higher numbers, and ganged together as one switch. You don't need to test those, they're for high power items that are hardwired, or not plugged into standard outlets, such as clothes dryers, ovens and heaters. If you're lucky, all the outlet breakers will be 20 amp circuits, and the 15 amp breakers are just assigned to ceiling lights. As it is, you may discover that all the outlets in the house are assigned to as few as two or three breakers.

Now that you know what outlets are available on each circuit, all that's left is to figure out how many of your lights you can plug in to each circuit, staying safe, and not popping a breaker!

I use this simple rule of thumb-- Each amp is "equal" to 100 watts of 120 volt power. Now this isn't correct-- but this rule of thumb leaves a nice safety factor, as it's not really a good idea to load a circuit all the way up to its maximum amperage limit.

So if you plug in a 600 watt light, that takes up 6 of the amps on a 20 amp circuit. And if you plug in two more 600 watt lights, the total is 1800 watts, or 18 amps; you're still under the limit, and won't pop a breaker. But if you start with a 1000 watt light, you only are safe for one more 600 watt light -- another 600 watt light would be 2200 watts, and that's over 20 amps, using our rule of thumb with a built in safety factor. But you could plug in another 400 watt light. If you need more than this, you should find a stinger (AC extension cable) and move on to the next circuit.

A word of caution-- Always keep an eye open for what else might be plugged into the circuit. They need to be taken into account too. So if there's a computer that needs to be left on, on the circuit you need for a light, you have to accommodate it's load also. If the computer draws 500 watts, and it's a 20 amp circuit, then you only have 1500 watts available on that circuit. I really really recommend against sharing lights and a computer on a circuit though.

Watch out for these appliances, that draw much more power than most people realize: Hair dryers and curlers, laser printers, coffee pots etc. Laser printers are particularly sneaky, as they don't draw much power till they print, then suddenly you will see a 600 watt surge, and your lights go out!

Most all of this applies also to offices and commercial bldgs-- These often have sub breaker panels on individual floors. Be vigilant about not overloading circuits that might be shared among rooms, or down a hall. You don't want to shut down someone's computer when they're trying to meet a deadline! A gotcha you'll find in offices is a coffee pot on the other side of a wall in a break room you don't know about, but is on the same circuit you've plugged a 1000 watt chimera into!


Shiv said...


I somehow found my way to your blog (from your website if I remember correctly).

Your blogs are very informative and in-depth. I've been in a situation where the circuit breakers have tripped. Now I'm actually an electronics engineer and so Omhs law is native to me. But at that time (setting up the lights and trying to get the shot to look right. Ohms law and everything else electronics went out the window.

I'm been in the computer industry now for 11 years and have recently taken up Videography. I own a Canon XHA1 and really enjoy this hobby. It taxes my cretive energies in different ways from my work and helps unwind as well.

I was hoping to see some of your work (videos). I've got a few of my videos posted on my blog if you care to check them out. Remember I've only been doing this for 6months or so now.


Steven Bradford said...

Thanks! Yes, my hope for this blog is to write practical information about lighting, cameras etc. There is a lot of advice out there on the digital and computer side of things, not nearly enough help for on the set.

I'll be putting my reel in my sidebar.

roostacat said...

mnnnnnnnnnnmn A standard 120 volt 20 amp circuit can safely power:
a) Two 1000-watt light, two 600-watt lights, and a 80 watt water pump
b)Three 400-watt lights, two 60-watt fans, and a 40-watt pump
c) Four 600-watt lights, a 20-watt fan, and a 60-watt pump.
I have tried to divide the total watts by the volts, but I can't figure what my answer should be.
Please Help me with this question

Steven Bradford said...


Actually, only "b" with a total of 1360 watts on one 20 amp circuit would be safe, the other two, at a: 3280 watts and c: 2480 would very quickly trip the breaker.

To use this rule of thumb, divide the (Amps X 10) to get the total number of watts that the circuit will support. This number is actually slightly less than the result you get using Ohm's law, but it builds in a safety factor. So add up the wattage of all the lights and devices, to see if they come in under 2000w, for a 20 amp circuit. A 15 amp circuit would have a limit of 1500 watts.

Where this rule doesn't work so well is figuring out very large loads on much higher limit circuits. But it's fine for standard household and office 110volt circuits.

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