I am installing a theater for a customer who asked me an electical question. It seems that the electrician installed a set of 13 lights (I think they're 75W) on a single switch in a 4 gang box. The other switches control more can lights. The one switch gets extremely hot (too hot to touch the screws). Is there anything wrong with this set up?

Well, just using simple Ohm's law, 13 bulbs at full power are drawing 975 watts, more or less 8 amps @ 120 volts.

I don't know what the average rating is of an average switch, but I suspect the switch is being run past it usable ratings.

I'm sure Larry can shed more "light" on the subject.


JJP

Also, keep in mind that even if the switch is rated at 1000 watts, it must be derated if it is ganged with other switches.

My thoughts exactly. Thanks for the confirmation. I don't think switches should be too hot to touch. Good joke JJ. What's the word Larry?

Here I come to save the dayyyyyyyyyyy!

Okay, back to reality. By switch, may we assume you mean a dimmer?

If not (!), then you should replace the switch with a commercial-grade, 20-amp rated switch. However, a residential-grade 15-amp switch should not get hot at all, but the contacts will not last forever, because....

Cold bulbs have a high 'inrush' current. (Ever notice that some X-10 switches (not dimmers) have a lower wattage rating than the equivalent current rating?) Filaments have a low resistance when cool, which rises within a few AC cycles as the filament 'incandesces'.

If you do mean a dimmer, it definitely should be rated at least 1000 watts. (If it's lower, I'm surprised it hasn't failed already.) Even then, with other dimmers in the same box, all of which should be derated, that isn't enough.

Just as 600-watt dimmers should be derated to 500 watts if two are in the same box, and to 400 with three or more, a 1000-watt dimmer should be derated to 800 watts with two, and to 650 with three or more. Many dimmers have heat-sink tabs that must be broken off to fit them side-by-side, which doesn't help.

My first suggestion would be to split the switched circuit to use two separate dimmers if possible, even if it means adding a gang or separate box; otherwise, you should find a 1500 watt dimmer, which will almost definitely require a two-gang box, which you will have to install above or below the existing box.

Now, if you're talking about X-10 dimmers, you definitely need to go a giant step farther, and find some PCS 2000-watt X-10 dimmers. I'm not aware of any other X-10 dimmers that exceed 1000 watts, which you are approaching with little margin. I always try to allow at least a 10% headroom above the manufacturer's rating.

Also, I'm surprised that this much lighting is on a single circuit. It sounds like the total wattage may well exceed the capacity of a single 15-amp circuit, which is 1440 watts. Even a 20-amp circuit is limited to 1920 watts. To wit:

15 x 0.8 = 12, & 12 x 120 = 1440.
20 x 0.8 = 16, & 16 x 120 = 1920.

Permanently-connected loads should never exceed 80% of the circuit capacity. (Actually, permanently-connected loads are not supposed to exceed 50% of the circuit's load if there are any receptacles on the same circuit.) Even switching to 65-watt halogens will help without losing light.

Even though I haven't done any lighting in my theater yet (movie room would be more accurate), I have reserved two 15-amp circuits in my sub-panel just for lighting, when the time ($) comes. I have two three-gang boxes inside the equipment closet for hidden dimmers, which will be operated remotely.

Okay, back to our show....

Larry
www.fineelectricco.com
My system

Thanks Mighty Mouse. I've almost got it, can you expand on that?

no

Seriously, thanks. That was extremely helpful.

Target has a wonderful selection of brass and aluminum decorative wall plates. Im sure that they could be ground and lapped to replace the heatsinks commercial dimmers tend use as faceplates.

That would defeat the whole idea of using a dimmer that uses a finned heatsink. Besides, it's not easy to find a two-gang-sized plate that is machined for a single, centered device.

Larry
www.fineelectricco.com