Electrical gurus'
 

I'm finishing my basement and put 12 recessed lights in the ceiling. I wired 6 lights to each switch, and the bulbs are 60 watts each. The switches I used are the ones with an on off switch as well as the little sliders on the side to dim the lights. Here is what has me confused....<br><br>The switches are rated for 600 watts each, I am running 360W through each one. When the rheostat is put in the full bright position, the metal plates on the front of the switches get pretty hot. As in hotter than I think they should. If you dim the lights, the switches run cooler.<br>I would think that on full bright, the rheostat is bypassed and there should be no resistance, which should mean no heat. Dimming the lights increases resistance (I'm assuming) and should cause the switches to heat up. <br>What do you guys think? I'm don't get it...

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[quote author=Spark Chaser link=board=10;threadid=8247;start=0#79536 date=1039727485]
Here is what has me confused....

The switches are rated for 600 watts each, I am running 360W through each one. When the rheostat is put in the full bright position, the metal plates on the front of the switches get pretty hot. As in hotter than I think they should. If you dim the lights, the switches run cooler.
I would think that on full bright, the rheostat is bypassed and there should be no resistance, which should mean no heat. Dimming the lights increases resistance (I'm assuming) and should cause the switches to heat up.
What do you guys think? I'm don't get it...
[/quote]

The rheostat allows the resistance in the circuit to be increased/decreased by moving a wiper across the windings of a resistor. The wiper must be connected to the plates. At max lighting, you have max current flowing through the rheostat wiper which is itself a constant resistance. It's low but it is not zero. So at minimum resistance you are flowing about 3 amps of current (360 watts/120 volts = 3 amps). That means that the total resistance of the six bulbs is 40 ohms. Now, increase the rheostat to dim the lights. Go all the way to dim and let's say that's 40 ohms provided by the rheostat. Now you have 80 ohms total in the circuit which cuts the current flow to 1.5 amps. Now assume the wiper is 5 ohms (for grins). At max lighting, the rheostat must dissipate 9 * 5 = 45 watts (I2 * R). At minimum lighting it must dissipate only 2.25 * 5 = 11.25 watts. See why it's hotter on the max setting? The power calculation using current and resistance is I*I*R.

You may need a more efficient rheostat or one rated for more current handling capability if they are getting hot.

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Thanks for the info...<br>Guess I shoulda sat down and thought about it a bit harder ;)

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Scary thing is I totally understood everything Alan just posted....electronic brain.......I can't stop it..... [eyecrazy]<br><br>Ohms Law....... ;D

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Ohms who? Yes, exactly correct. From an installers viewpoint, 6 cans per switch is a pretty heavy load. Another consideration is that while you as the homeowner intended to use only 60 watt bulbs, the next person might not. As to the type of switch/dimmer: Go to a commercial rating, they'll say 20 amp handling. For residential use most devices are rated for 120v/15a (amps). More and more custom homes are wired with 20 amp devices just because they can handle the load better. When I rewired much of my own house, I rewired for 20 amp circuits, 12 ga wire, etc. If it's not too late, as in you've covered your ceiling, split your load in half by wiring 3 cans per switch if you can't find the heavy duty rated switches. On another note: This is a basement application with can lights. How much clearance do you have between the top of the can light and the bottom of the floor above. Can lights generate alot of heat. That's why they have thermal cutoffs. However, in the more economical can lights, they may not be the most efficient, as in they may not cut off the electricity to an overheated device. Are these 12 cans in a big room lighting scheme? Maybe you can do more zonal as in breaking up into 4 quadrants? Just thinking off the top of my head here. I apologise for lack of specifics. I do mostly heavy industrial electrical work, so I'm digging up alot of old past work to get you some answers. By the way, how hot do the dimmer switches get? Warm to the touch or HOT? They are expected to get warm. However, hot is another story.<br><br>~Dave<br><br><br>

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Dave<br> The room is about 420 sg ft. I have 2 x 12 floor joists so the clearance above the light cans is good. I've ran them for several hours at a time when I am working and they don't get hot at all. The switches are Lutron [eyecrazy] rated at 600w each. Doesn't say on the box (mostly in spanish) what the ampere rating is- guessing 15 though. I ran all 12-2 wire off a new sub box that I put in the basement. I don't know who wired the basement first but :o scary... I've tore it all out and put in all new. The switches have big heat sinks on the front of them, as the mounting plates fill up a double box. They get warm enough that you can't keep your fingers on them for very long. I really dont want to rewire for 4 quadrants, wonder if a better switch would help?<br>Scott

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I put Lutron dimmers on several single lamp switches in my house. The one on my dining room chandelier is running 9 40 watt decorative bulbs. It does get warm enough to tell a difference in temp but nowhere near hot to the touch. If you have to remove any of the faceplate heat sink to make them fit the switch box they derate the power they can handle significantly. Another thought, if you are using plastic boxes you do not have the thermal transfer that you get with steel boxes. Now that I think about it my dimmers are all in the original metal boxes so my results may differ from yours.<br><br>Smoke Dog and NWDave - good explanations. <br><br>It amazes me at the kind of scary wiring you can run into in an older house. <br>

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I'll be over at Lowe's (Christmas shopping for myself ;D ;D ;D) today. I'll try to find some specific trade information for you, specifically, tradename and device size that may help you out. Redramnc makes a good point that I overlooked. All my dimmer switch boxes are metal. Later.<br><br>~Dave

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Thanks Dave ;D<br><br>My switches are in the heavy plastic boxes. Maybe the metal ones would help a bit as it would be a big heat sink. Worth a shot.<br><br>Scott

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Nope, nothing more from Lowes. All the Lutrons are rated for residential use. 120v/15a. Sorry, couldn't be more help.<br><br>~Dave