The load rating of the dimmer has no effect on how warm they get, a lager rated dimmers often has tabs on the faceplate edges oi increase the heat radiating area.

Dimmers get warm, and the larger the load on them (Watts) the warmer they get.

A plastic box and a plastic switch plate are going to make then get about as warm as they will get.

The amount of heat depends on both the load as Brick says (rather than the rating) but also the dimming level. They generate more heat at 50% than either full off or full on. But they are designed to get hot which is the major reason why they have ratings (and deratings n the case for multiple boxes).

Well it each dimmer was on 'full'. The 2 can one was warm. The 6 can was hot enough the screws holding the plastic cover plate were HOT. After removing the plate, the metal on the face of the dimmer was HOT...I could hold my finger there man it had me worrying. Now if you say I put the sucker at 50% and it gets hotter...I would think you could not even keep your finger there...how hot is to hot? Where is all this heat going? Am I safe with the 50 cent plastic coverplate I put on? Is the drywall safe?
Recall there is only 360 w on this...

" They generate more heat at 50% than either full off or full on. "

Not by much for a modern thyristor dimmer.

thy do not operate as a simple resistive item, buyt alter the voltge (and curret) goig to the load an turn it on ad off 120 times a second fr a shorter period of the 60 Hz waveofrm.

Less on time, dimmer bulb,
Close to full on is actually the highest since the device is conduction almost 100% of the time (if there is not a 'full on' bypass built into the dimmer).

When dimmed it might only be passing any current a few percent of the time.

The power is the trigger circuit power (very low) and then less than 1 to 2 volts of drop at the load current when the light is on.

Longer on time, brighter light, more power dissipated.
All at 120 times a second.

just found instructions for dimmer...says can handle 600W. It then has a chart for Multiple Device Applications...says 'In multi dimmer installations, the reduction of the dimmers capacity is required'...chart says single device = 600W, 2 devices = 500w and more than 2 devices = 400w. I assume this refers to how many dimmers I use with the 6 cans? Or does it refer to how many dimmers are in the blue outlet box...recall I have two in there. Note I rechecked my bulbs...the are fiet 65W bulbs for recessed lights.

Is it safe to say no matter what type dimmer I put here (or brand)...they are all going to get this hot?

Finally..I have rotary dimmer on my dining room chandelier. It powers 5 bulbs (could be 25w or 40w)...either way...does not get warm. I reaize that could be only 25% to 50% the watts the cans are using.

I just read this:
http://www.structuretech1.com/2012/05/hot-dimmer-switches/

guess the wattage of each drops to 500W....still I have 130 on the one and 390 on the other...under 500w each. I do not think these dimmers had fins. I guess, if I assume these are safe..ok. Is there another type/brand dimmer that would not get as hot though...or does it defy the laws of physics....or whatever...

Note I rechecked my bulbs...the are fiet 65W bulbs for recessed lights.

So these aren't incandescent bulbs??? Are these dimmers rated for the bulbs that you are using?

Brick, I wasn't talking about resistance dimmers. Nobody uses resistive dimmers which are problematic if you're not loading them to their rated wattage to begin with (believe me, I've done work in ancient 1930's theaters and there were rooms full of additional loads).

No, I am talking about the modern semiconductor TRIAC (or if you must) thyristors. They actually have greater internal loss at the mid-voltages because the thing has to fire and suddenly go from not conducting to conducting at the peak waveform voltage. When it is near off or near on, the thing starts conducting with very little voltage across it.

This is exacerbated by the cheap torrid they add to the circuit to cut down the RFI from the dimmer in this situation.

"They actually have greater internal loss at the mid-voltages because the thing has to fire and suddenly go from not conducting to conducting at the peak waveform voltage. "

Des not matter.

It is all about duty cycle.

longer on times result in more power being dissipated.

The thermal mass is more than enough you cannot see effects this small.

it does depend on the dimmer.
why?
zero crossing switching?
ie it turns on/off at a zero voltage..power VI..but if V is 0 then whats power? 0

but can they actually work at 0 volts..not SI is 0.7 volts for a junction. GE(Ga) is 0.3v so some heat must get dissipated

I think cheaper dimmers don't use 0 crossing switching

-dkenny

If the dimmer feels hot enough to still concern you, and you have taken into account any of the required deratings, you could always use a higher rated dimmer switch which has additional heatsinking to dissipate the temperature. Of course you need to check the requirements for that dimmer switch first.

From a technical standpoint, household dimmers use a TRIAC, which while a type of thyristor, is a single bidirectional device manufactured in an analogy of two non-parallel thyristors back-to-back. Higher end industrial dimmers will actually use two thyristors back to back to improve control, current, and harmonics created which require the use of inductors and capacitors to correct.

As far as heat, the 50% point does create more resistance temperature in the device (TRIAC) but due to the shortened duty cycle, the heat dissipation is reduced to close to that of the full on mode.

Also, the switching circuit does initiate at the zero crossing, however this is where the R/C network phase control (aka the dimmer control) begins it's cycle up/down through the phase until the TRIAC is triggered on by the required voltage at the gate.

"I think cheaper dimmers don't use 0 crossing switching "

All the common dimmers use a TRIAC (the equivalent of back to back SCRs) and a zero crossing circuit (a DIAC) and a phase delay circuit to move the turn on point away from the actual zero crossing the diac detects.

The thyristor will turn off when the current drops to zero if the gate drive is no longer present and of the correct polarity.
This means that only turn on must be controlled (delayed to produce dimming).

The current difference form turning on at any particular point in the cycle is dwarfed by the power during the peak of the cycle (1.4 times the current based solely on the load since we are concerned with peak currents and not RMS current).

The light output does NOT vary linearly with the load current, but is much closer to the power.

Many of the resistance tracks used in triac dimmers are not simple linear resistors.

There are newer (more expensive) dimmers that use other devices (gate turn on transistors, even FETs) and are designed to be more compatible with inductive loads (like motors, transformers, and AC-DC power supplies (that often have an inductive input).

By turning on at the zero crossing (instead of waiting like a thyristor) and then having a more controlled off slope later in the waveform harmonic production and its heating effects in inductors are reduced.

..all the techie stuff aside...was at my brother n law's who has 4 (yes 4) dimmers alongside each other...not sure if they are all in the same box. One has 8 - 65w 130V incandecent bulbs on it. It was warm but not as hot as mine. I think I am going to try to find his lutron brand and give it a try. My leviton was $10...his is $20 I think...if it is cooler, I will sleep better.

If you can touch the item and not poll back it is around 120 F.

If you pull back after a few seconds it is closer to 130 F.

If you pull back almost instantly, around 140 F.

While these may seem like high temperatures, modern wire insulation (older NM) is rated at least at 60 C (140 F) and newer stuff is 90 C (194 F).

Higher power dimmers have a larger faceplate to radiate more heat (and sometimes cannot be ganged in a single box the faceplates are so large) and some very large ones (1500+ Watt) even have surface fins and a cover to prevent contact with the fins) and do not use a cover plate on the box they are in (they cover the box themselves).

Try to stay below about 80% of the power rating for longer life.
Heat kills electronics (even is slowly).

I noticed you mentioned "double blue box" which typically means plastic. The metal plate on the dimmer that attaches to the box is the heatsink for the dimmer. When the dimmer is mounted to a metal box, the box itself will help to dissipate some of the heat from the heatsink.

Forget the "HD exclusive" stuff, which equals "cheap junk." (But does not mean it's unsafe.) If the heat worries you, go to an electrical supply house or a mom & pop hardware store and look for a quality brand dimmer, such as P&S Legrand.

Good point Yosemitebill. I have ALWAYS been a fan of metal
boxes, over plastic ones. I feel that if there is going
to be any arcing going on in a box, I feel as if I have
a better chance of my house not burning down if that arc
is contained in a metal box. Just my humble opinion.
In either case, I think I would put an ampmeter on this
dimmer to see how much is actually going through it.
Then convert those amps into watts and then you know
where you stand.

I looked at HD more carefully today. My brother-in-laws Lutron dimmer is a $30 I think (mine was $16). Its still rated 600W, a little fancier but the unit overall is larger and I think there is more metal surface area. However, I do not think I could fit two of these in my box without cutting off tabs on the sides...unless its ok for one tab to be flush with the outter (outside) edge of the blue box and the other edge to overlap the the edge of the other dimmer. I am not sure if I should try these or leave what I have in there. My brother-in-law may have a metal box...I will ask....I guess that would help dissapate the heat a lot and I assume the box would not get hot enough for the sheet rock or fiberglass to catch fire. The stuff you learn here....

I went with stainless steel cover plates for all my switches and outlets. Most outlet and switch boxes are plastic, but I suspect the stainless covers do aid in cooling.

I actually prefer the plastic boxes in some ways, with some of the smaller volume metal ones, the unused screw on the side of an outlet comes very, very close to the side of the box...which is why I make sure the unused screws on the sides of outlets and switches are screwed in.

"the unused screw on the side of an outlet comes very, very close to the side of the box..."

The breakdown strength of air is 30,000 V/cm.

so, will a metal outlet cover help?

Yes.

As would a metal box.

Heat conduction is very dependent on mass, and the plastic has less mass.

Heat conduction is also quite dependent upon the thermal conductivity of the material involved. Steel is typically about 100 times more thermally conductive than plastic. YMMV.

"Heat conduction is also quite dependent upon the thermal conductivity of the material involved."

Thermal conductivity is more strongly correlated with mass than any other thing.

Metals have a higher mass (density actually).
ALL of them.

Gasses have a lower mass (density) in any fixed space.

Only by doping plastics to increase their mass can their thermal conductivity be improved.

Powdered steel, carbon, and other metals are often used.

There are a few rare exceptions (carbon nanotubes conduct heat 'better' than they should).

Brickeyee, not sure what you're saying.

Aluminum with a density of 2.8g/cc has a very high thermal conductivity of 235W/mK. Iodine, which is not a metal, has a higher density of 4.93g/cc, but a much lower thermal conductivity of .449W/mK.

So I suppose that when comparing metals to plastics, the density thing is true enough, but if allowing other materials to be considered, I'll stick with my thermal conductivity answer.

To the OP, yes, a metal box will help to dissipate heat more readily than a plastic box.

I was reffering to replacing the outlet cover plate with a metal one...not the outlet box that is already dryalled over...

We do not make electrical boxes out of iodine.

Metal or plastic are the options

Stop grasping at straws.

It is not just conductivity either.

With enough mass, surface area, and cross section for heat movement the conductivity can be made a moot point.

We had the same problem and replaced the bulbs with 65W LED which only uses 10W each and now it is fine. Not hot at all.