If you load a 600 watt dimmer to 585 watts it will get very warm to the touch. If you load a 1000 watt dimmer to 585 watts, will it get just as warm as the 600 watt dimmer? The 1000 watt dimmer being the Lutron flush mount, no external fins.
it depends, does the 1000W dimmer have a greater heat dissapation capacity than the 600W, or just a higher heat tolerence, if the former than no, it will not get as hot, if the latter, than yes it will. the former is far more likely since a 1000W with the heat dissipation capacity of a 600W would get far to hot.
Dimmers work by firing SCRs in a delayed manner, called "phase angle control" which just means each SCR is on only part of each sine wave cycle. The longer it is on, the more RMS voltage gets through to the load.
SCRs are not perfect conductors when they are on; they reject approx. 1-1/2 watts per load amp. So if your load is 585W in either case, the heat rejected by the SCRs is going to be the same regardless of what the dimmer is rated for. The rating just denotes the amount of power that it can safely handle without overheating. If you tried to pass 990W through the 600W dimmer, it's heat sinks could not move the heat away from the SCRs fast enough and they would let the magic smoke out. 9900W through the 1000W dimmer however is not going to do it any harm because it has more heat sink and maybe larger SCRs (maybe not actually).
As Greg inferred, the dimmers have to be derated when you gang them together and have to break off one or both sides of the cooling plate surface. The dimmer has the derating written on it. As to the heat, try a plastic plate. Or go to a brassplate and burn your fingerprints off!
Thanks for the replies, Jeff I think you meant 990 watts? I have a customer with 585 watts on a 600 watt dimmer. They said it gets real hot. It is a P&S and is in a 2 gang box with a single pole switch. The P&S has no breakaway tabs.
I think this came up in a similar thread about a year ago. I had the same problem at a few locations - heavily loaded (but under max rating) units (Lutrons in my case) get hot or very warm to the touch. I researched it with Lutron and finally came to the conclusion that they just get hot, plain and simple, with anything over 400W, regardless if the unit is rated for 600W or 1000W.
A stainless plate actually helps disapate heat better, but transferred the heat to the clients mits, making the problem more noticable.
Sorry to you and your client, but until they come out with a product that keeps the heat away from the rocker switch, it's something we have to live with.