Bear in mind a dimmer drops at least .7v across the junction of the triac (maybe 1.4?). Multiply that times the number of amps the lights draw and you have the number of watts the device has to get rid of. (at 3.4 BTU per watt). A 600w light load (5a)will impose about 3.5 watts across the dimmer. That doesn't sound like much but if it is confined in a box it can still get pretty hot. Put a couple dimmers in a box and it can get real hot.
