I can't locate the Lutron D1500 dimmer switch on Lutron web site. Perhaps there is a different number for it.

Anyway, you are attempting to dim a total of 800 watts of light. A standard dimmer switch will not handle this much of a load as they are rated for 600 watts. A 1000 dimmer is the next size up and would be the minimum rating to be used. This means the dimmer would be operating at 80% of capacity and the switch may become warm - but all within acceptable limits.

A dimmer switch rated at 1200 watts or even 1500 watts may be a better choice, as the 800 watt load would present a load that would only be 66% and 53% (respectively) of rated capacity, and would likely run cooler and last a lot longer.

The down side to these higher wattage rated switches is their cost. It is not unusual for the price to double for a 600w vs a 1000w dimmer.

Sometimes, a 1000w dimmer is not sufficient to control a 600w load. This happens when 2 or more dimmer switches are installed in a single location under one wall plate. It is a fairly common arrangement for electricians to install 2, 3 or more "ganged boxes" so that there aren't 2, 3 or more individual switches clustered around a doorway. Even though a two ganged box has twice the area of a one gang box, the issue is about heat dissipation. A box will contain the heat. So the heat is given up from the front of the switch. The metal fins provide more area for cooling. When two or more dimmers are located in a multiple-ganged box, there is too much heat for the space. Two 600 watt dimmers would need to be derated to about 450 watts each (instructions for derating are included with the switch - each manufacturer has their own formulas), and if three 600 watt dimmers were in a single location, they might need to be derated to 300 watts each. So, simply moving to a 1000 watt or 1200 watt dimmer may not get you to the 600 watt level if there are several dimmers that require derating to 50%. Installing dimmers in boxes with standard "toggle" type (non-dimming) switches require no derating as toggle switches do not produce appreciable heat.

Make sure that the lighting load is a type designed for dimming. The popular CFL (compact fluorescent lamps) are not designed for dimming, unless the package specifically states otherwise. Lights that have a filiment but no transformer, ballast, starter, etc. are the only ones suitable for use with a dimmer (again - unless the package / fixture states otherwise). The dimmable types are typically "standard" incandescent, quartz, halogen and tungsten types.

Furthermore, a dimmer switch is not suitable for use as a fan speed control either. There are special switches to provide speed control of fan motors. Use of a dimmer on a motor load is a fire hazard.

I hope this helps & good luck!