First, determine what type of transformer is in the lighting fixture. Different characteristics of the two transformer types require special dimming considerations. If your low voltage fixtures use magnetic transformers, then a magnetic low voltage dimmer is required. The same will be true for Electronic Low Voltage, Florescent, ect.
Add the total wattage fo the bulbs that will be used by this dimmer. This is known as the 'load'. The dimmer must exceed the maximum 'load' of the bulbs to be used. Example;
If 10 bulbs are rated at 50 watts each. Your total load on the dimmer will be 500watts. A 600 watt dimmer will work, right? Not always.
If the dimmer is in the center of a 3 gang wallplate, we know that each side of the center dimmer must have its left and right sides respective tabs removed. If the dimmer is loriginaly isted as a 600 watt dimmer, but it is derated (tabs removed) on both sides, then the total wattage this dimmer will support will only be 400 watts. This will not be enough to support our 10 bulb application. We could lower the wattage for the bulbs, take bulbs out of the series, or increase our dimmers maximum 'load'. If we purchase a 1000 watt dimmer, but have to derate the dimmer as we did in the previous example, the dimmer will only support a 800 watt dimmer. This is enough for our 600 watt application.