Derating may be required if you have two or more dimmers sharing a wallbox to help dissipate the internal heat of the dimmer. Some dimmers will have fins (heat sink) on the side which will need to be removed. By removing the fin, it decreases the wattage capacity that the dimmer can control. Be sure to refer to the dimmer instruction sheet for fin removal and derating chart. Derating is NOT required when using dimmable LED or dimmable CFL bulbs.