The graph below indicates how to reduce the power dissipated by a potentiometer based on the increase in operating temperature. Three different materials are listed; Carbon Composition, Conductive Plastic, and Cermet. An additional curve for Wire Wound potentiometers is also shown. Cermet material is the best material to use for the widest range in operational temperatures, note the material works out to 125C before any derating is required. However there are a number of different types of packages, and each package will have its own operational characteristics. The graph is a general guide and in many cases a wire-wound device will operate up to 300C.
The power rating of the device must be decreased as the ambient temperature is increased. The link shows the derating curve for an Adjustable WireWound resistor.
Resistor Manufacturers, or Potentiometer Manufacturers
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Derating guidelines for components; Guideline for Derating Electronic Components;
How to Derate components as the temperature changes.
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