Power Rating

The power rating is the headline number on a resistor datasheet — but it is only meaningful together with the derating curve and the reference ambient temperature. A part labelled “100 W at 70 °C” delivers far less than 100 W when mounted on a small PCB with poor airflow, when the ambient runs at 85 °C, or when pulses exceed the average level.

The rating is set by the maximum allowable internal hot-spot temperature (typically 350 – 450 °C for wirewound, 155 °C for film, 200 – 275 °C for cement) divided by the thermal resistance from element to ambient. Because thermal resistance changes drastically with mounting style, datasheets usually publish ratings for several scenarios: free air, mounted on a copper-pour PCB, or bolted to an aluminum heatsink. Aluminum-housed resistors only achieve their full rating with the specified heatsink area and torque.

For pulse loads the average power must still be within the rated continuous power, while the peak single-pulse energy must stay within the pulse-energy curve published in the datasheet. Designers commonly derate by 50 % over the rated value to account for tolerance stack-up and lifetime, then verify with thermal measurement during prototyping.