Derating for Temperature and Altitude: Reading the Curve

A power supply’s headline wattage comes with strings attached. Run it hotter, higher or with less airflow than the datasheet assumes and it cannot deliver full output safely. That reduction is derating, and the datasheet shows it as a curve.

Temperature

Full output usually holds up to around 50 °C ambient. Above that the curve slopes down — often losing 2.5 % of rated power per °C. A 100 W supply at 60 °C might only be good for 75 W. Always read the curve at your worst-case ambient inside the enclosure, not the lab figure.

Altitude

Thinner air cools less well and insulates less, so high-altitude installs derate too — typically above 2000 m. Both the power and the isolation rating may need adjusting.

Airflow and orientation

Convection-cooled supplies assume a particular mounting orientation and clear space around them. Mount one flat, or box it in, and it runs hotter than the curve predicts. Forced air (a fan) buys back a lot of the lost output.

The safe habit: size the supply so it runs at 70–80 % of the derated figure at your real conditions. Send us the conditions and we will check the headroom.