Altitude Derating: What Happens to a Driver Installed Above 2000 Metres

A driver rated for 100 W at sea level will not deliver 100 W at 3000 metres, and the reason is not a power-loss mystery. It is the thinner air. At altitude the atmosphere is less dense, which does two things: it removes heat more slowly, and it holds off voltage breakdown less effectively. Both effects force the driver to be derated.

Cooling is the first constraint. A sealed driver dissipates heat through its aluminium housing to the surrounding air. Thinner air carries that heat away more slowly, so the driver runs hotter for the same output. The accepted practice, followed across the power supply industry, is to derate the output power by roughly 10 percent for every 1000 metres above 2000 metres, or to reduce the allowable ambient temperature by about 1 degree Celsius per 300 metres of altitude. Mean Well and others specify derating of about 3.5 to 5 degrees Celsius per 1000 metres above 2000 metres, depending on whether the unit is potted or ventilated.

Insulation is the second constraint. Air gaps and creepage distances are sized for the dielectric strength of air at sea level. At high altitude, the lower air density reduces the breakdown voltage, so the same spacing that holds off 3000 V at sea level may arc across at 3000 metres. Standards address this by requiring increased clearances for equipment used above 2000 metres, and some safety certifications are issued for a specific maximum operating altitude. A driver tested to 5000 metres has roughly 1.4 to 1.5 times the air-gap clearance of a sea-level design.

For an outdoor waterproof driver this matters more than it first appears. Many installations that seem ordinary are high altitude: alpine resorts, mountain lodges, high-desert facades, elevated transport infrastructure. A driver selected for full output at sea level and bolted to a wall at 3500 metres will run hot, derate, and possibly trip its OTP on warm afternoons. The specifier should apply the altitude derating curve at the actual installation site, not at the distributor's warehouse.

The distinction between potted and ventilated designs shows up here. A fully potted IP67 driver transfers heat through a solid thermally conductive block to the housing, which holds up better at altitude than a fan-cooled or convection-vented design whose airflow depends on air density. Mean Well's guidance reflects this: potted models derate about 3.5 degrees per 1000 metres, while unpotted convection models derate 5 degrees per 1000 metres. For high-altitude outdoor work, the sealed potted driver is the more forgiving choice.

The checklist for a high-altitude project is short but decisive: confirm the datasheet states a maximum operating altitude and an altitude derating curve; apply it to the actual site elevation; and verify the safety certificate covers that altitude. A driver that is derated to 70 percent output at 4000 metres still works, but only if the designer sized the wattage for that reduced figure in the first place.

Altitude also affects the surge environment, which matters for the outdoor drivers discussed throughout this series. Mountain installations often sit at the end of long overhead feeders, exposed to lightning-induced transients that the distribution network does not dampen well. The combination of thinner-air cooling and harsh surge exposure means a high-altitude outdoor driver benefits from both the altitude derating and a stronger surge rating, typically 4 kV differential and common mode. The two requirements are independent, but they tend to arrive together on a mountain-top sign or a highland façade, and both belong on the same datasheet review.