Derating motor capacity base on altitude

Altitude Standard motors are designed to operate below 3300 feet. Air is thinner and heat is not dissipated as quickly above 3300 feet. Most motors must be derated for altitude. The following chart gives typical horsepower derating factors, but the derating factor should be checked for each motor. A 50 HP motor operated at 6000 feet, for example, would be derated to 47 HP, providing the 40°C ambient rating is still required.

Altitude Deratring factor
3300-5000 0.97
5001-6600 0.94
6601-8300 0.90
8301-9900 0.86
9901-11500 0.82

50HP * 0.94 = 47HP

Power and heat dissipation is one problem. But there is a completely different issue of voltage breakdown. Even with running at no load there is a danger of voltage flashover (arc) if the machine is used above its rated altitude.

Some definition of the required distances are given in IEC 61010. However, a device designed for lower altitude might not be usable at all at much higher altitude - and no amount of derating will fix that. This is simply linked to the physical spacing between the conductors. The only case where this does not apply is if the conductors are completely enclosed in insulation (e.g. potted in resin or immersed in oil).

However, in the IEC 61010 the same dimension requirements are given up to 2000 m, so not sure what this means. Ideally one should ask the original manufacturer/designer.