Electrical power systems are affected by altitude. Standards indicate that an electrical device will operate satisfactorily at elevations between sea level and 1000 m. Per the IEEE document VOL. IA-20, NO. 2 a derate factor should be applied on equipment operated above 1000 m.
Per the IEEE document VOL. IA-20, NO. 2 the derating factor for dielectric strength are unity from sea level to 1000 m, at which point the dielectric strength is derated at a rate of approximately one percent per 100 m /1000m ASL. (3% per 1000ft)
Per Table Table III
1,500 m ||
3,000 m ||