IEC 726 includes a specific correction formula for installations above 1000 meters. Because air density changes affect cooling, the standard mandates a reduction in permissible temperature rise or an increase in test margins. This clause is often missing from pirated "summaries."
IEC 726 applies to dry-type power transformers with the highest voltage for equipment up to and including 36 kV. These transformers are typically used in indoor environments where fire safety is a priority, as they eliminate the risk of oil leaks and fires. Service Conditions iec 726 pdf