What is the normal life expectancy of a transformer?

When a transformer is operated under ANSI/IEEE basic loading conditions (ANSI C57.96), its normal life expectancy is 20 years. The ANSI/IEEE basic loading conditions are:

A. The transformer is continuously loaded at rated kVA and rated voltages

B. The average temperature of the ambient air during any 24-hour period is equal to 30 degrees C and at no time exceeds 40 degrees C.

C. The altitude where the transformer is installed does not exceed 3300 feet (1000 meters).