What is the maximum surface temperature of a transformers enclosure?

Frequently Asked Questions Transformer Design

Per NEMA ST-20 (2014):

Transformers <= 10 kVA can be up to 65C above ambient with a typical maximum ambient of 25C.
Transformers > 10 kVA can be up to 50C above ambient with a typical maximum ambient of 40C.
Transformers with lower than standard temperature rises will have lower maximum enclosure temperatures.