What is the maximum surface temperature of a transformers terminals wire leads or connections points?

Frequently Asked Questions Transformer Design 

Per NEMA ST-20 (2014):

  • Transformers <= 10 kVA can be up to 50C above ambient with a typical maximum ambient of 25C.
  • Transformers > 10 kVA can be up to 35C above ambient with a typical maximum ambient of 40C.
  • Transformers with lower than standard temperature rises will have lower maximum connection point temperatures.