Transformers are designed and intended for 100% loading.
Transformers are basically only affected by heat. You can overload any transformer as long as you give it sufficient time to cool down (i.e. 2 hours of 120% may then require 8 hrs at 60%). This is how the utility pole mount transformers are sized.
Other than possible reduced life, an overloaded transformer may have reduced voltage regulation, and the conductor losses go up by the 'square' of the loading (110% load = 121% losses).
