Mains power transformer ratings
Rated kVA and current are the values that can be delivered continuously without reducing the expected life of the transformer under rated operating conditions. Rated operating conditions would include the rated primary voltage, a maximum ambient temperature, a maximum altitude / minimum barometric pressure and a humidity specification. There would also be specifications about maintaining adequate cooling and condition of oil for an oil-filled transformer.

Any increase in current above the rated current would shorten the life of the transformer winding insulation according to a curve that is used for all electromagnetic devices.

The output voltage of a transformer is reduced with increased current by the effect of the transformer impedance. The output voltage will be a little above the rated voltage with no output current and reduced inversely proportionally to any current increase. The actual output voltage can be calculated form the transformer and load characteristics.

A transformer should be sized for the actual load current plus any anticipated increase or uncertainty in the load current determination. The size could also be increased to compensate fo any uncertainty about adequate cooling.