Neither one directly if you are a utility in the USA.
A facility is fully loaded when the load would reach the Facility Rating under any required contingency study.
The Facility Rating is required to be published by the FAC standards and can be based on the nameplate, IEEE 57.91, the seasonal weather conditions, historical company loading practices, normal/emergency loading or many other factors as long as the ratings are applied fairly and consistently to all assets.
It can be weird to visit a fully redundant substation on a moderate temperature day and see the "fully loaded" transformer at less than 25% load.
Alternately, a distribution transformer loaded to 150% might still have a bit more room on a cold winter morning.
It is hard to understand how a substation transformer be overloaded at 25% capacity with moderate ambient temperature (say 40 oC?). Could you please elaborate more about this statement?
Yes, each of the two transformers were loaded much less than the transformer rating.
The substation was "fully loaded" in the sense that adding additional connected load would have required additional transformer capacity to stay within the reliability requirements.
