Transformer Loads

The short answer is by calculating the load power factor in combination with supply de-rating factors (which are themselves a combination of engineering common practices and applicable electrical standards).

http://en.wikipedia.org/wiki/AC_power#Real.2C_reactive.2C_and_apparent_power

http://en.wikipedia.org/wiki/Derating

Also see numerous previous threads on CR4 on the subject.

The 63 kva does seem a bit larger than normal for 30 kw. However, some possible explanations might include:

Low power factor loads are anticipated.

The transformer has been upsized to allow for future addition of loads.

Perhaps a miscalculation.

Did whoever calculated this state the rationale?