Tesla,
You are quite correct that the line-neutral current on the secondary of the transformer is 4X the line to line current on the corresponding leg of the primary delta.
However you are making the error that the 'line' current feeding the primary is equal to the current on each delta leg in the primary; this is not correct as the current on each supply leg must split through _two_ branches of the delta.
Given the example transformer, fully loaded (but ignoring losses).
75000/480/root(3)=
90.2A flows on each of the three supply conductors.
75000/480/3=
52.1A flows on each of the delta legs of the primary.
(75000/480/3) * 4=
208.3A flows on each of the wye legs of the secondary.
-Jon
