Cooper Bussmann does not indicate how to calculate 3 phase transformer phase-to-neutral short-circuit current, indeed.
IEEE 80/2013 pp.76 [for instance] symmetrical one phase short-circuit
Ineutral=3*Io [in all three phase the same symmetrical component zero sequence current ]
The reactances X1,X2 and Xo are the total reactance of the supply system and the transformer. Xo is different and depends on transformer connection and the magnetic core construction [3 limbs or 5 limbs].
If we consider the short-circuit power of the supply system as infinite [Xsystem=0] then if the transformer connection is Delta/Yn then Xo≈X1 X2=X1[far from generators].
where VL_L=0.48 kV S=0.3 MVA
That means Isco[symmetric component per phase] is indeed Ifla/Z/3=2248.25 A
But you have 3 Io[one per phase] then IN=3*2248.25=6744.75
Also IEC 60909-0[formula 29]:
c=1 Un=380 V