Easiest way is ignore primary and secondary conductor impedances since the transformer impedance is usually quite large compared to the wire impedance. Then it's just Ohm's law, I = E/Z, or I = (1/%Z)(FLA).
In other words, just divide FLA by Transformer PU impedance(%Z/100)
Example: For a typical 25 kva, 12kv to 120/240v transformer, Z% = 2.4%. I (SC) = 104.2/.024 = 4340A
BTW, some new pole mounted and some padmount transformers have impedances less than 2 percent.
