You have to make assumptions.

Many designers assume an “infinite bus” — zero impedance on the high side of the transformer. Using this assumption results in a fault current that is slightly higher than a worst case actual value.

Other assumptions include:
- Assuming a “bolted fault” with zero ohms at the fault location.
- Nominal distribution voltage.
- Not adjusting conductor impedances for ambient temperature.
- Neglecting the impedance of inter-connections between conductors in the circuit.
- Even if you don’t assume an “infinte bus”, you still must compute the actual source impedance. Usually based on normal operating conditions. (ex. Is the bus tie open or closed? Is it fed from the normal or alternate source? etc.)