Too hard to tell you what to do without knowing more about the power system both on the primary and secondary side of the transformer. Find a good reference on doing this using per unit quantities. A good method recommended by ANSI involves creating separate R and X networks to simplify the complex impedance calculation. You should also take into account motor contribution if appropriate.
The secondary side short circuit current for a 3 phase transformer = Volt amp transformer rating /(secondary voltage *1.732*impedance).
For example, a 1500 kVA transformer with a 480volt secondary and a 5.75% impedance will have a calculated available short circuit secondary current of 31,374 amps. The short circuit current rating of the next circuit interrupting device and bus bracing should be at least that. For a double ended substation where the transformers can be operated in parallel double it. The primary side available current for this calculation is assumed to be infinite. For circuit breakers in series, use the circuit breaker manufacturer's rating for cascading. Breakers of more than one manufacture have no easy solution. Relying on cascading between the main secondary breaker and feeder breakers to combine to have adequate SC protection where the main secondary is insufficient is not an FM approved configuration and is not recommended.
The upstream medium or high voltage feeders should be sized by the requirement of the ampacity rating of the upstream feeder breakers or fuses protecting it and by the acceptable voltage drop to the loads, whichever is the greater size.
Where there is no main secondary breaker or fused switch and the NEC six throws rule applies, all circuit interrupting devices directly connected to the transformer must have at least the full SC rating a single device would have.
Failure to adhere to these requirements could result in a catastrophe if there is a short circuit on the load side of the breaker. An entire facility could be lost in such an accident as well as anyone near the exploding breaker being badly injured or killed. Even so, in my experience it is a surprisingly common mistake.
Note, some transformers have much lower impedances than 5.75%. Solid or cast core transformers and air cooled transformers may be much lower, in the vicinity of 2%. They are more efficient but require interrupting capacity devices with much higher SC interrupting rating.
The fault current on the supply side of your transformer is determined by the source of your power. If your transformer is served by a utility company, the utility company can give you the maximum available fault current or MVA fault. The utility company can give you the phase angle of the fault MVA or they can give you the fault current and the phase angle, also the fault impedance. Now you need to calculate the fault current on your transformer secondary. You can easily determine your fault MVA on your transformer secondary by dividing your transformer MVA capacity by your transformer per unit impedance (per unit impedance is your transformer % impedance divided by 100). This will be the maximum 3 phase available fault MVA on your transformer secondary. Multiply MVA by 1000 and you get KVA. If you divide this fault KVA by square root of 3 and then divide again by your secondary voltage expressed in KV then you will get your 3 phase fault current. Note this 3 phase fault current is maximum theoretical fault current. If you consider the fault impedance at the supply side of your transformer and add it to your transformer impedance, the actual maximum available 3 fault current will be smaller. This is because you are accounting for the utility (supply) impedance.