How to find the KA rating of Circuit breakers?

Before breaker's selecting for your electrical system, you need to calculate value of expected short circuit current at the place of breaker's installation. Then you need to calculate value of heat pulse and 1s current (expected value of current during one second). After that you need to calculate power of breaker and finally, after all, you can select appropriate breaker. Values of characteristics of selected breaker need to be higher from calculated values of characteristics of your power system.

1. The fault level of the upstream NW (Source) to be known, normally 500MVA or 250MVA.
2. Upstream impedance (reactance and resistor, capacitor to be ignored for Short cct calculation) can be determined accordingly.
3. The LV System starting from the secondary of the distribution transformer.
4. Short circuit percentage voltage for Transformer is known (normally 4% for 1000kVA and 6% for 1500 kVA) and hence reactance and impedance can also determined.
5. Impedance of Cables also can be determined from manufacturer TD sheet.
6. Subtotal impedance to be determined by conventional way (Submit if are in series/ (Z1+Z2+....Zn)/(Z1XZ2X...Zn) if are in parallel.
7. divide voltage by the Impedance up to the required location, will give you the fault current at that location.
8. Determine Maximum and minimum fault current. By the former you can decide the breaking capacity of CB and by the later the setting can be achieved.
9. verify the thermal constraints of the conductors(cables). ie

I²t ≤ S²K² , I short cct current, t time( < 5 s valid), K cable material Factor and S cable section area.
I²t Known as let through energy. accordingly breaking capacity of CB should be > than Circuit Maximum fault.

The MCB, MCCB, & ACB are all Low Voltage Circuit Breakers, where SF6 is a Non-active gaze used in Medium Voltage Circuit Breakers.

Now, to determine the value of Breaking Capacity of any circuit breaker, we should, by calculation, the Maximum Short Circuit Current Value " Isc3max " at the installation point of that circuit breaker, where we can calculate it by assuming a " Short Circuit between 3 phases at that point ", then after knowing " Isc3max " we can determine the Breaking Capacity value that should be " equal or bigger than Isc3max ".

Further:
1- The value " 250 ... 500MVA " is the short circuit power at Medium voltage side for up to 36kV.
2- About the Short Circuit Voltage percentage value:  we called " Ucc or Usc " and the value is " 4& for up to 630kVA transformers ", and " 6% for up to 2500kVA transformers ", but in all case, we can read it at the transformer's name plate.
3- Sorry Mr. Omar, we can't do, you mentioned, the sum of all Z, as these values aren't on the same vector, so, we should first calculate " R & X " for each component, then do the sum of all R " R total " and all X " X total ", then calculate the " Z total ".
4- By knowing the Minimum Short Circuit Current value " Iscmin ", we use it to determine the value of "Setting Value" of "Magnetic Protection or Short-time Protection".