1. Transformer Impedance

2. In Simple Terms
Transformer impedance is the % of rated voltage required to push 100% of the rated secondary current when the secondary winding is short circuited.

4. Simplified Equivalent Circuit
Transformer impedance
Z = R + jX
Z is represented in percent of rated primary voltage to cause rated secondary current in a short-circuited secondary.

5. Transformer Impedance
Transformer Impedance Is A Major Parameter To Enable Parallel Operation Of Transformers
For Proper Paralleling The Following Are Required:
Same Percentage Impedance For All Units (With Some Tolerance)
Same Phase Relationship Between Windings
Same Voltage Ratios
Similar Power Ratings

6. Maximum available fault current
Calculate the secondary (LS) current at the given MVA
Calculate what the impedance related voltage is based on the % impedance.
Calculate how many times the calculated voltage will go into the rated primary voltage.
Multiply this number by the full load current at the MVA

8. Now The Easy Method Fault Amps = Full Load Amps ÷ Impedance