if we known the pesentage of impedance then by using this 
formula we can calculate the fault Lavel.

fault lavel in MVA=  Rated KVa/% of Impedence

                 OR

Fault Lavel=  1.732xFault Currentx Rated Voltage

fault lavel in MVA=  Rated power/% of impedence


                  or

Fault Lavel=  1.732xFault currentxRated Secondry Voltage

fault level depend on system impedance
                                full load current*100
    Trasformer fault level=    ----------------------------
                            %impedance of the transformer

suppose transformer (11 KV/433) is of 1000 KVA & % 
impedance is 4 .LT side Fault level can be calculated as 
below :

KVA = 1.732 X V X I X Cos Phi - PF assumed : 0.95

LT Rated current (I) = (1000 * 1000)/(1.732*433*0.95)
                     = 1481 A

Fault current on LT side = FLC * 100/(% impedance)
                         = 1481 * 100/5
                         = 29.62 KA

hi,
Calculation of fault level of a distribution net work of 
complex in nature manually is laborious. Use network 
analyser for simulating the network and fault at the 
required bus to get the fault current. 
   for manual calculation use the  MVA method which is 
simple and gives fairly acurate results