You need to know the load type and calcuation of schort 
circuit

VOLTAGE & CURRENT

Total connected load of plant

first u should know 
1,The connected load of transformer or load factor
2,Highest connected load / motor in the transformer and its 
starting type
3, diversity factor
because
  KVA demand	at 0.8 p.f =connected load x LF x DF  
   				-------------   	 
                                efficiency X p.f
starting current of highest KW load takes 6 times of full 
load current. so we to check it whether the transformer has 
the capability to feed the load or not during starting. its 
the important thing while considering trafo selection..

first we calculate the industrial,ligthing & other load. on 
that basis we can calculated the total load factor,on that 
base we get a total kw ,and we get required total kva

1.Load factor
2.Diversity Factor

First we have to know about the load consumption, then  
diversity factor that is demand in KW and then in KVA

Load Current,Load Voltage and Load Power factor(Secondary 
Side). It may 3 Phase or 1Phase.

For 3phase Power in KVA = 1.732*V*I/0.8*1000

For 1 Phase in KVA = V*I/1000.

The rating of the transformer can be identified from the 
above formula.