Design of a 5 MW system

5,000,000 ÷ (33,000 x √3) ≈ 87.5A. Perhaps a 100A breaker?

Tornado has given you already how to determine the breaker size. If you are using NEC-Code please refere to this link and examples for sizing the breaker.

http://www.elec-toolbox.com/usefulinfo/xfmr-3ph.htm

Let me also give you some information about isolator.

The voltage rating and short circuit rating range of an isolator should be the same as that of the breaker. An isolator is selected based on the circuit current rating ( continuous current ), voltage, fault level( short circuit rating ) and BIL. The isolator provides safety during maintenance on the transformer. An isolator, when in series with the fuse, does not require short circuit rating. Isolator is an off load type dvice. The load switching is carried out by a breaker. The isolator can break the capacitive charging current of a cable and the no load inductive current of a transformer up to a certain magnitude.

This means that we should break at CB then use isolator to isolate circuit .?

where did you get this idea " isolator to isolate circuit" ?

yes u are right

guys i think we are deviating from the topic

my question is what should be the CB and isolator rating for a 5 MW system...

and whether we can use a 11 KV LA on the 33 KV side of the trafo.

for 33kV system minium available switchgear is of 630A you can use the same.