I am trying to build a transformer with:
Power: 10kw
Vrms:600
Frequency: 1Khz
AC source
turns ratio 1:1, 20 turns
I am using the formula:
Φ=BxA
E1=4.44xΦxfxN1
and I found Φ=6.75e10-3Wb
I can't find the flux density and the area of the core in order to build the transformer.
Any suggestions?

 

Your specs might exceed the Terms of Service here, relating to safety. 600v at 1kHz is truly dangerous.

How did you arrive at your turns ratio? And what are you trying to accomplish? Transform what to what?

Heat dissipation will be a much bigger problem than flux density.

 

is just in theory.... so how I will find the core area of the transformer?

 

There are lots of calculations you need to make to determine size and material of a transformer core.

Here are many of the needed formula gathered together for you.

http://info.ee.surrey.ac.uk/Workshop/advice/coils/terms.html

Searching google for more specific topics will return many more useful bits of data too!