current carrying capacity (can u solve this)

Take a look at the table listed at this site.

http://www.houwire.com/products/technical/article310_16.html

You need about

17 ckts in separate conduits of 500 kcm al which I think is ridiculous. You need to make some changes in the design. Maybe higher voltage.

A little information about your application would be helpful.

Typically, this amount of current is only seen at generator terminals and is handled by solid bus.

As stated by wareagle, you may need to rethink the design of your system. A couple of transformers could be in order.

As implied by earlier posts, 3 Megawatts at 420 volt system is not a practical instalation. Even if you could find suitable cable, your connections to the device would be unwieldy and the internal conductors and terminations would not be feasible.

Are the numbers correct? Power in those quantities is typical of power generation equipment or maybe extremenly large mining equipment (like draglines) that would normally run much higher voltage.

I helped install a complete switchyard for a coal mine and that only handled 100 Amp per phase on 11000 Volt.

Power losses in the conductors at your proposed current of 5155 Amp would represent significant expense and pose additional challenges for your system in maintaining it's own stability and not melting away.

Consult an appropriately-qualified Electrical Engineer!