Dear Mr. deetz (Electrical)
"... installing a 600 amp, 480v, 3 phase panel that will be fed from an 800 amp breaker from ... What size wire can I use to run from the 800 amp breaker to the 600 amp breaker. I was assuming to parallel feed 600mcm because of the 800 amp breaker, but now I'm wondering if I can go with 350mcm because the 600 amp breaker will trip, hopefully...."
Che. 1. The length of the cable from 800A to 600A breaker shall be sized to be protected by the 800A breaker. NOT the 600A breaker; whether it will trip or not is irrelevant.
2. It is the 800A breaker that protects the cable for (any over-current due to any fault or short-circuit) up to the [incoming terminals] of the 600A breaker.
3. See NEC for (exceptions) for short lengths which are [inside the same cubicle] etc...
Che Kuan Yau (Singapore)

Yeah, that was what I thought. Just looking for a loophole. Thanks. I appreciate the help.

If it's money, have you considered using the 90 degree tables and a three foot pigtail of 75 degree rated conductor for the connections ate each end?
Caution, recheck your voltage drops.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

They assume that you are in the USA?

if not, then your local codes regulations will apply.

I am assuming Minnesota, US.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

Thanks again guys. Yeah, MN. I'll just pony up and spend the money.

There are some exceptions for short runs and transformers with primary overload protection.
You have not given enough information for us to consider possible exceptions.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

I better adjust my thanks. Thank you all. Not just guys.