On a 400 amp service for a commercial building. I'm trying to figure out the conductor size using parallel runs. I think 3/0 is sufficient but not sure. 3/0 is good for 225amps under the 90 degree column. Do you then multiply by 2 for 450 amps?

#3/0 is correct but not because of the 90 Deg C column, but it is rated 200 amps in the 75 Deg C column. Cannot use 90 D column unless the terminals are rated 90 D.

So parallel #3/0 will be rated 400 amps.

Seeing that you said 225A in the begin, we can assume you are using copper:)