ECN Forum Forums Code Related Discussion NEC & other Code issues Parallel 3/0 or 4/0 for 400-amp

I plan to install a 400-amp 3-phase service in 4" PVC.
I believe that installing parallel service conductors (2 3/0 Cu THWN/THHN per phase) in the 4" raceway would total 6 current carrying conductors (neutral not counted) and require derating to 80%. Even using the 90 deg. column for derating purposes, the ampacity falls below 400-amps. Would 240.4(B) apply, or would I have to upsize to a pair of 4/0 per phase.

I will probably chech with AHJ, but was wondering what you guys thought about the installation.

Thanks

John