My 400 amp panel has (2) 500 MCM CU supply. What is the maximum current permited on each phase.

400 amps.

Thank you. The way my mind thinks, they (electrical gods) should have called it an 800 Amp panel if each leg has 400 amps capacity. It looks like my 150 amps on each leg of my panel leaves me plenty of room to grow. Paul

400A is 400A, if at 120v or 240v or some other voltage.

The wattage capacity is what is the actual capacity.

400A @ 120v is 48,000 watts. (This is not an accurate representation of a circuit or feeder though since you would NEVER see a 400A @ 120v circuit.)
400A @ 240v is 96,000 watts.
Both are 400A.