Bossanove5:
Heed these words...
"You are also going to have some serious derating problems with that many conductors in a single raceway so we are not even talking about #12s. All of the wires, including the ground, have to start getting bigger when you go over three 3 phase circuits in a pipe. (9 current carrying THHN)
In the 10-20 wire range you will need #10.
At a certain point more raceways ends up being cheaper."
I have seen what Greg is saying happen in the field a few times...results were downright ugly, expensive, and one cost a young guy an unwanted 'vacation.'
As to your question on the EGC...yes it is no issue to install a ground block within a wireway/troff/gutter and run a single EGC to the source panel. Common practice is to size the EGC from the panel to the block to the largest OCP of the circuits, although that is not an NEC requirement.
