A contractor has used a xmfr in an MCC as a pull section to get to the lighting panel. I do not believe it to be legal but can not find the correct NEC section to back it up.
I should also add the 300.3(C)(1) permits conductors of different systems to occupy the same raceway, enclosure, or box provided that the voltage rating of the insulation of the conductors is equal to or greater than the maximum voltage of any of the conductors in the enclosure.