I'm trying to understand why NEC 392.22(A)(5) and (6) are so limiting on the fill area that can be used for channel type cable trays. We often specify channnel tray for drops from ladder tray to motors for IAC cable or even larger TC cables. Based on NEC, if I'm installing a 3/C 500 IAC that is 2.4"OD, I would need to use a 6" channel. I don't know the reasoning for not allowing a 3" or 4" channel to be used. To take it further, if the motor feeder is parallel 500's, I would need two 6" channels when it seems one 6" channel should suffice. Heat dissipation should not be a problem as the cables would be single layer and typically run by themselves. Thanks for your input.