To erase the 2D graphics for a conduit or cable tray, go to
Ribbon: DM Electrical->3D-BIM-> 
Erase 2D Conduit or Cable Tray
Pulldown Menu: DM Electrical->3D-BIM->Erase 2D Conduit or Cable Tray
You will be prompted to identify from which conduit or cable tray the 2D graphics are to be erased.
Select conduit or cable tray in system to erase 2D from:
The 2D graphics for the entire run of the selected conduit or cable tray will be erased.