The Fillet Conduit or Cable Tray command is used to join two conduits or cable trays together. The commands works similar to the standard CAD FILLET command.
To fillet two conduits or cable trays, go to
Ribbon: DM Electrical->3D-BIM-> 
Fillet Conduit or Cable Tray
Pulldown Menu: DM Electrical->3D-BIM->Fillet Conduit or Cable Tray
You will be prompted to select the first conduit or cable tray to fillet.
Select first conduit or cable tray to fillet:
You will then be prompted to select the second conduit or cable tray.
Select second <conduit/cable tray>:
The two conduits or cable trays will be extended to each other and connected. If the conduits or cable trays are at different elevations, a vertical piece will be inserted at the elbow to complete the connection.