You have opened a layout space. The connections navigator is open. Tab Edit > Command group Phase busbar connection > Generate. Select the Assign phase busbar popup menu item.
You can assign a part to a phase busbar connection in this dialog. In addition, you can enter or select the DT for the phase busbar connection.
Overview of the main dialog elements:
Enter the desired DT for the phase busbar. The [...] button opens the dialog DT selection in which you can select a DT existing in the project.
Displays the part that is assigned to the phase busbar. The [...] button opens the part selection in which you can select the phase busbar part. A 3D macro that is used during the placement of the phase busbar can be entered at the assigned part.
See also