To connect items together, use the “DM Electrical->One-Line Devices->Connect” command. This defines the relationship between devices in your electrical model. In addition to being used to insert feeders between devices in your one-line diagram, it is also used when calculating fault and calculating voltage drop.
First, select the device on the left that is being fed. This is the downstream device. Then, select the device that it is being fed from on the right side of the dialog. This is the upstream device. You need to select both the upstream device, and the type of connection (top lugs, feed through, or connected to a circuit).
There is a special “Utility” option available to be chosen as the upstream device. Choose this when the downstream device you are connecting is at the top of your electrical one-line.