This is a very simple label for duct elements made with the MEP Modeler add-on. It's intended for use with any simple duct type, including Straight, Bend, Take-off, and the Obstruction Fittings (as far as I can tell). I'm using it solely for straight pieces, since no one is fabricating anything from my plans - I just want to coordinate with the HVAC engineer.
For round ducts, the label reads '5" dia'. For rectangular ducts, it reads 'Width x Depth'.
All our ducts are sized in inches, so I'm using a fractional inch format for the dimensions. On the off chance that someone else might find this label useful, I've added a parameter so you can choose to have the label's format match the current dimensions format in the project preferences.
It was convenient to use Select All for the (straight) Duct Tool, then label them all with the Label Selected Elements command (Document -> Document Extras).
