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This is where experience comes into play. Revit is a software package with a lot of powerful tools, however, Revit is NOT intelligent. You may be able to attach the duct in Revit, but in general the tap should be two (2) inched smaller that the dimension of the rectangular duct. For example if you need to make a 10" dia. tap in to a rectangular duct the rectangular duct dimension where the hole is made should be 12" minimum. If the need arises this can be done but creates a leakage problem as the system ages.