It varies by application, but 3/4" and 1" are the most common for branch circuits in many commercial and industrial situations. For feeders and the like, it can vary from 1.25" all the way up to 6", depending on the size of the wires, of course.
The available sizes of conduit are 1/2", 3/4", 1", 1.25", 1.5", 2", 2.5", 3", 3.5", 4", 5" and 6".
PVC is available in sizes up to 6". Metal conduit, such as EMT and Ridgid, is available in these sizes as well, although I am not sure if it is available in 5" and 6".
[This message has been edited by CTwireman (edited 12-10-2002).]
