For a WM a properly plumbed 2″ standpipe, trap, and drain line will always be sufficient, unless they redesign WMs to eject larger volumes of waste water. But in DWV plumbing, larger pipe is always better. There may be a suds issue, as RP says. If so, going to 3″ should be overkill. In DWV, overkill is never a bad thing.
» This message has been edited by nicktheplumber on 04 September 2003