: I am remodeling my master bath and I am switching from a single vanity to a double. The supply line that is there is 3/4" OD copper that is reduced down to 1/2" od. The question is with a shower (feeding in the opposite direction) a vanity in another a room and now two new vanities should the secondary trunk line be 3/4" then reduced down to 1/2" at each outlet location or is 1/2" OK for all? Thanks!
If you're only adding an additional lavatory basin to the existing supply piping, it will be fine.
The pipe sizes that you stated are strange; for instance 3/4" copper has an O.D. of 7/8" and 1/2" copper has an O.D. of 5/8"