I have custom shower with 3/4" thermostatic valve, three 3/4" volume
controls (one for fixed shower arm, the second for hand-held shower and
the third one for body sprays) and 3 body sprays. I have 2" shower
drain and even when all shower units are fully open which represents a
wall of water shower drain works completely OK. I have never had entire
shower bottom which is 5' X 3' under the water.