77 Ill. Adm. Code 890.620
Overflows
Section 890
Section 890.620 Overflows
a) Design. When any fixture is provided with an overflow, the
waste piping shall be so designed that the standing water in the fixtures
cannot rise in the overflow when the stopper is closed or remain in the
overflow when the fixture is emptied.
b) Connection. The overflow pipe from a fixture shall be
connected on the house or inlet side of the fixture trap, except that overflows
of flush tanks may discharge into the water closets or urinals served by them.
No overflow shall be connected to any other part of the drainage system.