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From the 2018 UPC Illustrated Training Manual

419.2 Drainage Connection.
Approved wye or other directional-type branch fittings shall be installed in continuous wastes connecting or receiving the discharge from a food waste disposer. No dishwasher drain shall be connected to a sink tailpiece, continuous waste, or trap on the discharge side of a food waste disposer.

Any type of force or pumped drain unit such as food waste disposers must enter the system through a wye type or a directional fitting (see Figure 419.2a). This will provide a smooth path for the waste rather than a tee connection, which would cause the waste to flow against the back of the receiving pipe and flow both upward and downward because of the force of the pumping action.

Dishwasher drains are not permitted to be connected to a sink tailpiece, continuous waste or trap on the discharge side of a food waste disposal unit. This is because of the possibility of the pressured waste from either the disposal or the dishwasher backing up into the other. If a disposal is present, then the dishwasher waste must connect to the disposal inlet supplied with the disposal (see Figure 419.2b).

The 2018 Uniform Plumbing Code Illustrated Training Manual is available for purchase here.