The short answer: Foodservice operations that put fats, oils, and grease (FOG) down the drain must capture it before it reaches the sewer, using a grease interceptor (a small under-fixture "grease trap," or a large in-ground gravity interceptor for higher volumes). Under the IPC (Section 1003) and local sewer-authority rules, fixtures that discharge grease — pot sinks, prep sinks, dishwashers, mop sinks in kitchens, floor drains in cooking areas — route through the interceptor, while non-grease fixtures (restroom lavatories, toilets) do not. The device is sized to the flow and grease load it must handle, and the local sewer/wastewater authority frequently sets its own sizing and type requirements that are stricter than the model code — so the AHJ and the sewer district both matter. Hand sinks and toilets are kept off the interceptor so it isn't overwhelmed with non-grease flow.
Why interceptors are required
Grease congeals in sewer lines and causes blockages and overflows. Codes and sewer authorities require foodservice FOG to be intercepted on-site. The interceptor slows flow so grease floats and solids settle, letting cleaner water pass to the sewer; the captured grease is pumped out on a maintenance schedule.
What connects (and what doesn't)
Connect (grease-bearing): pot/pre-rinse/prep sinks, dishwashers, wok stoves, kitchen floor drains and floor sinks serving cooking areas, mop sinks used for kitchen cleanup.
Do NOT connect: restroom water closets and lavatories, and other non-grease sanitary fixtures — adding them wastes interceptor capacity and can violate local rules.
Sizing basics
The IPC provides a framework for sizing based on the fixture/flow served and grease-retention capacity. In practice, local sewer-authority requirements often govern and can dictate interceptor type (hydromechanical vs. gravity), minimum size, and sometimes mandate an in-ground gravity interceptor for full kitchens. Because of this, sizing is confirmed against both the plumbing code and the local wastewater authority — don't size from the model code alone.
Common review comments
No interceptor where grease-bearing fixtures discharge.
Non-grease fixtures (toilets, restroom lavs) tied into the interceptor.
Interceptor undersized for the fixtures served, or wrong type for the jurisdiction.
Model IPC framing; local plumbing code AND the sewer/wastewater authority govern, and are often stricter. Verify against your codes of record and local utility.