Traps, Interceptors, and Special Wastes
Trap fundamentals
A trap holds a water seal of 2 to 4 inches and must be self-cleaning. Prohibited arrangements include S-traps, bell traps, drum traps (except where specifically allowed), double trapping, and crown venting in place of a proper vent. Each fixture is separately trapped, with narrow exceptions such as a continuous set of sinks or a battery of fixtures with a common waste. Two distances matter: the fixture outlet is limited in how far it can be from the trap - commonly no more than about 24 inches vertically and 30 inches horizontally from the trap weir.
Interceptors
Grease interceptors are required for commercial kitchens, oil/water separators for garages and repair shops, plus sand, plaster, lint, and dental amalgam interceptors. They must be sized to the fixture load and flow rate, installed where accessible for cleaning, and properly vented. Exam traps: burying a grease trap so it cannot be serviced, or routing a mop sink through a floor drain instead of an air-gapped receptor.
Special wastes
Corrosive and high-temperature wastes need neutralization or cooling, and many appliances discharge indirectly to a receptor. An air gap at an indirect connection should be at least twice the effective outlet diameter, and never less than 1 inch. Indirect waste piping must drain by gravity, is commonly limited to about 15 feet of developed length, and must not create a cross-connection.
Worked example
A 2-inch condensate line dumps into a floor drain through a 3/4-inch air gap. Too small: the required gap is 2 × 2 = 4 inches, so the line must terminate at least 4 inches above the receptor rim.