Trade Exam · Module 4

Traps and Interceptors: Seals, Trap Arms and Separators

Lesson 4 of 9~20 minFree previewCurrent as of September 2026Leer en español

CPC Chapter 10 in plain language: one approved liquid-seal trap per fixture, a tailpiece no longer than 24 inches, a trap seal of 2 to 4 inches, trap arm lengths from Table 1002.2 at 1/4 inch per foot, the prohibited traps (S, bell, crown-vented, moving-part and double traps), trap seal primers for floor drains, and how grease, sand, lint and oil interceptors are required, sized and maintained.

Key Takeaways

CPC 1001.2: each fixture is separately trapped, not more than one trap per trap arm, and the tailpiece never exceeds 24 inches.
CPC 1005.1: the trap seal is not less than 2 inches and not more than 4 inches unless the Authority Having Jurisdiction requires a deeper seal.
Table 1002.2 maximum trap arm lengths: 1-1/4 inch 30 inches, 1-1/2 inch 42 inches, 2 inch 60 inches, 3 inch 72 inches, 4 inch 120 inches, never less than two pipe diameters, at 1/4 inch per foot. A water closet's trap arm is limited to 6 feet.
CPC 1004.1: S traps, bell traps, crown-vented traps, traps that rely on moving parts, and double trapping are prohibited.
CPC 1003.2 and 1003.3: one slip joint at most on the outlet side of a trap, and a trap is the same size as its trap arm.
CPC 1007.1: floor drains subject to infrequent use get a trap seal primer unless the Authority Having Jurisdiction decides it is not needed.
CPC 1014.1 and Table 1014.3.6: grease interceptors are required where the Authority Having Jurisdiction determines pretreatment is needed; toilets, urinals, disposers and dishwashers do not discharge into them (with listed exceptions for disposers).
CPC 1009.3 and 1016.4: sand and heavy-solids interceptors have a water seal of not less than 6 inches.

Why This Matters

Every fixture drain ends in a trap, and every trap has a job: hold a water seal that keeps sewer gas out of the building. Chapter 10 of the California Plumbing Code (CPC) sets how traps are chosen, sized, vented, and protected, and when an interceptor has to catch grease, sand, oil, or lint before it reaches the sewer. The numbers in this chapter are few and exact, so learn them as written.

One Trap per Fixture (CPC 1001.2)

Each plumbing fixture is separately trapped by an approved liquid-seal trap. Fixtures with integral traps, like a water closet, are excluded.
Not more than one trap is permitted on a trap arm.
Food waste disposers installed with a set of restaurant, commercial, or industrial sinks connect to a separate trap.
Each domestic clothes washer and each laundry sink has its own trap, except a laundry sink trap may also take a clothes washer set next to it.
The tailpiece, the vertical distance from the fixture outlet to the trap weir, is as short as practicable and never more than 24 inches.
One trap may serve up to three single-compartment sinks or laundry sinks of the same depth, or three lavatories, immediately adjacent in the same room, with waste outlets not more than 30 inches apart. With three compartments, the trap is centrally located.

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