Traps and Interceptors Both Codes Agree On — Fixture Traps, Trap Seals and Primers, Prohibited Traps, Trap Sizing, Grease Interceptors, Oil Separators, Sand and Laundry Interceptors (UPC ch. 10 / IPC ch. 10)
A trap is a water seal; everything in the trap sections is about keeping that seal intact (2 to 4 inches deep, vented, not siphoned, not evaporated, not defeated by a bad trap design). An interceptor is a tank that holds back what the sewer cannot take — grease, oil, sand, lint, glass. Both codes agree on the trap rules almost word for word; on interceptors they agree on the principles and the key numbers (30-minute retention for gravity grease interceptors, the 6-in sand seal, the 2-ft / 18-in oil separator) while each keeps its own sizing tables. The differences — building traps, drum traps,
2018 UPC and 2018 IPC · Reviewed 2026-09-20
1. Every fixture gets its own trap
- Each plumbing fixture is separately trapped by a liquid-seal trap (UPC 1001.2; IPC 1002.1); fixtures with integral traps (water closets) are the exception.
- No fixture is double trapped (UPC 1004.1; IPC 1002.1).
- The tailpiece — the vertical distance from the fixture outlet to the trap weir — may not exceed 24 in (UPC 1001.2; IPC 1002.1). The IPC adds a 30-in horizontal limit from the outlet centerline to the trap inlet.
- One trap may serve a multi-compartment sink of not more than three compartments whose waste outlets are not more than 30 in apart (UPC 1001.2 — three single-compartment sinks or laundry tubs of the same depth, or three lavatories, adjacent in one room, trap centrally located; IPC 1002.1 exception 2 — a combination fixture with compartments not more than 6 in different in depth and outlets not more than 30 in apart).
- A trap serving a fixture must also be vented (UPC 1004.1 "no trap without a vent"; IPC 901.2.1) — see the vents lesson for the trap-to-vent rules; the trap-arm length tables differ (§9).
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