UPC Table 1002.2: Maximum Trap Arm Length by Pipe Size

The table measures the trap arm. That is the run of drain pipe between the outlet of a fixture's trap and the point where the vent connects. The table caps the trap arm's "developed length," meaning the pipe's actual routed distance, not a straight line. The cap depends on the pipe's diameter:

Each fixture trap shall have a protecting vent so located that the developed length of the trap arm from the trap weir to the inner edge of the vent shall be within the distance given in Table 1002.2, but in no case less than two times the diameter of the trap arm.

In practice, that table works out to these limits by pipe size:

Bigger pipe is allowed to run farther. It takes more falling water, and more time, to build the kind of vacuum that would pull a trap dry. A narrow 1-1/4 inch line under a bathroom sink loses that protection much sooner than a wide 4 inch line serving a floor drain. That is why the smallest pipes get the shortest leash.

This limit governs one layout choice every time a fixture drain is roughed in: how far the trap can sit from the point the vent ties into the line. Picture a remodel that moves a bathroom sink a few feet from where it used to sit, but reuses the old vent connection. The new trap arm distance has to be checked against the table for the pipe size in use. It cannot be assumed fine just because the old layout worked.

One exception matters in almost every bathroom. Water closets and other self-siphoning fixtures are not bound by the standard distance table. A toilet's design already forces a strong, controlled flush. It does not depend on the same suction-avoidance logic that protects a sink or tub trap.

If a fixture's trap arm runs longer than the table allows, the trap seal can pull dry between uses. The classic symptom is a bathroom that starts smelling faintly of sewer gas with no visible leak anywhere. A gurgling toilet or drain during a nearby fixture's drain cycle is often the earlier warning sign of this same venting problem, showing up before the seal actually fails. The fix usually means moving either the trap or the vent connection, not just adding a fitting. That makes this a layout problem best caught at rough-in inspection, not after the wall is closed up.

Table 1002.2 is part of the Uniform Plumbing Code. IAPMO publishes and holds the copyright on it. The table above restates its values in a new format; it is not a copy of the copyrighted table graphic itself. Phoenix enforces the 2024 UPC with local amendments. Confirm the current adopted figures at up.codes before finalizing a rough-in that sits close to a table limit.