Running the connection sideways before reaching the trap leaves waste standing in unscoured pipe outside the trap. Under the IPC, what is the greatest horizontal distance permitted between the fixture outlet and the trap inlet?
- A24 inches
Twenty-four inches is the vertical limit from the fixture outlet down to the trap weir, taken from the same subsection. Applying the vertical figure to a horizontal run is the mirror image of the usual mistake.
- B48 inches
Forty-eight inches exceeds the limit by more than half and would leave a substantial length of unscoured waste pipe ahead of the trap on a laboratory sink.
- C60 inches
Sixty inches appears in this chapter in a different context, as part of a specific allowance involving grease interceptors serving food preparation sinks. It does not govern ordinary fixture trap placement.
- 30 inches
Why D is correct
Thirty inches, measured centerline to centerline between the fixture outlet and the trap inlet, is the horizontal limit. The pipe in that stretch is not protected by the seal and does not benefit from the trap's self-scouring action, so the code keeps it short.
What this question is testing
Recall of the horizontal limit and the measuring points, and whether the candidate can keep the two dimensions in this subsection separate under time pressure.
On the job
Laboratory sinks, bar sinks and any fixture set into a long island counter are where this dimension gets stretched, usually because the trap has to land over a floor penetration that is already poured. The practical consequence of a long horizontal run ahead of the trap is standing water, odor and, in a lab, chemical residue sitting in a pipe nobody can flush. Fixing it means relocating the drain, so it is worth checking before the slab.
Memory technique
Thirty inches sideways is as far as the trap may sit.
Exam tip
Both numbers sit in the same paragraph. Read the question for the word vertical or horizontal before choosing.
Where to look it up
Chapter 10, the fixture traps subsection. UPC differs on this point — follow the code your state has adopted.