Traps and Interceptors Practice Questions
Trap seal depth, prohibited trap types, trap arm limits and interceptor selection.
14 questions · 2 diagram exercises
A trap exists to hold a water seal that sewer gas cannot cross, and almost every question in this section tests one of the ways that seal can be lost. The permitted seal depth is a range, not a single number: not less than 2 inches and not more than 4 inches under the IPC, except for traps built to serve a specific purpose with a deeper seal. Too shallow and the seal blows out; too deep and the trap stops being self-scouring, so solids settle in it.
Prohibited traps are a short list that repays memorising, because the reasoning behind each one is the examinable part. An S-trap is prohibited because the descending leg lets the discharge siphon the seal straight out. A bell trap is prohibited because its seal evaporates and its removable top is routinely lost. A crown-vented trap, a vent taken off the top of the trap bend, is prohibited because that location fouls and blocks. Traps with interior partitions are prohibited because the partition corrodes away and the seal disappears without any visible sign. Only one trap is permitted per fixture, with the exception for a set of not more than three single-compartment sinks or laundry tubs on one trap within a limited distance.
The trap arm is the horizontal run between trap weir and vent, and its maximum developed length is diameter-dependent: the IPC's table runs from 3½ feet on a 1¼ inch arm up to 12 feet on 4 inch and larger. The arm also must not fall more than one pipe diameter over its length, which is the rule candidates forget. Exceed either limit and the fixture is effectively unvented no matter what the drawing shows.
Interceptors shift the section from physics to process. Grease interceptors are sized by flow and retention: the common hydromechanical method works from fixture volume, drainage period and a sizing factor, while gravity grease interceptors are sized on retention time and are specified in gallons. Oil and sand interceptors serve garages and washdown areas, and the exam expects you to know which waste stream gets which device and that an interceptor is itself vented and provided with access.
What this topic tests
- Permitted trap seal depth and the consequence of being outside the range
- The prohibited trap list and the failure mechanism behind each
- One trap per fixture, and the limited multi-compartment sink exception
- Maximum trap arm developed length by diameter, and the one-diameter fall limit
- Grease, oil and sand interceptor selection, sizing basis and access requirements
Mistakes that cost marks
- Giving a single trap seal figure where the code sets a 2 to 4 inch range
- Treating a deeper seal as safer, when it stops the trap being self-scouring
- Measuring the trap arm from the fixture outlet instead of from the trap weir
- Ignoring the limit on trap arm fall, which can unvent a fixture that meets the length limit
Traps & Interceptors practice questions
Answer, check, and read why. Your result is saved in this browser, and anything you get wrong is queued for retry.
What trap seal depth does the IPC require for an ordinary fixture trap?
Calculators for this topic
Check your hand calculations against the same arithmetic, with the working shown.
Traps & Interceptors. Frequently asked questions
What trap seal depth does the code require?
Not less than 2 inches and not more than 4 inches, unless the trap is a special design intended for a deeper seal. Shallower seals are blown out by pressure fluctuations; deeper ones lose their self-scouring action and accumulate solids.
Why are S-traps prohibited?
Because the outlet leg descends. A full discharge continues to flow after the fixture empties and siphons the seal out behind it, leaving the fixture open to the drainage system. A P-trap vented within the permitted trap arm length cannot self-siphon the same way.
How many fixtures may discharge into a single trap?
One, as a rule. The recognised exception is a set of not more than three single-compartment sinks or laundry trays of the same depth, adjacent to each other, with the waste outlets within a limited distance of the trap. Separate fixtures otherwise each need their own trap.
What is the maximum distance from a trap to its vent?
It depends on the diameter of the trap arm. The IPC's table permits 3½ feet for a 1¼ inch arm, 5 feet for 1½ inch, 6 feet for 2 inch, 10 feet for 3 inch and 12 feet for 4 inch and larger, measured as developed length from the trap weir, with total fall limited to one pipe diameter.
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