According to the California Plumbing Code, what is the maximum horizontal distance allowed between a fixture trap and its vent for a 1.5-inch drain pipe?
Correct Answer
D) 3.5 feet
California Plumbing Code Table 1002.1 specifies that for 1.5-inch drain pipes, the maximum horizontal distance between the fixture trap and vent is 3.5 feet. This distance ensures proper trap seal protection and prevents siphonage that could allow sewer gases to enter the building.
Why This Is the Correct Answer
California Plumbing Code Table 1002.1 (Trap Arm Requirements) specifies that for a 1.5-inch drain pipe, the maximum horizontal distance between the fixture trap weir and the vent pipe is 3.5 feet. This short distance ensures the trap arm slope does not allow the trap seal to siphon, which would permit sewer gases to enter the building.
Why the Other Options Are Wrong
Option A: 6 feet
6 feet exceeds the code limit for 1.5-inch pipe. A 6-foot trap arm on a 1.5-inch drain would create excessive slope or allow sufficient flow velocity to siphon the trap seal. The 6-foot distance is associated with larger pipe sizes such as 2-inch pipe.
Option B: 8 feet
8 feet is the maximum trap arm distance for 3-inch drain pipes, not 1.5-inch pipes. Using the wrong pipe-size row from Table 1002.1 leads to this error.
Option C: 5 feet
5 feet is the maximum distance for 2-inch drain pipes, not 1.5-inch. This is a common off-by-one pipe-size error.
Memory Technique
Remember the sequence: 1.25" → 2.5 ft, 1.5" → 3.5 ft, 2" → 5 ft, 3" → 6 ft. Notice the pattern: each step up in pipe size adds roughly 1-1.5 feet of allowable trap arm length. For 1.5-inch pipe, think '3.5 feet — the number 3.5 has a 1.5 in it.'
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