According to the California Plumbing Code (CPC), what is the minimum trap seal depth required for plumbing fixtures?
Correct Answer
D) 2 inches
The California Plumbing Code Section 1003.1 requires a minimum trap seal depth of 2 inches for all plumbing fixtures. This water seal prevents sewer gases from entering the building while allowing proper drainage. The maximum trap seal depth is 4 inches to prevent siphoning.
Why This Is the Correct Answer
CPC Section 1003.1 requires a minimum trap seal depth of 2 inches. This water seal blocks sewer gases (including hydrogen sulfide and methane) from entering the living space while still allowing drainage. The maximum of 4 inches prevents siphoning.
Why the Other Options Are Wrong
Option A: 1 inch
1 inch is insufficient to maintain a reliable gas-blocking seal. A 1-inch water seal would evaporate too quickly and could be siphoned away by normal drainage activity, allowing dangerous sewer gases to enter the building.
Option B: 4 inches
4 inches is the maximum allowable trap seal depth, not the minimum. Exceeding 4 inches increases the risk of the seal being siphoned out during high-flow events.
Option C: 3 inches
3 inches falls within the acceptable range (2–4 inches) but is not the minimum. Selecting 3 inches as the minimum would be incorrect because 2 inches is the actual code floor.
Memory Technique
Minimum trap seal = 2 inches = 'two fingers deep to keep gases asleep.' Visualize two fingers submerged in the trap water — just enough to seal off sewer gas.
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