A sanitary rough-in for a floor-mounted water closet is coordinated with the underground plumber. Which minimum drain size is commonly used for the water closet branch?
Correct Answer
C) 3 in.
A water closet branch drain is commonly at least 3 in. Other choices are typical smaller fixture trap or drain sizes, while 4 in. may be used but is not the common minimum stated here.
Why This Is the Correct Answer
A water closet branch drain is commonly at least 3 in. Other choices are typical smaller fixture trap or drain sizes, while 4 in. may be used but is not the common minimum stated here.
Why the Other Options Are Wrong
Option A: 1-1/2 in.
In mechanical and plumbing, 1-1/2 in. is the wrong selection for Water closet drain size: 1-1/2 in. is a different drainage slope than the pipe size in the item; the accepted slope is 3 in.
Option B: 2 in.
The trap in B is 2 in. That selection is wrong because 2 in. is a different drainage slope than the pipe size in the item; the accepted slope is 3 in.
Option D: 4 in. only
Choice D (4 in. only) is rejected because 4 in. only is a different drainage slope than the pipe size in the item; the accepted slope is 3 in. The cited answer is 3 in.
Memory Technique
Treat water closet drain size like a labeled shelf; only the matching condition belongs with 3 in.
Reference Hint
Open Pipe and Excavation Contracting, 2011; use the index entry or section for "Plumbing fixture drain sizing" to confirm that A water closet branch drain is commonly at least 3 in.
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