The isometric below shows a bathroom group (water closet, lavatory, bathtub) draining into a 3-inch horizontal branch. Per the UPC's drainage sizing table and fixture rules, is the 3-inch branch shown appropriately sized, oversized, or undersized for the 6 DFU load?
LAV WC TUB
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1-1/2" 3" 2"
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+------------+------+------+
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3" BRANCH TO STACK
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vNot quite. The answer is B: Appropriately sized — 3 inches is the minimum for a branch carrying a water closet, and 6 DFU is well within a 3-inch drain's capacity
By load alone, 6 DFU would fit a 2-inch horizontal drain (rated 8 DFU), but the 2-inch row excludes water closets, and a water closet's minimum trap and trap arm is 3 inches. So 3 inches is the code minimum for this branch, and it is rated for 35 DFU — far above the 6 DFU load. The pipe is neither undersized nor oversized in a way a plan reviewer would mark.
Reference: UPC 2021 Table 703.2 (note 3); Table 702.1 (Maximum Unit Loading and Maximum Length of Drainage and Vent Piping)
Checked against the official text
Table 703.2 is not supplied at the exam desk and must be memorized. The 2015 edition's grid and note 3 were not read directly; confirm against your 2015 book.