In the isometric below, the 3-inch stack connects into a 4-inch building drain downstream. Is this transition — a smaller pipe connecting into a larger one — generally acceptable?
[3" WASTE STACK]
|
|
CO ----> +---+---+ ----> 4" BUILDING DRAIN --->
(1/4 bend)
Base-of-stack cleanout
Not quite. The answer is B: Yes — drainage piping routinely steps up in size downstream; each segment is simply checked against the sizing table for the load it carries
Drainage systems step up in size moving downstream as more fixtures and branches join the flow. A 3-inch stack draining into a 4-inch building drain is normal: the code sizes each piece of drainage piping from the total fixture units connected to it, so each segment is checked against Table 703.2 for the load it actually carries.
Checked against the 2021 UPC text. Washington's exam is written against the 2015 edition, which we have not read for this section, so confirm it against your 2015 book.
More Washington Journeyman Plumber practice questions
Trade Code Wizard is not affiliated with, endorsed by, or licensed by the International Code Council (ICC), the National Fire Protection Association (NFPA), IAPMO, the ESCO Institute, NATE, HVAC Excellence, NASCLA, NICET, the U.S. Environmental Protection Agency, PSI, Prov, Pearson VUE, or any state or local licensing board. Exam and code names are used only to identify which examination this study material prepares you for.
These are original practice questions written to publicly published exam content outlines. They are not actual exam questions, and they do not reproduce code text. Explanations cite code sections and state requirements in our own words — always verify against the code edition your jurisdiction has adopted, which may differ from the edition cited here.
Passing is not guaranteed. This is study material, not a substitute for the adopted code, for your licensing board's requirements, or for qualified instruction.