When reading a water-sizing isometric under time pressure, what is the most common error that causes an otherwise-correct WSFU calculation to produce the wrong pipe size?
Not quite. The answer is A: Using the correct WSFU total but checking it against the wrong developed-length column, missing that a longer run can force a larger pipe even when the load alone would fit a smaller one
The WSFU addition itself is usually the easy part — the mistake that actually changes the final answer is checking that correct total against the wrong length limit, or skipping the length check entirely. Every pipe size in the table is bounded by both a fixture-unit ceiling and a length ceiling at once, and missing either one produces an undersized pipe on paper even when the arithmetic was never wrong.
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.
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