How does the UPC treat the S-trap configuration as a fixture trap?
Not quite. The answer is D: The UPC prohibits S-trap configurations because they self-siphon
The UPC lists S-traps, bell traps and crown-vented traps as prohibited. An S-trap's straight vertical drop below the trap outlet, with no vent close enough to break it, pulls the water seal out behind the falling waste on nearly every discharge — a physical problem that doesn't depend on which code family is in force.
Reference: UPC 2021 §1004.1 (Prohibited traps)
Checked against the official text
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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