Vertical Wet Vent - Why restricted to trap arm?

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Long time reader, thank you for all the great information.

I am curious about section 908.1 Vertical Wet Venting. The section reads,

"Wet venting is limited to vertical drainage piping receiving the discharge from the trap arm of one and two fixture unit fixtures that also serves as a vent not exceeding four fixtures. Wet-vented fixtures shall be within the same story; provided, further, that fixtures with a continuous vent discharging into a wet vent shall be within the same story as the wet-vented fixtures. No wet vent shall exceed 6 feet in developed length"

My local authority is adhering strictly to the "trap arm" limitation.

What is the downside of using a vertical wet vent to receive the discharge of a vented drain, within the same floor? i.e. In the case that the trap arm cannot be longer than X feet, why not vent close to the fixture and then drain to a vertical wet vent?

Thanks for helping me to better understand this restriction.

Cool Blue Harley

One Dumb Plumber
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San Francisco, CA
In the vertical wet vent only trap arms are connected to the wet vent, not waste branch piping. By strictly adhering to the trap arm limitation, and this is very important, the code is effectively limiting the connection to the wet vent section to single fixture connections.
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