I'm designing a bathroom vanity for the first time and have to figure out the length of the P Trap arm and the distance of the Vent Pipe opening to the sink's drainage hole.

Are there standard lengths for the Trap Arms? What about the P Trap itself? If i'm not mistaken, CA code requires A P Trap with a 1 1/4" opening, but are there variations in length?

The sink's drain hole has nothing to do with distance to the vent(only maximum vertical drop of 24" to the trap). The critical dimension is from the weir of the trap to the vent opening in the drainage piping. The entire length of piping from the weir to the vent is considered the "trap arm". The average length of a tubular trap arm that attaches to the trap is about 7". The minimum/standard size of the lavatory drain tailpiece is 1¼", but often times an 1½" trap and trap arm is used.

table 1002.2 discusses "Horizontal lengths of trap arms"

forget 1 1/4 unless it's for a pedestal sink.

MAX length 1 1/2 trap arm is 3' 6" or 42"

this allows the plumber to put the san tee in adjacent stud bay and "dirty arm" the trap arm in the wall.

most important if a recessed medicine cabinet is a consideration.
BTW, trap to vent distances vary across the country. mine (MA) for 1 1/2 pipe is 60 inches..

Not exceeding a 1/4" per foot.

Thanks for all the information.



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