P trap size

I am installing a new tub. The sewer line and hookup is new also... The waste and overflow items are joined together by a 1 1/2 inch San Tee. The trap is positioned directly beneath... The trap arm (horizontal piece after the P trap) will be connected to a nearby 2 inch comby which is part of the main 4 inch drainage line. I am assuming it would be a good idea to increase the drain size from 1 1/2 to 2 inches BEFORE the trap, rather than increasing the pipe size in the trap arm. Correct? Thought I'd better ask just in case there's some wrinkle to this I would never have anticipated. Thank you in advance!

I think you've got the right idea. One thing you should never do is to decrease the size of a drain, you should either stay the same or increase as you go toward the main drain.

In my area of NJ shower traps are required to have 2" drains and traps. Is yr tub drain 2"?

Prashster,

I was sort of surprised to see that the waste, overflow, drain pop up mechanism came equipped with 1 1/2 inch fittings. Also included was a 1 1/2 inch santee. The brand is Geberit. This all fits onto a Kohler Villager. This is all fine, I guess, except that I configured the comby to be 2 inch... So it's just a matter of where I place the fitting that expands the pipe size. Sounds like my plan to expand the pipe size above the trap will work.

Thanks for your input.

it is normal for Geberit to have made 1.5" pipe; this is the only size for a tub.

Where you increase to 2" will not make a difference, in my opinion. It may be true that the drain will drain more quickly when 2".

I have a Geberit drain assembly, and a 2" P-trap. The pipe gurgles more (when draining) than the previous tub did. Maybe it is because of the wider diameter pipe. Maybe it is the new shape / configuration. I'll never know.

David

A shower requires 2", but your post said this was a tub, no mention of a shower. In the real world, there are many tub/shower combos that have only a 1-1/2" drain and work OK. I'd got to 2" as soon as possible, but you can't change the size of the tub's outlet.