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Kitchen Drain Size. Max is 2 inches. 1.5 is also OK

Posted by davidro1 (My Page) on
Mon, Jun 21, 10 at 13:48

Drain pipe sizing. Based on what I've learned. How it applies to kitchens with multiple diswashers + faucets. (Instant Hot, filtered drinking water, etc)

About plumbing.
Know your groups.
Know your DFU's.
Know your branches.
Know your arms.
What is an arm?
The trap arm is the segment of pipe you see between the P trap and the connection to the vented pipe (in the wall).
The trap arm remains the same size as the kitchen sink drain opening, or it can be increased to one size larger.
A kitchen trap arm can be 1.5" in diameter. 1-1/2" is the Kitchen Sink Drain size.
A kitchen trap arm can be 2" in diameter. It's one size larger than the Kitchen Sink Drain size.
A kitchen trap arm cannot be made larger.

In the wall is the venting. Vented pipes have sizes governed by DWV tables in plumbing codes. Trap arms are not governed by this. The Tables say "not Trap Arms". The Codes say "here are sizes, except for Trap Arms." I've seen people with last-minute confusion because someone just told them the drain size has to be increased. It's a false alarm, because the sometimes larger size of a DWV pipe does not apply to the Trap Arm.

P traps have a "Trap Arm" before reaching the vented portion of the DWV line ("Drain, Waste, Vent" plumbing).

The trap arm in a kitchen is either 1.5" or 2".
Either size is code-O.K. because of the concept of "group" (explained in ).
Although one could make a clear argument for the 2" size, it is not a Code-determined thing.

"Contrary to popular belief an oversize line will clog much faster than an undersized line." in posted by a man who knows a lot about plumbing codes, lazypup, on Wed, Feb 21, 07 at 0:28

HTH, if there are others here who would like to know the same information, and to prevent last-minute confusion if one hears that the drain size has to be increased. It's not true.

Take no action based on having learned this from me here. Find out more elsewhere. Post if you wish to share. Key concepts:
Trap Arm is the key word.
Never increase it beyond 2" diameter.

Here is a link that might be useful: Grouping DFU's reduced total #

Follow-Up Postings:

RE: Kitchen Drain Size. Max is 2 inches. 1.5 is also OK

David, have you looked into the appropriate drain plumbing of dual sinks, each w/ a disposal, and a dishwasher? the DWV will be mid-way between the two sinks.

My reading suggests you want the two trap arms to meet at a wye connector at the DWV. I'm thinking the wye should be a triple, so that the center outlet can be fitted w/ a threaded plug and used as a cleanout. I gather the dishwasher can simply be drained into the D/W drain connection at the nearest disposal, via a high loop. I will check if Portland OR is adamant about air gaps.

I'm trying to avoid the situation where running one disposal causes waste to come up into the other sink.

More Info

Oh, the sinks will be appx 30'' apart.

RE: Kitchen Drain Size. Max is 2 inches. 1.5 is also OK

Advising others is iffy, in an internet forum. Perhaps the precise term needed goes missing, or a key concept is missed. There comes a point when it is better not to play plumber. I know enough to be aware of the limits of my knowledge for my own use. In a public forum I'll advance cautiously. Here we go.

johnliu you are in California. For future reference 2007 California UPC is here
It was manhatten42 who led me to it.
There is nothing in there about a Group as large as the one you just described.
Either your AHJ will consider it all still part of the Group, or not...
My bet is yes, it's going to be do-able as a Group, for the reasons described in the other thread linked to in the OP about Groups.

It can be done with one P trap.
It can be done with two P traps.
If you have two P traps or want two P traps, the connection point fitting to use is very very particular.
It's described in these recent threads.
e.g. Double fixture cross, "back to back fixture fitting", e.g. (R&G Sloane #91491 in ABS)

In the second thread it is explained why not to add a wye horizontally and then connect each sink (with its own P-trap) to the two ends of the wye fitting. If connected that way, then if a draining sink got stoppered, the water draining already in the DWV pipes would siphon the other sink's P trap out, break the seal and allow sewer gases into the house. I hope that explanation meets the post-posting pick-apart scrutiny test.

If your Wye you wrote about was intended to go vertically, it's also a no-go. A Wye would make the trap arm do downhill at 45 degrees, and that is a 3/4 S-trap (a no go). The reason is that you want to ensure your trap arm always remains in the horizontal plane (with a minor slope) until it reaches the vented DWV pipe. Remaining horizontal allows the trap arm to self-vent.

So, no to the Wye connection. An exception is with Venting between the Trap and the Wye, but nobody ever vents first and then "Wye-combines" as it's not efficient use of wall cavity space.

In some situations it is a solution to add an AAV to vent the DWV of a kitchen P trap.

Learn more and hire a plumber if you need two P traps connected in new plumbing. And take no action based on learning anything from any smart person on the internet. Plumbing requires thousands of hours of training and shaping the geometry of new drains is one of the hardest subjects.


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