Kitchen Drain Size. Max is 2 inches. 1.5 is also OK
Drain pipe sizing. Based on what I've learned. How it applies to kitchens with multiple diswashers + faucets. (Instant Hot, filtered drinking water, etc)
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 http://ths.gardenweb.com/forums/load/bath/msg060009418756.html ).
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 http://ths.gardenweb.com/forums/load/plumbing/msg022153292886.html 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 #