I'm rennovating my kitchen and I'd like to replace some ancient pipes in the process. For a 2 basin kitchen sink, with a garbage disposal, and a dishwasher, what size drain line should I use? I have about 30ft distance and 6 feet of drop to reach the sewer line, just before the trap. The sewer line is 4" cast iron where I will join it. I'd like to use PVC for the new drain line. Thanks in advance for all suggestions. ~Ross

We need to talk about venting...
And what is this trap you are talking about?
Deb
The Pipewench

I would use 2" pvc for the drain and 1 1/2" for the vent.

Oh yes...venting...nearly forgot...the old sink wasn't vented other than the main soil stack that goes up to the roof and that was 20 ft down the drain line.
The house has vertical 4" pipe that goes straight up 3 stories and sticks out of the roof. All drain lines on the first and second floors tie into this pipe at some point. The pipe has an elbow in the basement and then it runs to wall, where it exits to the street. Just before it exits, there is a 180 degree loop that hangs down. I thought this was called the trap. The elbow prevents sewer gas from backing up into the house because there is always water sitting in it. Does this sound like an odd scenario?

Sorry I lost my place. I see in books that there should be seperate vent lines that tie from each fixture all the way back to the main vent. My house doesn't have any of these. Any recommendations?

Call a LICENSED PLUMBER.

That's a pretty simplistic approach isn't it? We could all save a lot of time reading and typing if that was the answer to all technical discussions. I'd like to at least gather as much information as possible before making the call.

Many plumbing codes permit 1 1/2" drain pipe and a 1 1/2" vent. On a kitchen layout such you have I perfer 1 1/2" rather then 2" because you will get a better scouring action through the smaller pipe. Over the years I have seen more 2" kitchen sink drain pipe stoppages compared to 1 1/2" drain pipes.

The Uniform Plumbing Code requires 2" horizontal drain pipe for a sink and or dishwasher. An 1½" vent needs to be within 5'
of the trap, before the drain pipe runs vertically. You need to find out what codes are for your location.

If a 1 1/2" drain line is permitted, it is only for a very short one. Once it does stop up, it is going to be very difficult to snake properly.

It is a simplistic solution, because we cannot "see" your system, so we do not know exactly what you plan to do, whether there are in fact vents on all the fixtures, or anything else about your drains and vents. Trying to describe them can be an exercise in futility, because what you describe we may not interpret properly, and vice versa.

the way i learned it, minimum size pipe and 1/4" per foot pitch. the water moves at 2 feet per second and scours the pipe by running thru in a circular motion.
1 1/2" pipe is all i have ever used for kitchen drains and i have never had a problem. in actuality you could vent the 1 1/2" pipe with 1 1/4" but it costs more than 1 1/2".

It does sound like you have a trap inside your house. Where do you live? Most systems do not have a house trap. My jurisdiction has some areas that are trap districts but they are out at the main.
Getting vents in the wall or furring out around them is a big project that requires some planning. Sometimes getting bids from plumbers helps to understand the scope and alternatives.

I always go 2" for kit line's. And 1 1/2" Vent min.

UPC limits 1-1/2" trap arms to 42" max.
Deb
The Pipewench

Oops. I was just thinking about the 2" drain...