Explain the procedures used to size drain pipes ?

I can only give you code for my area. Minor fixtures, Lavs, tubs, sinks , washers, etc, =2"
Major fixtures such as a toilet= 3" 2nd. floor and above the slab, 4" under the slab and sewer line to the street or septic tank. Does that answer your question? Cheers, Tom

Code in my area goes by fixture count, Lavs are 1 fixture count, tubs, shower's, kitchen sinks 2 fixture count, toilet 4, and bathroom group consisting of toilet, lav and tub or shower 6 fixture count.3 fixture count total on 1 1/2" pipe, 6 on 2", 20 on 3" and not more than 3 toilets,90 on 4". You can run 3" under slab as long as you don't exceed 20 fixture count. You would have to check on code in your area, as you can see code differs in states, cities and suburbs. Ohio has state code but each city or suburb can upgrade from state code but not lower standards from state code.

Just to clear something up in my prior post, no matter what the fixture count is 3" pipe is the minimun size pipe to receive discharge from a toilet regardless of fixture count.

The inspectors in my area are unhappy with 1 1/2" drain pipes or vents. Cheers, Tom

@ Tom, in this area 1 1/12 waste and vents are required, if that's what code calls for. Inspectors here feel oversizing is as bad as undersizing, for instance they don't feel a lav waste that is 2" will wash out as well as an 1 1/2" waste. In my opinion a national plumbing code would be great, but because of weather and soil conditions in different parts of the country it would be impossible to have. Regards, Puff

Hey Puff,
My area is about to mandate the kitchen drain to increase from 2 to 3 inches. I imagine this is to accommodate a dishwasher plus a garbage disposal. Going on your logic the old 2" drain should be scoured much better then the 3". And if you've ever snaked rotting hair clotted with rancid grease I'll bet you'll agree they ought to increase lav drains to 3" also. (Just kidding! And your thoughts on this! Cheers, Tom

I believe a 2" kitchen sink drain with disposal and diswasher is sufficent, that's what I would want in my house depending on the developed lenght of the waste line, I belive 3" for a kitchen sink is overkill, but I have been wrong before. But by code here you can run 1 1/2" waste to kitchen sink, disposal and dishwasher with no limit on developed lenght, but there is a limit of developed lenght of a vent for pipe size. I have seen many blockages in 1 1/12 kitchen waste lines, and not many inspectors here would argue the point for a 2" waste. Our code here has also been mandated that the horizontal waste to a washing machine box be 3" until it turns to the vertical and then can become 2" with a 2" trap

Our code here has also been mandated that the horizontal waste to a washing machine box be 3" until it turns to the vertical and then can become 2" with a 2" trap Gee! Wonder why they did that. Thanks Puff. Tom