3 inch to 4 inch waste line

At what point in the house can the drainage system go from 4 inch main drain down to a 3 inch pipe ?

 

The required pipe size is determined by the number of fixture units draining into the pipe. Most common single-bath, single family homes would not require a 4" drain anywhere. Local plumbing codes may require 4" from the home to the city sewer connection.

 

The way your question is worded, it sounds like you want to reduce the out going drain from 4" to 3". I hope this is how I am reading the question and not what you want to do. (sorry, I'm just an old school teacher)

 

Problem solved, thanks to all who answered.

 

you do not say HOW the problem was solved, but the common answer to the original question would be "you can use 3 inch up to the point where you connect a fourth toilet. Then it has to increase to 4 inch pipe". UNLESS, you live in an area which requires the vent out the roof to be the same size, undimished, as the incoming line, then you need to run 4" all the way.

 

The inspector allowed 3 inch as I only have 2 WC. Increase to 4 inch just before it leaves the building.

 

by code you should be 4" building drain to your 3" stack which remains 3" all the way to open air
2 toilets on 3" if horizontal, 4" downstream of 3rd
3" stack can be up to 6 toilets, after six must be 4" pipe