I'm not really sure what you are asking? It's important to have a trap in the drain that will hold some water and block sewer gasses from re-entering your space
I'm sorry after I posted this ,I realized I asked the question wrong. Redoing the plumbing and piping in my home.got the kitchen sink,hooked up and draining into septic. Now I have the bathroom sink in ,Problem is.... The shower drain is bigger than the bathroom and the kitchen. all flow into the same pipe to the the pipe to the septic. now my question?? should I make all my drain pipe the same size as the shower, to the septic pipe?
The drain lines should always flow to larger pipes as they exit the house. As more pipes mean more flow the main drain should should always be able to handle the volume.
If you had a larger pipe leaving lets say your kitchen sink and it flows into a smaller pipe you risk plugs at the point where the pipe gets smaller. But going from smaller to larger is fine.
Now for proper sizing. You simply do not want to flow for any length of time out of a sink for instance on an over sized pipe. The volume of water that drains out of the sink will not be enough to carry any of the possible solids along with it, The result is bits of soil or in the case of kitchen sink bits of food will separate and begin to stick to the pipe. Over time the drain will become sluggish and being to plug up.
It sounds as though your doing this under the radar without permits. But someday the house will be looked at by a buyer and inspected. So I would suggest you consult with a professional plumber in your area and pay them a few dollars to lay out the pipe sizes for you and show you the proper types of fittings required for sewage systems.
You simply cannot put a Y connection in. It must be set at a special angle and connected correctly. Done wrong, you not only risk drain issues, but the inability to properly snake the pipe in the future if a clog does occur.
If you go to your local big box stores purchase a field guide for plumbing. It is called Code Check. This little booklet is not to expensive, but it will provide you with enough information to at least get an understanding on how to do this properly. http://www.codecheck.com/cc/index.html
Thank You! You have given me a very good education. My kitchen sink is at one end of the house and the rest is at the other. I will get a code check book. We have no inspectors where this house is it's not a town . It is a unorganized territory. A plantation. The house is over 100 Yrs old. It is the family home.I am redoing it from previous people living here and freezing pipes and tearing out rooms. I have the "honor" of redoing the hole house to make it liveable.This is the only problem I have had so far. All of the pipes i have replaced with Pex pipe. Just wanted to be sure on the drainage.