I am trying to estimate the size for waste water pipes ("Arctic Blue", TIS. 17--2532) for residential use. I have found this useful website giving available standards: http://www.thaipipe.com/products&services/upvcpipe.htm This is my current project:

1. Waste water from two kitchen sinks, one washing machine, and one dishwasher: Would 2" (55mm) be sufficient? What thickness PVC 5, 8.5, or 13.5, mainly outside installation above ground about 8m?
2. Water water from two existing rain pipes 3" feeding into one pipe: Would 4" (55mm) be sufficient? What thickness, partially embedded in concrete floor partially outside above ground, possibly exposed to direct sunlight, about 12m total?

OK, less convoluted.

Would a 4 inch drain pipe be sufficient for taking the rain water from two 3 inch down pipes?

Here an example showing two down pipes (3"); they would connect to a pipe in the ground that would be (4").

We use all 2" pipes leading into 4" mains to take the water to the waste water tank or tanks Remember to vent the lines to keep it flowing to the waste water tank if you haven't done that already. Last build we were short of 1 vent & it sucked. New house has a vent to the sink & another one to the washing machine. Cheap to add at the beggining of the build & hateful if it is forgotten.