A QUICK way to fail a plumbing system is getting your drain and vent sizes wrong.
Now of course you could (and should) scroll through the massive 642 pages of the UPC (Uniform Plumbing Code) and locate Table 702.1. Because in that table, you’ll find the proper p-trap size for each plumbing fixture….you’ll also need to calculate the maximum number of fixture units each drain pipe (and vent pipe) can handle. This can be referenced using Table 703.2.
Or perhaps your plumbing code is the IPC (International Plumbing Code)?
Well, if that’s the case, you’ll need to reference its equally massive code. But instead locate Table 709.1, Table 710.1(1), and Table 710.1(2), to make similar calculations.