This fixture serves a variety of purposes such as dish washing, cleaning of food, rinsing out articles of clothing, and several others. As a result, waste water discharges vary in composition, and the pipes drawing off this waste must be prepared to handle maximum drainage problems. A I 1/2"-diameter waste pipe is considered to be the minimum size to use for a kitchen sink, but common practice is to allow a margin of safety by using 2" pipe. The nature of the wastes from this fixture also makes the need for adequate cleanouts apparent.
The problem here is similar to that described above, except that solid waste does not occur in sufficient quantity to require a very large waste pipe. The minimum may be 1 1/4", but 1 1/2" pipe will provide a safety margin.
Since waste from this fixture is mostly water with few solids present, little trouble occurs with its waste piping. A minimum 1 1/2" pipe will suffice for the bathtub in normal circumstances.
Here again you have the same kind of waste as found in the bathtub, but due to the large amount of water released over a short period, a minimum of 2" pipe is suggested.
Common practice has long been to use pipe with a 4" internal diameter, but in recent years 3" copper tubing has been finding increased use.
Washing machine frequently use the same drain as the kitchen sink, especially when they are a combination fixture. If they are in combination, use 2" pipe; if not, 1 1/2" pipe will probably work successfully.