No matter the material from which it is constructed, a sewer drain is certainly an item you want to have work correctly. When it comes to sewer drains, bigger does not always mean better. Correct pipe sizing ensures quick flow of sewer waste away from the house.
A standard rule of thumb is that sewer pipes leading away from a toilet are 3 inches in diameter. Sewer drains from laundry sinks or washing machines are 2 inches in diameter and those from sinks in the kitchen, bathroom or powder room generally use a 1.5-inch pipe. The main sewer pipe leading to the septic tank or public sewer is usually 4 inches. It is important to look to the local plumbing code to see if it varies from these standards.
Some people think they can decrease the likelihood of clogs by using larger pipes. This can be a mistake. The water pressure helps solid wastes move through the pipe and large pipes spread that pressure out. So a larger pipe may slow the flow of the drain water and cause waste to pool.
Tim Carter of Ask the Builder recommends a 2-inch sewer drain on a bathtub just as most plumbing codes require on showers. Although code may allow you to get away with a 1.5-inch drain, a 2-inch pipe better accommodates a plumber's snake and prevents clogs from hair.