What is the fixture unit for a 1 1/2" waste pipe?

The correct answer reflects the standard fixture unit allocation for a 1 1/2" waste pipe, which is commonly recognized as 2 fixture units (fu). Fixture units are a way to quantify the probable demand that a plumbing fixture imposes on a drainage system.

In this case, a 1 1/2" waste pipe is typically associated with smaller fixtures, such as sinks or lavatories, which have a lower drainage load compared to larger fixtures. Assigning it 2 fu aligns with established plumbing codes and practices, indicating it can handle the flow rate and usage expected from smaller plumbing fixtures effectively.

Larger waste pipes, indicated by higher fixture unit values, are designated for fixtures that generate greater waste water, such as toilets or commercial drains, which require more significant drainage capacity. This system of measuring helps ensure that plumbing systems are adequately designed to handle various fixture demands without risk of overflow or backup.