Which term refers to the maximum allowed flow of a fixture based on its nominal size?

The term that refers to the maximum allowed flow of a fixture based on its nominal size is known as a "fixture unit." Fixture units are a standardized measure used in plumbing to express the load (in terms of flow) that various plumbing fixtures impose on a drainage system. Each fixture-type has a specific number of fixture units assigned, which correlates to its expected flow capacity.

Fixture units are essential because they help determine the size of pipes needed in a plumbing system, ensuring that the system can adequately handle the maximum expected flow from multiple fixtures without risking overflow or backflow issues. This concept is critical for effective plumbing design and is widely used in codes and standards to ensure systems operate efficiently and safely.

Other terms mentioned, such as drainage area, water pressure, and volume rate, relate to plumbing but serve different purposes. The drainage area refers to the surface or space that collects and directs water. Water pressure pertains to the force exerted by water within the pipes, influencing how effectively water flows out of fixtures. Volume rate describes the quantity of fluid flowing per unit of time but does not directly refer to the maximum allowed flow based on fixture size.