The maximum discharge capacity for one fixture unit is equal to _____ gpm.

The maximum discharge capacity for one fixture unit being equal to 7 1/2 gallons per minute (gpm) is rooted in the guidelines established by the Uniform Plumbing Code. Fixture units are a method of assessing the demand that various plumbing fixtures exert on a drainage system.

One fixture unit corresponds to a flow rate that reflects a professional consensus on average water usage within a plumbing system over a given period. Specifically, the 7 1/2 gpm rate allows for a standardized measure to ensure that plumbing systems can effectively handle the cumulative flow from multiple fixtures without becoming overloaded or causing backups.

Utilizing this standardized capacity helps in the design and sizing of drainage systems, ensuring they can accommodate the expected flow. This is also essential in plumbing calculations for sizing pipes and determining pump specifications, making it a critical component for plumbing design and installation.

The other options provided do not align with the established rate of fixture units and would generally lead to either overestimating or underestimating the actual flow capacity required for proper plumbing function.