Which factor needs to be considered when determining the size of a water supply line?

When determining the size of a water supply line, the fixture served is a crucial factor because different fixtures have varying flow requirements. Each fixture, such as sinks, toilets, or showers, has a specific flow rate measured in gallons per minute (GPM). To ensure that each fixture receives adequate water pressure and flow, the supply line must be sized appropriately based on these requirements.

For example, a shower may require a higher flow rate than a toilet, and if the supply line is too small for the demand of the fixtures it serves, it may lead to inadequate performance, such as low water pressure or even insufficient flow when multiple fixtures are used simultaneously. Hence, while factors like the location of the building, length of the line, and type of valves can influence the overall plumbing system, the actual demand created by the fixtures is the primary consideration when sizing the supply line.