The minimum size of a supply line shall be ___-inch (9.5 mm) inside diameter nominal pipe.

The minimum size of a supply line being 3/8-inch (9.5 mm) inside diameter nominal pipe is based on the need to ensure adequate water flow for various applications, particularly in residential plumbing systems. Supply lines must be appropriately sized to deliver adequate flow rates without excessive pressure drops, which can lead to insufficient water supply at fixtures.

Choosing 3/8-inch as the minimum size allows for a balance between flow capacity and ease of installation. It is especially relevant in systems where space constraints may prevent the use of larger pipes. For example, fixtures like sinks or faucets often operate effectively with this diameter, minimizing both water velocity and noise or turbulence while still meeting user demand.

Larger diameters would provide more flow but are not always necessary for every application, potentially leading to increased material costs and complexities in installation. Hence, the designation of 3/8-inch as a minimum size reflects the code's emphasis on practical, efficient plumbing design.