Which of the following represents a common structural rule in plumbing?

The principle that hydraulic loads must match fixture ratings is fundamental to ensuring that a plumbing system functions correctly and efficiently. Each plumbing fixture has a specific rating that indicates its water flow and discharge capacity, which correlates with the hydraulic load it exerts on the system. By ensuring that the hydraulic load imposed by the fixtures is appropriately matched to the design and capabilities of the piping system, it prevents issues such as flooding, slow drainage, and pressure problems. This alignment is critical for maintaining the necessary flow rates and ensuring the plumbing system operates safely and within code requirements.

In contrast, while cleanouts are typically installed in accessible areas, they do not all necessarily have to be above ground, which can lead to varying practices. Pipes can be made of different materials that work together in a system, depending on their function and the environment in which they are used. Furthermore, vents can be installed both horizontally and vertically, depending on design requirements, so stating that they must be installed horizontally is not accurate. This understanding of fixture ratings and hydraulic loads is vital for effective plumbing design and installation.