According to plumbing standards, what is the rule regarding pipe size reduction?

The correct answer emphasizes the importance of maintaining proper flow and pressure within plumbing systems. Reducing pipe size in a plumbing system can lead to increased velocity of the fluid, which can cause turbulence, noise, and potential wear on the pipe over time. The guideline to never reduce pipe size by more than one size ensures that systems can continue to function efficiently without compromising the hydraulic performance.

When pipes are reduced by more than one size, it can lead to issues such as increased friction loss, which negatively affects the system's ability to deliver water effectively. This guideline is rooted in established plumbing codes and practices aimed at optimizing both functionality and longevity of plumbing installations.

The other options do not align with standard plumbing practices. Allowing unrestricted reductions in size or mandating a reduction by at least two sizes would not consider the implications on system performance and could lead to significant problems in water delivery or drainage. Restricting reductions by material type is also not a consistent practice since the need for maintaining flow is a universal principle that transcends material considerations.