What is the appropriate liquid seal measurement for a plumbing trap?

In plumbing systems, the liquid seal in a trap is important for preventing sewer gases from entering a building while allowing wastewater to flow through. The standard measurement for the liquid seal in a plumbing trap is commonly found within the range of 2 inches to 4 inches. This range is effective because it ensures that there is enough liquid volume to create a barrier against gases without obstructing the flow of wastewater.

A liquid seal of 2 inches is generally considered the minimum required to maintain an effective barrier, while a seal of 4 inches is often used in larger traps or special applications where greater protection is necessary. This is why the correct choice includes both 2 inches and 4 inches; it reflects the variability in standard practices for different applications and types of plumbing fixtures. Understanding this concept is crucial for plumbers to ensure proper system function and safety when installing or maintaining plumbing traps.