What is the maximum number of fixture units allowed on a 3-inch-diameter vertical drain stack?

• A. 36 fixture units
• B. 42 fixture units
• C. 48 fixture units
• D. 54 fixture units

Answer: (C)

The maximum number of fixture units allowed on a 3-inch-diameter vertical drain stack is based on plumbing codes that specify how many fixture units can be safely serviced by pipes of different sizes. A 3-inch diameter drain stack is capable of handling a larger volume of drainage due to its increased cross-sectional area compared to smaller sizes.

In this case, the capacity of the 3-inch stack is determined mainly by the potential for drainage flow and the requirements to prevent blockage and ensure an efficient drainage system. The plumbing code typically allows up to 48 fixture units on a 3-inch drain stack, which provides a balance between adequate drainage capacity and the risk of overloading the system.

Understanding the concept of fixture units is essential—each fixture contributes a certain number of units depending on its expected usage and drainage characteristics. By adhering to the maximum limits set in the plumbing code, you can ensure that the drainage system functions effectively without risk of clogs or backups.

Thus, the correct answer is that a 3-inch vertical drain stack can accommodate a maximum of 48 fixture units.