What is the minimum size cleanout allowed for a 4 inch drain line?

For a 4-inch drain line, the minimum cleanout size is determined by plumbing codes which stipulate that the cleanout should be at least as large as the pipe it serves. In the case of a 4-inch drain line, the cleanout must be sized to accommodate blockages that could occur in the same diameter as the drain line.

The correct minimum cleanout size of 3-1/2 inches allows for effective access for clearing clogs or blockages in the 4-inch pipe connected to it. It provides sufficient space for a service technician to insert tools and remove any stoppages without compromising the integrity of the plumbing system.

Larger cleanouts can also be installed, and many professionals may choose to go with a cleanout that is the same size as the drain (4 inches) to ensure maximum access for maintenance. However, the plumbing code allows for 3-1/2 inches as the minimum, which is why it is considered the correct answer in this context.