What is the maximum number of water closets on a 3-inch horizontal branch drain?

• A. 1
• B. 2
• C. 3
• D. 4

Answer: (B)

The maximum number is two because the size of a horizontal branch drain limits how many large fixtures it can serve while still maintaining proper drainage and venting. A water closet discharges a relatively large volume, and a 3-inch branch has enough capacity to handle two of these fixtures along with any smaller fixtures without risking inadequate venting or trap siphon. Adding a third toilet would push the flow beyond what a 3-inch branch can reliably manage, which is why the code or standard practice sets two as the limit. If more toilets are needed, you’d either upsize the branch to a larger diameter or provide additional branches or venting arrangements to maintain proper drainage.