The waist tube of the drain outlet for a bathtub shall be not less than _____ inches in diameter.

The waist tube of the drain outlet for a bathtub must have a minimum diameter of 1 1/2 inches to ensure adequate drainage. This size is established based on plumbing codes and standards that account for the volume of water that needs to be discharged efficiently. A diameter of 1 1/2 inches helps in preventing clogs and allows for a smoother flow of water, accommodating the typical output from bathtubs during use. It strikes a balance between being large enough to handle waste and water without causing delays in drainage while not being excessively large that it could complicate installation or require larger connecting plumbing infrastructure. This specification is crucial for maintaining proper plumbing function and hygiene standards within residential and commercial plumbing systems.