A clean-out may be omitted on a horizontal drain line less than ____ in length.

The specification regarding the omission of a clean-out on a horizontal drain line relates to plumbing code requirements that aim to ensure adequate access for cleaning and maintenance of the drainage system. A clean-out is typically needed in drainage lines to provide access for clearing blockages, allowing for maintenance without disassembling the piping.

In many plumbing codes, including the Uniform Plumbing Code and the International Plumbing Code, the requirement to install a clean-out can often be waived if the horizontal run of the drain line is short enough. Specifically, if the horizontal drain line is under 1.50 meters in length, it's generally considered accessible for maintenance without the need for a clean-out. This distance ensures that any potential clogs can be easily reached through adjacent fittings or terminal clean-outs.

Thus, the identification of 1.50 meters as the threshold reflects the balance between practicality and the necessity for maintenance access in plumbing installations. If the horizontal drainage line exceeds this length, installing a clean-out becomes more critical to facilitate proper upkeep and avoid potential blockages that can cause significant issues in the plumbing system.