Where is a cross fitting not permitted?

A cross fitting is not permitted in a drainage system primarily because it introduces potential issues related to flow direction and can create problems with drainage performance. In drainage systems, maintaining the correct flow path is crucial to ensure that waste and wastewater move efficiently toward their disposal points without obstruction. The design aims to prevent clogs and the backflow of sewer gases, which could happen with a cross fitting due to its configuration.

In contrast, cross fittings may be used in vent, water supply, and rainwater systems where flow dynamics allow for greater flexibility in direction without compromising the system's overall function. Vent systems often require changes in direction, and water supply systems can handle a variety of configurations due to the pressurized nature of the flow. Similarly, rainwater systems can typically accommodate cross fittings without adversely affecting performance as long as proper design principles are followed.