What limitation is placed on the use of 90 elbows in sanitary drainage systems?

The use of 90-degree elbows in sanitary drainage systems is restricted by the limitation that they cannot be used to change direction in a way that would create undue stress or create a buildup of waste materials. This is particularly important in drainage systems, where maintaining a smooth flow is crucial to preventing blockages and ensuring effective waste removal.

While elbows are designed to allow for directional changes in piping, using them excessively or inappropriately can lead to issues such as turbulence in flow or accumulation of waste at the bend, which could result in clogs. Therefore, proper guidance indicates that other forms of fitting might be more suitable for such changes, particularly in low-flow areas or where there is potential for sediment buildup.

Considering the context of the other choices, they either misinterpret the application limits of elbows in sanitary drainage systems or impose restrictions that are not typically stated in the code. For instance, elbows can be used both above and below ground, depending on the design and function of the system, and there is no singular restriction based on size or connection to vertical pipes alone. The focus is primarily on how they are used within the system to promote effective drainage.