Which type of elbows are prohibited for joining sanitary drainage systems of 4" or less?

When considering the requirements for joining sanitary drainage systems that are 4" or less in diameter, the prohibition of 90-degree elbows with a centre-line radius less than the pipe size is based on the need to maintain optimal flow conditions within the drainage system.

Using such elbows can create excessive turbulence and flow resistance. Essentially, a radius that is less than the pipe size can lead to increased risk of blockages and hinder the smooth transfer of wastewater, which is critical for maintaining sanitary conditions. The design of drainage systems aims to minimize sharp turns and abrupt changes in direction, which can significantly impact the efficiency of wastewater removal and result in potential clogs or overflow issues.

In contrast, a 90-degree elbow with a centre-line radius greater than the pipe size—being more gradual—allows for a smoother transition that can reduce turbulence and promote better flow characteristics. Similarly, 45-degree elbows generally present less resistance than 90-degree elbows, and concrete elbows, while not as commonly used, have their own design guidelines tailored to specific applications but do not directly relate to the issues concerning radius in this context.

Therefore, by prohibiting 90-degree elbows with a tighter radius than the pipe size, the code aims to ensure effective drainage performance and reduce the likelihood