For sanitary drainage systems of 4" or less, when are 90 elbows permitted?

The correct answer highlights that 90-degree elbows are permitted for changing directions from horizontal to vertical in the flow direction. This is crucial in sanitary drainage systems, particularly because they help facilitate the necessary flow of wastewater through the pipes and prevent blockages.

In a properly functioning drainage system, maintaining a smooth flow is vital for the effective removal of waste. The use of a 90-degree elbow in this context allows for a seamless transition between the horizontal layout of the pipes and the vertical movement required when connecting to the drainage stack or reaching different levels within a building.

Using elbows in the specified manner aligns with both efficiency and the guidelines set by the National Plumbing Code of Canada, ensuring that the system can effectively handle shifts in direction without increasing the likelihood of clogs or reducing the flow capacity.

Options that suggest restricting the use of 90-degree elbows to only specific connections do not acknowledge the broader function and flexibility needed in sanitary drainage design. This nuanced understanding of application and allowability is key in plumbing practices as laid out by the National Plumbing Code.