How is it assumed that the drainage branch flows at design conditions?

The correct approach in plumbing design assumes that the drainage branch flows at design conditions when it is flowing completely full. This assumption is vital for ensuring that the system can effectively handle peak flows and avoid issues such as backing up or insufficient drainage. Designing for full flow allows for accurate calculations of capacity, velocity, and potential friction losses within the drainage system.

Choosing a design flow of half-full, one third full, or completely empty does not accurately reflect the demands on a drainage system during peak usage, where it is essential to account for the maximum anticipated flow. Therefore, the most reliable and safe practice is to design for the scenario where the drainage branch is operating at full capacity. This ensures that the plumbing system can properly function under all expected conditions.