What is the nominal internal diameter of a fixture branch for a toilet and basin?

The nominal internal diameter of a fixture branch for a toilet and basin is typically 4 inches. This size is standard for the waste lines that serve toilets, providing adequate capacity to handle the volumes of water and waste without risking clogging or inadequate flow. Toilets and basins generate larger amounts of waste that require a more substantial pipe diameter to ensure sewage can flow efficiently to the main sewer line or septic system.

In plumbing systems, a 4-inch diameter is also designed to accommodate the unique flush and drain requirements of a toilet, which needs more diameter than sinks or lavatories that are usually serviced by smaller branches. Using a larger diameter helps in managing backflow or blockages, ensuring the system operates effectively.

Smaller diameters, like 1¼" or 1½", are typically used for lavatory sinks or washing machine drainage rather than for toilets. A 2-inch diameter can sometimes be used for some applications, but it does not meet the typical code requirements for toilet branch drainage, reinforcing why the selection of 4 inches is crucial for adherence to plumbing standards and operational effectiveness.