How to find the inlet dimensions of a distribution box

The inlet diameter of a distribution box typically matches the upstream pipe, commonly ranging from 4 inches (100 mm) for small HDPE boxes to DN50–DN75 (50–75 mm) for PVC or concrete systems.

Standard Inlet Dimensions

• HDPE Distribution Boxes: For 4-inch (100 mm) HDPE single-wall boxes, the inlet is designed to press-fit into a 4-inch smooth wall pipe, with internal or external fittings depending on the box type (regular or hillside). The internal capacity ranges from 7.25 to 8 gallons to overflow .
• PVC or Masonry Boxes: The inlet diameter should match the last section of the upstream pipe. For example, a DN50 (50 mm) pipe is commonly used for the inlet, with larger systems using DN75 (75 mm) if required. A drainage tee is often installed at the inlet to allow cleaning operations .
• Concrete Boxes: Precast concrete boxes, such as the DB200 model, typically use flexible watertight connectors for 100 mm PVC pipes, with one inlet and multiple outlets .

Installation Considerations

• The inlet should be positioned to maintain a minimum fall of 50 mm between the septic tank outlet and the distribution box inlet to ensure proper flow .
• For raised or hillside boxes, the foundation excavation should be at least 60 cm deep to protect the pipe and allow proper compaction .
• All pipes should be installed on the ground or slightly elevated to prevent damage during slab casting, with PVC or HDPE fittings glued or press-fitted as appropriate .

Summary

In general, the inlet diameter is determined by the upstream pipe size, with common dimensions being 4 inches (100 mm) for HDPE boxes, DN50–DN75 for PVC, and 100 mm for concrete boxes. Proper installation includes ensuring correct slope, secure fittings, and protection during construction to maintain system efficiency and longevity .