Configuration of three-level distribution box for construction site

A three-level distribution box system on a construction site consists of a primary (main) distribution box, secondary (sub) distribution boxes, and tertiary (switch) boxes, arranged hierarchically to ensure safe, balanced, and efficient temporary power distribution.

Hierarchical Structure

1. Primary Distribution Box (Level I)
   • Serves as the main hub, receiving three-phase power, ground, and neutral from the transformer or external supply .
   • Equipped with main circuit isolation, main circuit breaker, main fuse, and shunt isolation with leakage protection .
   • Features a copper busbar for reliable connections and a metering system for monitoring .
   • Typically installed near the power source to facilitate power introduction .
2. Secondary Distribution Box (Level II)
   • Receives power from the primary box and redistributes it to specific construction areas .
   • Configured with main circuit general isolation, fuses, circuit breakers, and shunt isolation with additional protection .
   • Designed with inner and outer doors, rainproof tops, and spray-painted exteriors for safety and durability .
   • Positioned centrally within areas of concentrated electrical equipment to maintain three-phase load balance .
3. Tertiary Distribution Box / Switch Box (Level III)
   • Located under the secondary box, directly controlling individual electrical equipment or small groups of devices .
   • Equipped with isolation and leakage protection switches to ensure two-level protection across the system .
   • Installed as close as possible to the equipment it serves, typically within 3 meters horizontally .
   • Each electrical device should have its dedicated switch box, following the principle of "one machine, one switch, one leakage, one box, one lock" .

Installation and Safety Considerations

• The distance between distribution boxes and switch boxes should not exceed 30 meters .
• Boxes should be made of cold-rolled steel (1.5–2.0 mm for distribution boxes, ≥1.2 mm for switch boxes) and treated for corrosion resistance, rainproofing, and dustproofing .
• Load capacity of a single distribution box typically ranges from 50–150 kW, with main switch ratings determined by the number of lower-level boxes and equipment power consumption .
• Ensure separate circuits for power and lighting, maintain three-phase load balance, and clearly label all switches and boxes .
• Fixed boxes should be fenced and secured, and power should be cut off and locked when not in use .

Summary

The three-level distribution box system ensures safe, organized, and efficient temporary power distribution on construction sites. The primary box handles incoming power, secondary boxes distribute it to concentrated areas, and tertiary boxes control individual equipment with proper protection. Compliance with national standards, proper placement, and adherence to safety principles are essential for reliable operation .