Construction Standard Three-Level Distribution Box

A standard three-level distribution box system consists of a main distribution box, secondary distribution boxes, and tertiary switch boxes, forming a hierarchical power distribution network with integrated protection at each level.

First-Level Distribution Box (Main Distribution Box)

The first-level distribution box serves as the primary hub of the power system at a construction site. It is typically installed near the incoming power source and receives three-phase power, neutral, and grounding wires from the transformer or main supply ( ). Key features include:

• Bottom-in/bottom-out wiring for easy cable entry and exit ( )
• Copper busbar connections for reliable current distribution
• Internal metering and protection devices, such as main circuit breakers, main leakage protectors, voltmeters, and ammeters ( )
• Outdoor suitability, with rainproof tops and robust enclosures
• Centralized control, managing power distribution to all secondary boxes The first-level box ensures one machine, one switch, and one protection, forming the first layer of safety and control ( ).

Second-Level Distribution Box (Sub-Distribution Box)

The second-level distribution box is installed in specific construction areas or near large electrical equipment. Its main function is to receive power from the first-level box and redistribute it to tertiary boxes or branch circuits ( ). Construction features include:

• Double-door design with powder-coated exterior for durability and safety ( )
• Rainproof top for outdoor use
• Branch circuit protection switches and branch leakage protectors to safeguard each circuit
• Load capacity typically between 50–150 kW, depending on the number of tertiary boxes and equipment power requirements ( ) This level ensures two-level protection, combining upstream and branch-level leakage protection ( ).

Third-Level Distribution Box (Tertiary or Switch Box)

The third-level distribution box is the final junction point, directly supplying power to individual electrical equipment or groups of similar devices ( ). Key construction and operational points:

• Contains circuit breakers, isolation switches, and residual current devices (RCDs) for direct control and protection
• One switch box per equipment or equipment group to prevent multiple devices from being controlled by a single switch ( )
• Can be fixed or movable, depending on the equipment layout
• Ensures the last layer of protection, with the leakage protection switch installed at this level

Installation and Layout Considerations

• Tree-like network: Power flows from the main box to secondary boxes, then to tertiary boxes, forming a hierarchical distribution network ( )
• Proximity: Distribution boxes should be located within 30 meters of concentrated electrical equipment ( )
• Outdoor and temporary site suitability: All boxes should have weatherproof enclosures and robust construction for field conditions ( )
• Load planning: Box selection and protection devices should be based on the project's power consumption plan, with adjustments made for large-scale or formal construction projects ( )

Safety Principles

• One machine, one switch, one leakage, one box, one lock: Each electrical device must have independent control and protection ( )
• Two-level protection: Leakage protection is installed both at the main and branch levels
• Compliance: All boxes must meet construction electricity standards and relevant safety codes ( ) This three-level distribution system ensures efficient power management, safety, and flexibility for construction sites, accommodating both fixed and temporary electrical equipment.