Construction site diagram of the three-level power distribution box

A three-level power distribution system on a construction site consists of a main distribution box (first level), sub-distribution boxes (second level), and switch boxes (third level), forming a hierarchical network to safely deliver power to all equipment.

First-

Level Distribution Box (Main Distribution Box)The main distribution box serves as the primary hub of the construction site's power system. It is typically installed near the power source and receives three-phase power, neutral, and ground wires from the transformer. Key features include: Main circuit breaker, main leakage protector, voltmeter, and ammeter for centralized monitoring and controlCopper busbars for reliable main connectionsBottom-in, bottom-out structure for efficient wiringInternal metering system to ensure stable power supply to downstream boxesRainproof and durable design suitable for outdoor construction environments Second-Level Distribution Box (Sub-Distribution Box)The sub-distribution boxes are installed in areas with concentrated power consumption or near large equipment.

They receive power from the main distribution box and redistribute it to the third-level switch boxes. Key aspects include: Branch circuit breakers and branch leakage protectors for each circuitPlastic-sprayed, rainproof enclosures with inner and outer doors for safetyLoad capacity typically between 50–150 kW, depending on the number of connected switch boxes and equipment Third-Level Distribution Box (Switch Box)The switch box is the final level, directly supplying power to individual electrical equipment. Each switch box usually controls one piece of equipment or a group of similar devices.

Features include: Isolation switches, circuit breakers, and leakage protectors for direct control and protectionPlacement close to the equipment, generally within 30 metersEnsures the principle of “one machine, one switch, one protection” to prevent overloading and improve safety Layout PrinciplesThe system forms a tree-like hierarchical network, with the main distribution box at the root, sub-distribution boxes as branches, and switch boxes as leavesDedicated distribution boxes are used for fixed equipment like tower cranes and construction elevators, while temporary facilities may share a single boxTwo-level protection is implemented: leakage protection is installed both at the switch box and at the upper-level distribution box to enhance safetyProper spacing, grounding, and load calculation are essential to prevent overheating and ensure reliable operation This hierarchical layout ensures efficient power distribution, safety, and ease of monitoring across the construction site, while complying with construction power standards and regulations.