National Standard for Construction of Three-Level Distribution Boxes

Three-level distribution boxes on construction sites must comply with "three-level power distribution, two-level leakage protection, one machine one switch, one leakage one box," using certified materials and proper installation distances.

General Requirements

• Purpose: Distribution boxes provide temporary power at construction sites and must ensure safety, reliability, and compliance with national standards .
• Certification: All distribution boxes must pass CCC certification and comply with relevant national standards .
• Materials: Boxes are made of cold-rolled steel plates or flame-retardant insulating materials. Steel plate thickness: 1.5mm for distribution boxes, 1.2mm for switch boxes. Surfaces must be rainproof, dustproof, and anti-corrosion treated .

Three-Level Configuration

1. First-Level (Main Distribution Box)
   • Located near the power supply.
   • Equipped with N-line and PE-line terminal boards; N-line insulated from metal board, PE-line connected to metal board.
   • Must include CDM3T series 160A-630A circuit breakers with DZ20L-T160A-630A leakage switches for overload, overcurrent, short circuit, leakage, lightning, and disconnection protection.
   • Monitoring devices: watt-hour meter, voltmeter, ammeters, current transformers, fuses, and surge protectors.
   • Leakage protection: rated leakage current 75–150mA, action time 0.1–0.2s, delayed action type .
2. Second-Level (Sub-Distribution Boxes)
   • Installed near concentrated electrical loads.
   • Distance from main distribution box: ≤30 meters.
   • Horizontal distance from switch box to controlled equipment: ≤3 meters.
   • Must include appropriate circuit breakers and leakage protection devices.
3. Third-Level (Switch Boxes / Terminal Boxes)
   • Located close to individual equipment or machinery.
   • Each machine should have one switch and one leakage protection device.
   • Ensures localized protection and easy maintenance .

Installation and Maintenance Channels

• Front operation channel width:
  • Single-row or double-row back-to-back: ≥1.5m
  • Double-row face-to-face: ≥2m
• Rear maintenance channel width:
  • Single-row or double-row face-to-face: ≥0.8m
  • Double-row back-to-back: ≥1.5m
• Channels may be reduced by 0.2m in areas with protruding structures .

Safety and Compliance

• All electrical conductors and equipment must be approved and free from recognized hazards .
• Installation must follow OSHA and NEC standards, including proper grounding, insulation, and protection against electric shock, overcurrent, and arc flash .
• Temporary wiring should not exceed 90 days unless extended by code, and GFCIs must be used in damp or wet locations . These standards ensure that construction site power distribution is safe, reliable, and compliant with national regulations, minimizing risks of electrocution, equipment damage, and operational delays.