A standard three-level distribution box system consists of a main distribution box (ACB), secondary distribution boxes (MCCBs), and tertiary switch boxes (MCBs/RCBOs) arranged hierarchically for selective protection and reliable power distribution.

First-Level Distribution Box (Main Distribution Box)

The first-level distribution box, also called the main distribution box, is installed near the power source and serves as the central hub of the system. Its main functions include receiving electrical energy from the transformer, distributing it to secondary boxes, and providing centralized monitoring. Typical components include:

• Air Circuit Breaker (ACB) with short-time delay (0.1–0.5 s) for selective coordination with downstream devices
• Main leakage protector for overall safety
• Voltmeters and ammeters for monitoring
• Rated current is usually 1.2–1.5 times the transformer capacity, and the short-circuit breaking capacity exceeds the maximum short-circuit current at the installation point .

Second-Level Distribution Box (Sub-Distribution Box)

The second-level distribution box is installed near large electrical equipment or in power consumption areas. It receives power from the main distribution box and distributes it to tertiary boxes. Key features include:

• Molded-Case Circuit Breakers (MCCBs) for branch circuits
• Branch leakage protectors for safety
• Thermomagnetic MCCBs are used for general loads, while electronic MCCBs are suitable for precise protection and communication requirements
• Selectivity with the upstream ACB must be verified to ensure proper coordination .

Third-Level Distribution Box (Switch Box)

The third-level distribution box, or switch box, is located close to the end-use equipment and provides direct power to devices. Its configuration typically includes:

• Miniature Circuit Breakers (MCBs) mounted on DIN rails
• RCBOs or separate RCDs for leakage protection
• MCB tripping curves are selected based on load type:
  • Type B (3–5 In) for resistive loads and lighting
  • Type C (5–10 In) for mixed loads and socket circuits
  • Type D (10–20 In) for inductive loads such as motors and transformers .

Additional Considerations

• Busbars, neutral bars, and earth bars are arranged to facilitate wiring and ensure safety
• Surge Protective Devices (SPDs) may be installed according to graded lightning protection principles
• Enclosure protection (IP rating, material, UV and corrosion resistance) should comply with local standards
• The system should follow IEC 61439, IEC 60898, IEC 61008/61009, or regional codes for low-voltage distribution . This hierarchical configuration ensures selective protection, efficient power distribution, and safety compliance across residential, commercial, or construction site installations.