The rated power of a secondary distribution box typically ranges from a few kilovolt-amperes (kVA) for residential use up to several hundred kVA for commercial or industrial applications.

Overview

A secondary distribution box (DB box) is a low-voltage assembly that receives power from a distribution transformer and distributes it safely to various circuits within a building or facility . It is designed to handle the load of connected devices while providing protection against overloads and short circuits through circuit breakers, fuses, and residual current devices (RCCBs), .

Typical Ratings

• Residential DB boxes: Usually fed by a distribution transformer rated 10–50 kVA, with the DB box itself designed to handle currents corresponding to 100–200 A at 230/240 V single-phase or 415 V three-phase systems .
• Commercial DB boxes: Often connected to transformers rated 50–200 kVA, supporting multiple three-phase circuits for lighting, HVAC, and machinery .
• Industrial DB boxes: Can be part of a secondary unit substation, with transformers rated up to 500 kVA or more, supplying high-demand equipment and multiple distribution panels . The rated power of the DB box is determined by the transformer capacity, the maximum current rating of the bus bars, and the circuit breakers installed. For example, a DB box with a 100 A main breaker at 415 V three-phase can handle approximately 72 kVA (calculated as 415 V × √3 × 100 A).

Design Considerations

• Voltage level: Secondary DB boxes operate at low voltage, typically 230/240 V single-phase or 400/415 V three-phase .
• Number of phases: Single-phase for small residential loads; three-phase for commercial or industrial loads .
• Load distribution: Proper sizing ensures that each circuit receives adequate power without overloading the DB box.
• Safety devices: Circuit breakers, RCCBs, and isolators protect the system and allow maintenance without interrupting the entire supply .
• Proximity to load: Secondary DB boxes are often located close to the load to minimize cable lengths and voltage drops .

Summary

The rated power of a secondary distribution box depends on the application: residential boxes are typically rated for 10–50 kVA, commercial boxes for 50–200 kVA, and industrial boxes can exceed 500 kVA. The rating is influenced by transformer capacity, bus bar current rating, and protective devices, ensuring safe and reliable power distribution to all connected circuits .