Power parameters of the secondary distribution box

A secondary distribution box typically handles low-voltage power, commonly 400/230 V three-phase, and its capacity depends on the connected load, often ranging from a few kVA for residential use to tens of kVA for commercial or industrial applications.

Voltage and Configuration

Secondary distribution boxes, also called sub-distribution panels, receive power from a distribution transformer and supply it to a specific area or equipment . They usually operate at low voltage levels, such as 400 V between phases and 230 V between phase and neutral in a three-phase, four-wire system . Single-phase loads, like domestic appliances, connect between one phase and neutral, while three-phase equipment, such as motors, connects across all three phases .

Typical Power Ratings

The power capacity of a secondary distribution box depends on the transformer rating and the expected load:

• Residential areas: Often supplied by transformers rated 10–50 kVA, with secondary distribution boxes handling a portion of this load .
• Commercial or industrial areas: Secondary boxes may handle tens to hundreds of kVA, especially when supplying motors, HVAC systems, or concentrated loads like shopping malls or high-rise buildings .
• High-reliability networks: In urban centers, secondary spot networks can supply up to 25 MVA through multiple transformers feeding a secondary bus, though this is for specialized high-demand applications .

Components and Protection

Secondary distribution boxes typically include large three-phase circuit breakers to protect the connected loads and ensure safe operation . The exact rating of these breakers is selected based on the total load and the type of equipment served. Proper sizing ensures that the box can handle peak loads without tripping while maintaining safety margins.

Factors Affecting Capacity

The actual power a secondary distribution box can deliver depends on:

• Transformer kVA rating feeding the box
• Number and type of connected loads (residential, commercial, or industrial)
• Voltage level and phase configuration
• Load diversity and expected peak demand
• Protection devices and wiring size In summary, a secondary distribution box is designed to safely distribute low-voltage power to a defined area, with capacities ranging from a few kVA for homes to tens or hundreds of kVA for commercial or industrial applications, depending on the transformer and load requirements .