A secondary distribution box is connected to a primary distribution box to receive stepped-down power, distributing it to specific areas or equipment while the primary box serves as the main distribution point from the transformer.

Functional Relationship

The primary distribution box acts as the main distribution panel, typically located near the transformer or in a central electrical room. It receives low-voltage power from the transformer (commonly 0.4 kV in low-voltage systems) and distributes it to secondary distribution boxes or directly to large loads if necessary . It usually contains isolation switches, circuit breakers, and earth leakage protection devices to ensure safety and control. The secondary distribution box, also called a sub-distribution panel, is connected to the primary distribution box via three-phase or single-phase feeders. Its role is to supply power to a specific area, floor, or building section. It typically contains larger three-phase circuit breakers and may handle motors, machinery, or other high-demand equipment . The secondary box ensures localized control and protection, reducing the load on the primary box and improving safety and reliability.

Typical Connection Setup

1. Feeder Wiring: Power flows from the primary box to the secondary box through insulated cables or busbars. The connection is usually three-phase for industrial or commercial loads, and single-phase for smaller residential areas .
2. Voltage Levels: The primary box receives power at the transformer's low-voltage side (e.g., 0.4 kV), and the secondary box distributes it at the same voltage but to localized circuits .
3. Protection Devices: The primary box protects the entire system with main breakers, while the secondary box provides localized protection with its own breakers and earth leakage devices, ensuring that faults in one area do not affect the entire system .
4. Physical Layout: The primary box is usually larger, centrally located, and may have bottom entry/exit wiring. Secondary boxes are smaller, distributed throughout the site, and may have front or side access for maintenance .

Practical Example

In a residential complex, a 10 kV line feeds a transformer. The transformer steps down the voltage to 0.4 kV, which enters the primary distribution box. From there, feeders connect to secondary distribution boxes in each building. These secondary boxes then supply power to individual floors or units, and finally, tertiary boxes or circuit points deliver power to appliances .

Key Points

• The primary box is the main hub; the secondary box is a sub-hub for localized distribution.
• Connections are made via feeders or busbars, maintaining proper voltage and phase alignment.
• Both boxes include protective devices, but the secondary box focuses on area-specific safety and control.
• This hierarchical setup ensures efficient, safe, and reliable power distribution across large sites or buildings .