Power is allocated to a secondary distribution box by connecting it to the main panel or transformer via a properly rated feeder, ensuring correct voltage, phase alignment, and protective devices.

Understanding the Secondary Distribution Box

A secondary distribution box, also called a sub panel, is a smaller breaker panel that extends power from the main electrical panel to specific areas or circuits within a building . It allows localized control, additional circuit capacity, and easier maintenance without overloading the main panel. Secondary distribution typically operates at low voltage (below 1 kV) and can be single-phase or three-phase depending on the load requirements .

Steps to Allocate Power

1. Determine Load Requirements

   • Calculate the total expected load for the circuits that will be connected to the sub panel. This ensures the feeder cable and breakers are appropriately sized to handle the current without overheating or tripping .

2. Select the Feeder Cable

   • Choose a cable rated for the voltage and current of the sub panel. For three-phase systems, ensure all three phases plus neutral and ground are included. The cable size should comply with local electrical codes and account for voltage drop over distance .

3. Install Protective Devices

   • The sub panel should have its own circuit breakers for each downstream circuit. A main breaker may also be installed in the sub panel to allow complete isolation. The feeder from the main panel should be protected by a breaker sized to the sub panel's capacity .

4. Connect to the Main Panel or Transformer

   • Run the feeder cable from the main panel or secondary transformer to the sub panel. Connect the phases, neutral, and ground to the corresponding bus bars in the sub panel. Ensure proper phase alignment to maintain balanced loads .

5. Separate Neutral and Ground Bars

   • In a sub panel, the neutral and ground bars must be isolated from each other to prevent ground loops and ensure safety. Only the main panel should have the neutral bonded to ground .

6. Label and Test Circuits

   • Clearly label each circuit in the sub panel. After installation, test the panel for correct voltage, proper breaker operation, and grounding continuity before energizing the circuits .

Additional Considerations

• Voltage and Phase Matching: Ensure the sub panel voltage matches the main panel output (e.g., 120/240 V single-phase or 208/480 V three-phase).
• Distance and Voltage Drop: For long feeder runs, consider upsizing the cable to minimize voltage drop.
• Compliance: Follow local electrical codes and standards for safety, grounding, and breaker sizing .
• Future Expansion: Leave space in the sub panel for additional circuits if future load growth is expected.

By following these steps, power can be safely and efficiently allocated to a secondary distribution box, providing reliable electricity to the designated circuits while maintaining protection and compliance with electrical standards.