The distance between a secondary distribution box and the main distribution box depends on voltage drop, cable type, accessibility, and code requirements, typically ranging from a few meters to tens of meters in low-voltage installations.

Key Considerations

1. Electrical Code and Safety Requirements The National Electrical Code (NEC) specifies that panelboards must have adequate working space: at least 30 inches wide, 36 inches deep, and 6.5 feet high in front of the panel, with clear access for maintenance and operation (NEC 110.26) . This ensures that the secondary distribution box is installed in a location that is safely reachable from the main distribution box. 2. Voltage Drop and Cable Length Secondary distribution boxes are fed from the low-voltage side of a transformer or main distribution panel. To maintain proper voltage levels, the distance should be minimized to reduce voltage drop. For typical low-voltage systems (120/240 V or 230/400 V), distances of up to 30–50 meters are common in residential or commercial buildings, but longer runs may require larger conductor sizes to compensate for voltage drop . 3. Installation Layout

• Primary distribution box: Main panel located in a central distribution room or electrical room, often with bottom entry/exit wiring and copper busbars .
• Secondary distribution box: Sub-panel supplying a specific area, floor, or building section. It can be located in a mechanical room, corridor, or near the load center .
• Tertiary distribution: Final connection to individual devices or units, typically very close to the load. 4. Practical Guidelines
• Keep secondary boxes as close as possible to the load to reduce cable length and energy loss.
• Ensure the route between main and secondary boxes avoids obstacles and allows for future maintenance.
• Use proper cable sizing and protective devices (MCBs, RCDs, RCBOs) according to the load and distance .
• In multi-story buildings, secondary panels are often installed on each floor, fed from the main panel via riser cables. 5. Typical Distances
• Residential buildings: 5–20 meters from the main panel to sub-panels.
• Commercial buildings: 10–50 meters depending on floor layout and load distribution.
• Industrial sites: Can exceed 50 meters if large conductors are used and voltage drop is accounted for.

Summary

The distance between a secondary distribution box and the main distribution box is not fixed but should be determined based on voltage drop, cable sizing, accessibility, and compliance with electrical codes. Proper planning ensures safety, reliability, and efficient power distribution throughout the building or facility .