A pre-reserved outgoing circuit in a distribution box is a dedicated provision for future or specific outgoing feeders, typically protected by MCCBs or cutouts, with proper busbar and cable arrangements.

Concept and Purpose

A pre-reserved outgoing circuit is essentially a slot or provision in a distribution box designed to accommodate an outgoing feeder in the future or for a specific load. This allows for flexibility in electrical distribution without the need to modify the main busbars or the box structure later. It ensures that the distribution system can expand safely while maintaining proper phase sequencing and electrical clearances .

Design Considerations

• Busbars and Droppers: The distribution box should have busbars sized according to the load rating (e.g., 50x8 mm for 63/100 KVA, 50x15 mm for 160/315 KVA) and properly insulated with PVC sleeves for phase identification . Feeders for outgoing circuits are connected via droppers from these busbars.
• Protection Devices: Outgoing circuits are typically protected by MCCBs (Molded Case Circuit Breakers) or porcelain cutouts, depending on the box rating and application. For example, a 100 KVA LT distribution box may use a 4-pole MCCB for the busbar and a 2- or 3-pole MCCB for outgoing feeders .
• Cable Glands and Connections: Heavy-duty, double compression cable glands are provided for both incoming and outgoing cables to ensure secure and safe connections .
• Compliance with Standards: Distribution boxes must comply with IEC 61439 / EN 61439 for low-voltage switchgear assemblies, ensuring safety, proper current-carrying capacity, and temperature rise limits .

Installation and Safety

• Phase Sequence: The outgoing circuit must maintain the correct phase sequence (RYB) to ensure proper operation of connected loads .
• Clearances and Insulation: Adequate spacing between busbars, droppers, and the box body is required to prevent short circuits and overheating. Insulating barriers may be used between MCCBs to manage heat during faults .
• Future Expansion: Pre-reserved circuits allow for easy addition of new feeders without major modifications, reducing downtime and installation costs.

Summary

A pre-reserved outgoing circuit is a planned provision in a distribution box that ensures safe, expandable, and standard-compliant electrical distribution. It involves proper busbar sizing, protective devices like MCCBs or cutouts, secure cable connections, and adherence to IEC/EN standards, enabling future load connections without compromising safety or performance .