Architectural drawing of secondary distribution box

A secondary distribution box is a compact assembly housing transformers, low-voltage switchgear, and protective devices, designed to distribute power efficiently to local loads.

Overview

A secondary distribution box (or secondary unit substation) is a close-coupled assembly that includes:

• Primary high-voltage equipment
• Three-phase power transformers
• Enclosed secondary low-voltage switchgear This configuration minimizes the length of secondary distribution cables or busways, improving efficiency and reducing installation costs . The box is typically located near the load center to optimize power delivery.

Typical Layout

The architectural drawing of a secondary distribution box generally includes:

• Primary incoming feeders entering the high-voltage compartment
• Transformers positioned centrally or adjacent to the primary compartment
• Secondary switchboards connected directly to the transformer outputs
• Busways or cable trays for routing low-voltage circuits to downstream loads
• Protective devices such as circuit breakers, fuses, or tie breakers for selective isolation Designs may follow different configurations:
• Simple Radial: Single primary circuit feeding the secondary bus
• Primary Selective Radial: Two primary circuits with switching to limit outage duration
• Secondary Selective: Two independent buses with a tie breaker for continuity during faults

Symbols and Drafting Practices

Electrical construction drawings use standardized symbols to represent components:

• Transformers, breakers, and busbars are depicted using NEMA or ANSI symbols
• Cables and conduits are shown with lines and junction boxes
• Future circuits are indicated with dashed lines
• Drawings are typically to scale, with electrical items drawn darker than architectural elements for clarity

Integration into Architectural Plans

• The secondary distribution box is usually shown on site plans and floor plans
• Dimensions, clearances, and access points are included for maintenance and safety
• Separate plans may be used for power, lighting, and communication systems to avoid clutter

3D Visualization

For practical visualization, 3D models of distribution boxes are available, showing the enclosure, breakers, and internal layout. These models can assist in planning space, cable routing, and installation . By combining these elements, an architectural drawing of a secondary distribution box provides a clear, standardized representation of the electrical distribution system, ensuring safety, efficiency, and compliance with design standards.