Standard enclosure dimensions for secondary distribution boxes

Typical secondary distribution enclosures range from 26 inches deep for smaller conductors to 36×60 inches for larger installations, with variations depending on traffic load and cable size.

Wall-Mounted and Small Enclosures

For light-duty or wall-mounted secondary distribution boxes, typical dimensions follow common manufacturer size families. These enclosures generally range from 200×200×120 mm up to 800×600×300 mm, with intermediate sizes available to accommodate internal components, cable entry, and bend radius requirements . Small plastic or FRP boxes are often used for low-power gear, terminals, or splicing, and may include IP55 to IP66 protection for outdoor use .

Underground and Concrete Enclosures

For underground secondary distribution, enclosures are designed to meet both electrical and mechanical requirements. Non-concrete enclosures are used for incidental loading, while concrete enclosures are required in areas subject to vehicular traffic . Typical dimensions include:

• 26-inch depth: Required for installations with conductors larger than 4/0 .
• 36×60 inches: Common for larger secondary enclosures, installed level with adjustment bolts to meet grade requirements . Enclosures must allow for proper cable layout, minimum bending radius, and duct entry spacing. Conduits are typically stubbed 1.5–2.5 inches from the inside surface of the enclosure, and swedge reducers are used for smaller conduits . Concrete enclosures must comply with ASTM C-857 for full-traffic areas, and covers are often polymer concrete with slip-resistant surfaces .

Sizing Considerations

When selecting an enclosure, consider:

• Internal usable space: Wall thickness, gaskets, and hardware reduce internal dimensions compared to external measurements .
• Future expansion: Allow 20–30% extra space for airflow, cooling, and additional components .
• Traffic load: Concrete enclosures are required for vehicular areas, while non-concrete enclosures suffice for pedestrian or incidental loading .
• Cable management: Ensure adequate space for bending radius, duct entrances, and grouping of conduits to avoid exceeding minimum cable bend limits . By following these guidelines, secondary distribution enclosures can be properly sized to meet both electrical and mechanical requirements, ensuring safe and reliable operation.