What are the requirements for long spans of cable trays

Long-span cable trays require careful consideration of support spacing, load capacity, splice placement, and cable securing to ensure structural integrity and compliance with NEC standards.

Support Spacing

For long spans, cable trays are typically supported at intervals ranging from 14 to 20 feet, with 20 feet being the most common for outdoor or heavy-duty installations. Extra-long spans may exceed 20 feet, especially for outdoor runs crossing roads or open areas, to reduce the number of supports required. The support span is the distance between structural supports, and the tray length should always be equal to or longer than this span.

Only one splice plate should be placed between supports to maintain structural integrity.

Load Considerations

The tray must be designed to support the weight of the cables plus any environmental loads such as wind, snow, or ice. The distance between supports affects tray strength exponentially, so selecting a tray rated for the intended span and load is critical. Manufacturers provide load capacity charts to determine proper support spacing based on the total cable weight.

Splice Plates and Expansion

Splice plates connect two tray sections and are essential for long spans. Proper placement ensures mechanical stability and electrical grounding. For outdoor or long horizontal runs, expansion splice plates may be required to accommodate thermal expansion and contraction.

Cable Securing and Safety

Cables should be fastened to the tray's transverse members in vertical or angled runs to prevent movement. In horizontal runs, cable weight often keeps them in place, but cable ties may be used to maintain spacing and improve heat dissipation. Ties must be approved for cable tray use to meet fire-safety standards.

Unsupported cable spans from tray to equipment should not exceed 6 feet; otherwise, additional supports are required.

Material and Environmental Considerations

Select tray materials (aluminum, steel, or FRP) based on environmental conditions and load requirements. Aluminum offers excellent corrosion resistance, while steel provides higher mechanical strength. The choice of material affects long-span performance and durability.

Compliance

All installations must comply with NEC Article 392 and manufacturer specifications. This includes maintaining proper clearances (at least 12 inches above trays for access), separation of high- and low-power cables to prevent EMI, and adherence to fill limits (40% for power cables, 50% for control cables),. By following these guidelines, long-span cable tray installations can achieve structural stability, safety, and code compliance while minimizing the number of supports and optimizing installation efficiency.