Cables are bundled inside cable trays

Cables are often bundled inside cable trays for organization and support, but bundling reduces their current-carrying capacity due to heat accumulation, requiring ampacity derating.Purpose of Bundling

Cables are bundled in trays primarily for mechanical support, organization, and ease of installation. Bundling keeps cables neatly arranged, prevents sagging, and allows for efficient routing between equipment, cabinets, or across ceilings and walls . Ladder trays, ventilated trays, and solid-bottom trays are commonly used to support these bundles, with ladder trays providing good airflow and mechanical support for long runs .

Thermal Considerations

When cables are bundled, mutual heating occurs because each cable generates heat (I²R losses) that is partially trapped by surrounding cables . This reduces the ability of each cable to dissipate heat compared to a single cable in free air. As a result, the ampacity—the maximum safe current a cable can carry—must be derated to prevent overheating and insulation damage . The IEC 60364-5-52 standard provides correction factors for grouped cables to ensure compliance and long-term reliability .

Best Practices

• Spacing: Where tray width allows, cables should be spaced rather than tightly bundled to improve airflow and reduce thermal penalties .
• Tray Selection: Perforated or ladder trays are preferred for high-current power feeders because they allow better heat dissipation . Solid trays are suitable for low-current or small instrumentation cables .
• Layering: Keeping cables in a single layer minimizes thermal derating. If multiple layers are unavoidable, the derating factor must be applied .
• Support and Protection: Use support blocks or cable ties to maintain spacing and prevent sagging, especially for large power cables .

Summary

Bundling cables inside trays is a practical method for cable management and mechanical support, but it introduces thermal challenges that reduce the safe current-carrying capacity. Proper tray selection, spacing, and adherence to derating standards are essential to maintain safety, performance, and longevity of the cable system .