Temperature rise standard for cables laid on cable trays

Cables in trays experience temperature rise due to mutual heating, limited airflow, and environmental conditions, with center cables often running 8–15°C hotter than edge cables under identical loads.

Factors Affecting Temperature Rise

Mutual Heating: When multiple cables are grouped in a tray, each cable contributes heat to its neighbors. Thermal imaging shows that center cables can be 8–15°C hotter than edge cables, highlighting the limitations of uniform derating factors in standards like IEC 60364-5-52 and AS/NZS 3008.1.1 . Tray Type and Ventilation: Open or ventilated trays allow better heat dissipation, while solid bottom trays restrict airflow, increasing cable temperatures. Solid bottom trays with covers provide EMI/RFI shielding but require careful thermal analysis since standard ampacity tables are limited . Load and Environmental Conditions: Higher current loads, ambient temperature, and poor airflow increase cable temperature. Humid or still air further reduces cooling efficiency . Cable Position and Insulation: Cables in the center of a densely packed tray heat more than those at the edges. Insulation type (e.g., XLPE, PVC) affects thermal resistance and permissible temperature rise .

Calculating Temperature Rise

Standard Derating: Traditional tables assume all cables are equally loaded and experience the same temperature rise. This is conservative for edge cables but may underestimate center cable temperatures . Advanced Thermal Modeling: IEC 60287-2-1:2023 allows calculation of thermal resistance for each cable individually, considering position, surrounding medium, and neighboring heat contributions. This method provides more accurate permissible current ratings but requires detailed modeling . Analog Thermal–Electrical Models: For solid bottom trays, thermal–electrical analog circuits can estimate temperature rise for any number of cables, validated by finite-element simulations .

Mitigation and Monitoring

Ventilation and Heat Dissipation: Use open trays, heat release holes, or layered cable arrangements to improve airflow and reduce mutual heating. Materials with high thermal conductivity, like aluminum, help dissipate heat . Temperature Monitoring: Sensor cable systems can detect localized overheating along trays, enabling early intervention before insulation degradation or fire risk occurs . Design Considerations: Ensure tray width and fill factor allow sufficient spacing, consider future cable additions, and apply derating factors based on cable grouping and position .

Summary

Cables in trays heat up due to mutual heating, limited airflow, and environmental factors, with center cables being most affected. Accurate assessment requires position-specific thermal analysis or monitoring systems. Proper tray design, ventilation, and temperature monitoring are essential to maintain cable longevity, performance, and safety.