Current carrying capacity of cables in cable trays

The ampacity of cables in cable trays is generally lower than in free air due to restricted airflow, mutual heating, and tray material effects, requiring derating according to standards like NEC 392.80 and IEC 60287.

1. Heat Dissipation and Tray Type: Cables in trays experience reduced convective cooling, especially in solid-bottom or covered trays, which restrict airflow. Open ladder trays provide the best ventilation, while ventilated troughs and solid-bottom trays progressively reduce heat dissipation, lowering ampacity .

2. Cable Grouping and Mutual Heating: Closely spaced cables generate mutual heating, which increases conductor temperature and reduces current-carrying capacity. Multiconductor cables in bundles require adjustment factors to account for this effect .

3. Proximity and Electromagnetic Effects: Metal trays can induce eddy currents and proximity effects, slightly increasing conductor resistance and further reducing ampacity .

4. Ambient Temperature: Ampacity tables assume a standard ambient temperature (typically 30°C or 86°F). Higher ambient temperatures require derating using correction factors specified in NEC 310.15(C)(1) or IEC 60287 .

NEC 392.80 provides rules for cables rated 2000V or less:

• Multiconductor cables: Start with ampacity from Table 310.16 and apply adjustment factors for more than three current-carrying conductors within the cable. The number of conductors in the tray does not affect this adjustment .
• Single conductors: Free-air ampacities from Table 310.17 can be used if spacing and single-layer installation conditions are met .
• Covered trays: If a tray is continuously covered for more than 6 feet, ampacity is limited to 95% of the table values .

IEC 60287 provides analytical and numerical methods to calculate ampacity, considering thermal resistance, cable grouping, and tray geometry. IEC 61537 also guides tray sizing based on mechanical strength, ventilation, and fill capacity .

• Tray Fill: Maintain recommended fill ratios to avoid excessive heat buildup. Open ladder trays allow higher fill percentages than solid-bottom trays .
• Separation: Keep power and signal cables separated to prevent interference and additional heating. For VFDs, maintain at least 12 inches vertical separation or use shielded cables .
• Temperature Derating: Apply ambient temperature correction factors and consider local environmental conditions.
• Installation: Ensure proper spacing, avoid tight bundling, and follow manufacturer and code recommendations for tray type and cable arrangement.

By considering these factors and applying NEC or IEC guidelines, designers can determine the safe current-carrying capacity of cables in cable trays, ensuring both safety and compliance.