Double-Banked Cable Trays: What is the ampacity derating factor for NEK 606 RFOU 0.6/1kV 4G120 mm² cables installed in vertically stacked (double-banked) offshore platform configurations?
The ampacity of NEK 606 RFOU 0.6/1kV 4G120 mm² cable in free air at the NEK 606 standard reference condition (45°C ambient, 90°C conductor temperature) is approximately 260 amperes for a single cable installed on an open support structure with unrestricted air circulation. When this same cable is installed in a two-tier (double-banked) configuration where one 4G120 cable sits directly above another with minimal vertical spacing (typical spacing 50–100 mm between outer sheaths), the ampacity of each cable must be derated to approximately 200–215 amperes, representing a derating factor of about 0.78–0.82 or roughly an 18–22% reduction from the free-air rating. When three cables are stacked vertically (three-tier configuration), the derating becomes more severe: the bottom cable sees approximately 0.68–0.72 factor (177–187 A), the middle cable experiences approximately 0.70–0.74 factor (182–192 A), and the top cable maintains approximately 0.80–0.85 factor (208–221 A). The fundamental reason for these derating reductions is that vertically stacked cables cannot dissipate heat as effectively as cables in free air because the upper cables partially shield the lower cables from direct air circulation, creating a thermally interactive system where the waste heat from upper cables warms the ambient environment around lower cables, reducing their cooling effectiveness and thereby their safe operating current capacity.
