Conductor Derating

The standard conductor sizing requirements of the National Electric Code (NEC) is calculated for a maximum of three current-carrying conductors in raceway, cable or earth (directly buried). The derating also applies to situations where single conductors or multi-conductor cables are not installed in raceways are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.). The derating provides a correction factor based on the conductor insulation temperature rating, new ambient conductor temperature, the conductor ampacity and the number of current carrying conductors in a raceway. The calculation base temperature is either 30°C (86°F) or 40°C (104°F).

The National Electric Code (NEC) specifies the minimum size for cable tray systems which includes ladder, ventilated trough, ventilated channel, solid bottom and other similar structures. It’s important to note that cable trays are mechanical support systems and not raceways. Raceways are included in another section of the code.

Proper cable tray sizing is complicated and often existing cable trays are overloaded per the code. If you have questions or would like some engineering assistance to properly size your cable tray installation please contact us.