Derating Cables

[SparkyLarks]

I have an application in the UK where a 3.3kV AWA cable is being installed. The Length is 250m total and most is being installed in a Cable tunnel or outside on tray, but the Last 14m passes through a duct.

The Max current is 560A.( this would assume that 2 normally open points have been closed on the LV system. The normal current Max would be 230A

400sqmm cable when installed in duct has a current rating of 535A. (The rating in the trench is approx 840A x .8)
We have increased the size requirement to 500sqmm.

However my question is a general one. Is there any length of duct at which you would ignore it when cable sizing?

 

[jghrist]

In the USA, the National Electrical Code has an exception if the distance is less than 3 m or 10% of the circuit length, whichever is less.