The cables run in ladder indoors, on passing to outdoors it runs above ground in conduit. When it runs indoors, there are other cables (9 cables) laid in a single layer along with it, on passing to outdoors it runs alone. It runs in same length in both indoors and outdoors.
How do you select the derating factor for this? I could not find any derating factor in IEC standard for this situation.
You should consider the worst situation
If Derrated ampacity on conduit > Derrated ampacity on ladder, use first value, if not, use the other.
Above is a snippet.