Cable Ampacity - IEC for multiple layers

[cherryg]

IEC 60364-5-52 provides tables of cables ampacity derating factors (in single layer in trays). Highest number of cables is 9 with 6 layers of trays (spaced 300mm or more vertically). Now my query is: Has anybody worked out the derating factors for 2 layers/3 layers/random fill of cables in tray? Why is there a fundamental difference in cable laying conditions between IEC and NEC/NEMA world?
Interestingly, IEC 60364-5-52 Sec. 523.3 says: The appropriate values of current carrying capacity may also be determined as described in IEC 60287 series , or by test, or by calculation using recognized methods. This opens up an window for methods used by research publications or mathematical models.
One thing to be noted: Single layer of power cables results in large number of trays. Now, it is a techno-commercial decision to adopt two or three layers of cables (with larger derating factor) resulting in larger cables, and reduced length of trays.

 

[juleselec]

All of the current ratings and derating factors in IEC 60364-5-52 are calculated based on IEC 60287 (which in itself is based on the Neher-McGrath equations) for the most common cable types and installation conditions found in IEC countries (mainly Europe).

Now if you've got weird configurations that don't fit in with the standard tables, then IEC 60364-5-52 basically tells you to work it out for yourself from IEC 60287. This is basically what software packages such as CYMCAP do.