De-rating of Groups of Equally Loaded Parallel Cable Circuits Arranged in Trefoil Touching (Buried)

[NateSparks]

Hi,

Does anyone know how to calculate the de-rating factor for multiple parallel circuits of buried trefoil groups? The situation that I'm investigating is shown in the attached image.

I've been trying to use IEC 60287-2-1 to figure out how to calculate the modified external thermal resistance (T₄), with no luck. Using this standard, two sets of equations come to mind, the first refers to groups of equally loaded buried cables (touching) and the second refers to groups of equally loaded buried cables (not-touching). However, since I'm considered the effects of groups of parallel circuits in trefoil, we have a combination of touching cables (in one trefoil group) and non-touching cables (where the cables of one trefoil group don't touch the cables of another trefoil group). Does anyone have any ideas/ has anyone done this before? I'm looking for a precise calculation method to calculate the modified external thermal resistance (T₄) of a particular cable; to then use in the general ampacity formula for directly buried cables.

Thanks!

 

[7anoter4]

At first what is bedding made from [material] and what is above the concrete slab-air, earth or else. It seems to me between trefoil groups it is prisoner air [or is it connected with open air?]