Grouping factors for circuits with different conductor CSA

[GaryMo]

I appreciate this isn't really within keeping for the majority of this forum but as there's numerous knowledgeable people here I thought I would ask for opinions.....

Looking at Darrell Lockes 'Guide to the wiring regulations BS7671:2008' he mentions that tables 4C1 to 4C5 are for grouped circuits with conductors of the same CSA.
He goes on to mention that to calculate a grouping factor for conductors of differing sizes we can use:

F = 1/sqroot n

F = group rating factor
n = number of circuits

Using this example, a small industrial workshop wired in 70 degree PVC singles through a 4" galvanised trunking (method B) having a number of welders, pillar drills, ring finals and lighting circuits it wouldn't be a surprise to find 9 circuits (for example) running together in one section of trunking.

Using the above formula a grouping factor of 0.33 is obtained meaning a 32A ring final would require 16mm conductors and 10A lighting circuits would require 6mm conductors. Cable sizes for 50A welding circuits would be a total joke.

Am I missing something here?
Maybe the 30% rule, if so how is that applied to grouped circuits of multiple sizes?

 

[Pensdown]

I maybe reading your post incorrectly but the grouping factors in tables 4C1-4C5 don't apply to single core cables in steel trunking, even if they are the same size?

 

[GaryMo]

I maybe reading your post incorrectly but the grouping factors in tables 4C1-4C5 don't apply to single core cables in steel trunking, even if they are the same size?

See Note 3 for 4C1:

The same factors are applied to:
-groups of two or three single core cables
-multicore cables