Well it is fair to say that accurate grouping formulea, rather like the maximum demand calculations, are a bit of a dark art, and the book guidance errs on the side of safe in all cases, but also oversized in many.

However, the real problem is one of cooling, and there is a lot less air on the move in trunking, compared to in a panel.

The assumption that 30% current, which is more like 10% of the heating - I squared R and all that, can be neglected is a bit arbitrary, but works well in practice. But if other cables are de-rated to other fractions, then their heating load can also be estimated.

It may be worth crunching a few worst case probably cases, of cable resistances and power at high loads to see what the total heating is.

I presume you cannot do the obvious and run a bit more trunking along side that which is already there to space things out  better ?

There are cables that can run a lot hotter than PVC and XPLE, and would probably get you out of jail, but they may cost more than the alternatives.

I have seen an occasional 'special' installation where liquid filled cooling tubes are run alongside critical cables, but I suspect that too would blow your budget - maybe the trunking could be fan vented though - and how hot is the ambient, is the normal assumption about the environment actually valid ?

If you did something like that you'd need to know the volume of air on the move and its heat capacity to show if it would be OK or not. (rule of thumb, 1 cubic metre of air, about 30 cubic feet, carries about 700 joules with every degree C - so if you could pump air round at the rate of the typical bathroom extractor of about 1 cubic metre per minute, and it came out 20 degrees hotter than it went in that would be removing 14kJ/min, or about 400-500 watts) But you need to know sharpish if the cooling stops.

Do not tie the cables in tight bundles in the panel ;-)

Mike

This sounds like a new project?

If so how is the person who designed going to sign the Electrical Installation Certificate to certify that they have designed it to BS 7671 when clearly it does not? Also how are the other 2 parities going to sign the Electrical Installation Certificate to say they certify that it has been installed and inspected the installation being compliant with BS 7671?

You ask for a good engineering engineering suggestion in my view would be, there is not one, and anything else would be a bodge. The answer is put it right before any other work is done so it will be compliant with BS 7671.

Not the answer you were hoping for but a truthful one.

JP

I do not think it is quite that strong. You may or may not be able to comply with '7671; you will need to show that the cables will not be damaged by the extra heat due to grouping for cases  where the simple table is not adequate. That would need some rigorous mathematical analysis of the thermal situation, and possibly some test jig measurements to back it up.

In terms of the paper trail, as a non-standard design it would  need to indicate how that is to be accomplished- "see attached analysis doc XYZ for derivation of project specific grouped cable ratings"

Rather like using the ERA tables for current rating instead of the ones in BS7671,  or makers data for some exotic materials but you would be providing the intellectual back up yourself. And that attached doc needs to be able to stand full scrutiny - neck on the block and all that. 

But this is  a lot of work, likely to raise eyebrows, causes trouble at future inspections, and unless there is a really good reason not to  do so, I'd recommend you to be putting some extra trunking in to augment that already purchased and spacing things out so the grouping effect is less onerous and it then meets the more normal way of looking at it.

That said, if you like to be able to refer back to a 'standards approved' approach for cables that are neither loaded  to 100% nor to less than 30% one could adapt the method of BS IEC 60287 for calculating the current rating of groups of  non bundled cables sharing a closed channel . This method calculates an effective increase in ambient temperature from the total power dissipation of all the cables in the channel and and the perimeter length  through which heat is to be lost. The effective increase in ambient temperature is added to the external ambient, and the cable size is then selected on the basis of the de-rating factor for this higher temperature . The semi-empirical equation for the effective increase in ambient temperature, dt in degrees C given in
BS IEC60287-2-1 is

dt = 3*Ptot /perimeter

where: Ptot is the total heat dissipation from all of the cables in watts per metre of length P is perimeter length channel, 2 × width + 2 × depth, in metres the heat dissipation can be calculated from the voltage drop values given in BS 7671. The heat dissipation, per core, is given by I²R, where I is the load current and R is the conductor resistance in ohms per metre length. Take acre with the voltage drop values given in BS 7671 for a single-phase circuit - it equates to 2 × R. as it is for a 'there and back' path.

As it is usually used for mains supply cables in big ducts, I suspect this method is better for very large cables and very large channels, and underestimates the cooling ability of thinner structures - much as a constant current per copper area under estimates cable current carrying capacity. as an aside pre-war MOD designs based on a constant 1000A per square inch oversized cables less than 50A and ran far too hot on larger ones.

Mike.

Thanks Mike - that's helpful information.

In response to John Peckham  - I'm not trying to circumvent the requirements for the certification (which are well understood) but as has been discussed in this forum previously there are parts of BS7671 where the tabulated figures are given for a particular set of circumstances that don't always apply exactly and then some alternative methods may need to be used.  I think this is one of those cases.

I'm sure many of us have also been presented with situations where earlier mistakes by others get discovered way too late and you have to look at creative ways to solve the situation you find yourself in.

Jason.

Thanks Mike - that's helpful information.

In response to John Peckham  - I'm not trying to circumvent the requirements for the certification (which are well understood) but as has been discussed in this forum previously there are parts of BS7671 where the tabulated figures are given for a particular set of circumstances that don't always apply exactly and then some alternative methods may need to be used.  I think this is one of those cases.

I'm sure many of us have also been presented with situations where earlier mistakes by others get discovered way too late and you have to look at creative ways to solve the situation you find yourself in.

Jason.