Graham, thank you for your thought-provoking thread, but I am bound to wonder why you were paddling up that long river in Brasil. 

For some reason, I am now visualising somebody poking both the live conductors of some T&E into the same ferrule. 

It does seem to be an odd way of expressing the size: I think that it is perfectly reasonable to assume that, "twin 2.5 mm²" means that the capacity is 5.0 mm². And it is no use going up one or two sizes to find one which will fit because only the most tiddly cable sizes double up: 0.5/1.0 and 0.75/1.5. Then again, I suppose that you could combine one 1.5 mm² cable with another 1.0 mm² one and so on.

I think plain and simple, ferrules are ONLY suitable for stranded conductors Class 2, 5 and 6, as clearly stated in DIN 46228-4 (insulated entry type) or DIN 46228-1 (non-insulated type).

Which is why the standard has the tube dimensions for each set of wire sizes.

BUT

Twin-entry ferrules, whilst permitted by manufacturers of specialist terminals (e.g. Weidmüller) are not specifically covered in the Table of preferred sizes in DIN 46228-4.

Hence, the tubes for twin-entry ferrules are one (either the next size above or two sizes above) of the single-entry tubes (it's the insulating collar that's differently dimensioned, of course, to an oblong shape, because of the 2 x diameter issue I explained above). For example 2x2.5 uses the 1x4.0 tube sizes.

gkenyon said:

Hence, the tubes for twin-entry ferrules are one (either the next size above or two sizes above) of the single-entry tubes (it's the insulating collar that's differently dimensioned, of course, to an oblong shape, because of the 2 x diameter issue I explained above). For example 2x2.5 uses the 1x4.0 tube sizes.

I thought about that - the manufacturers would stick with a set range of sizes of metal tubes and then fix single or double entry plastic bits. However, if they can make different plastic bits, there is no reason why they cannot make different metal bits.

All in all, I don't think we are talking about power transmission, but more likely, control cables.

Chris Pearson said:

All in all, I don't think we are talking about power transmission, but more likely, control cables.

In Machinery (BS EN IEC 60204-1) and Switchgear and Controlgear Assemblies (BS EN IEC 41649 series), it's both power and auxiliary/control circuits.

I'm also seeing ferrules used more and more for Class 2, 5 and 6 conductors in electrical installation practice across the board. I have to admit, I do like them for stranded conductors in 'fixed installation', especially Class 5 and Class 6 (flex and extra-flex) but as you'll know most of my working life hasn't been domestic and definitely includes experience with control panels and some more highly regulated environments.

I replaced an outdoor light switch in the garage at home, which had failed. My garage is all singles in plastic conduit and trunking, and of course the "same old" bits of strand dropped out when I undid the light switch terminals - so I fitted ferrules to help maintenance next time the switch needs to be replaced. I always use uninsulated ones when wiring plugs and trailing sockets-outlets too when I can these days.

Chris Pearson said:

Then again, I suppose that you could combine one 1.5 mm² cable with another 1.0 mm² one and so on

Yes, seen this, but not sure it's been strictly considered, and would wonder whether ferrule manufacturers would warrant it'd be OK