you can use the adiabatic equation to determine the minimum CPC size

1.5mm CPC in twin and earth has been acceptable for a good while now.

So your 2.5mm is probably oversized.

I have heard of same size cpc in Irish twin and earth, with i think insulated 2.5mm CPC. Not sure whether it is recognized by BS7671 though. No disrespect whatsover to the Irish of course.

If you think of it as an increased CPC on one of the legs, does that solve the problem?

I reckon it is the minimum size that is important in a cpc. Providing the min size is adequate in all places then yes fine.

"oversized" does not hinder that. Are we bothered in balances on the ring (or a radial part of it) having a lower impedance? I think not.

I can not see any issue with it.

Likewise with parallel paths, doing more good than harm.

Is its appearance or its measurement likely to mislead anyone? Probably not in practice to anyone with a reasonable amount of competence

In the UK it is fine to construct a ring with either a standard T and E,  which of course has the reduced diameter bare CPC., or equally  a cable with 3 equal cores - no-one would blink if NYY-J or similar was used. A mixture is unusual, but unless the installation fails the test for ring-round' continuity, there is nothing to worry about.

It could be  a concern, perhaps, if both legs of the ring had been extended to a remote peninsula, in a single 4 core and earth cable where the 4 cores were the 2 lives and 2 neutrals. such an arrangement might be unacceptable if loads with high earth leakage were expected, as there is then  not the redundant earthing that the ring architecture provides.

Note that many continental countries have a blanket ban on cables where the earth is smaller than the live, and the Southern Irish have come into line with that, by having their T and E with a full size CPC.  The objection is based on the far end touch voltage during a fault - rather than half the mains voltage as the resistance of the two cores is equal, the voltage the case of the equipment rises to until the ADS kicks in is more like 150-170.  Actually it is particularly noticeable on 10mm.

For two and three conductor cables:

• 1.0 sqmm cable has 1.0 sqmm CPC
• 1.5 sqmm cable has 1.0 sqmm CPC
• 2.5 sqmm cable has 1.5 sqmm CPC
• 4 sqmm cable has 1.5 sqmm CPC
• 6 sqmm cable has 2.5 sqmm CPC
• 10 sqmm cable has 4 sqmm CPC
• 16 sqmm cable has 6 sqmm CPC

Of course that is only a question in a TN system, in TT the voltage rise is higher due to the electrode resistances but the permitted disconnection times are significantly shorter.

Then How do you record this correctly on the schedules of circuit details ? same with the  hybrid ring circuit ?

Personally

"cpc >= 1.5mm" instead of "CPC = 1.5mm"   - i.e. record the 'weakest link / worst case' and then note that some of it is fatter than that - much like cable routes - it may be a mix of open air and through insulation but the rating is set by the through insulation section.

Mike

Hi mapj1 I meant size .Especially when you use software like fastest or Napit. I understand but you have to record what you see. Do you record for r2 1.5mm or 2.5mm??

Not familiar that SW,  but does that not have the option to put 1.5mm with a plus sign or a greater than or equals equals sign ?
If not then put the lower of the two and add a note elsewhere - and if the SW does not allow you to attach notes then it really is no use, and you should send it back !

record the r2 you measure. unless you know the split geometry well enough thta you have the two lengths, then there is not a lot of point in trying to generate an accurate  predicted value. all it needs to be is low enough that when combined the Zs the RCD will trip.

Mike