UK Selecting bonding conductor size

[Pretty Mouth]

Thanks for all your replies so far. I did use the adiabatic equation to calculate the earthing conductor size, and I agree that this calculation is somewhat irrelevant when you consider the BS7671 regulations on PME main and bonding cable sizes. (132.16 etc.) Which give a value of 10mm csa for both.
Now this particular installation is a new consumer unit only. The property was wired according to the 14th edition and the requirements then were for the bonding cables to be at 6mm csa according to table D2M. According to the NICEIC, there is an allowance, when working on existing installations wired to a previous edition of the regs, for the bonding cables to be left as they are and not increased to 10mm. This is provided that a) continuity is verified and at an acceptably low value (in this case both are <0.05 Ohms) and b) there is no sign of thermal damage to the cable. There is no sign of overheating of the bonding cables in this instance.
So basically the existing 6mm bonds can be kept in place, with a note on the EIC to confirm that they are only 6mm but have not been subject to thermal damage. This avoids a large amount of disruption running new 10mm cables which aren't in fact needed.

This tallies with guidance in best practice guides 1 and 4. If the main bonding conductors are at least 6mm, and no thermal damage present, then it's not considered potentially dangerous.

However, if they are less than 6mm, then it's considered a C2, potentially dangerous. This is a problem, as in your OP you said the bonding conductors are only 4mm.

 

[westward10]

Have the NIC technical desk actually told you that? Or have you heard it from a 3rd party?

Because as far as I’m aware, you need to upgrade the bonds to the current regs before carrying out any additions or alterations, CU upgrade included.

Now don’t get me wrong, and I’m happy to be corrected, but I can’t for the life of me imagine taking my assessor to look at a job where the earthing & bonding has not been upgraded, and not getting a major non compliance.

As a disclaimer, I’m with NAPIT not the NIC.

I agree for TN-S or TT it may be acceptable but not TN-C-S. BS7671 does not give specific guidance on this except a general note in 132.16 and as such my opinion is 4.0 would not be adequate when consulting this Regulation and has never been so.
And whilst schemes give opinions I am not one to have any faith in them.

 

[MMecca]

This tallies with guidance in best practice guides 1 and 4. If the main bonding conductors are at least 6mm, and no thermal damage present, then it's not considered potentially dangerous.

However, if they are less than 6mm, then it's considered a C2, potentially dangerous. This is a problem, as in your OP you said the bonding conductors are only 4mm.

You're right, I did. On closer inspection they are both 6mm.

 

[MMecca]

Does Table D2M of the 14th Ed actually quote a minimum size of bonding for TN-C-S.

Yes, it's 6mm, subject to consulting with the supplier

 

[MMecca]

I agree for TN-S or TT it may be acceptable but not TN-C-S. BS7671 does not give specific guidance on this except a general note in 132.16 and as such my opinion is 4.0 would not be adequate when consulting this Regulation and has never been so.
And whilst schemes give opinions I am not one to have any faith in them.

Apologies, both bonding cables are 6mm, not 4mm as I first stated.

 

[Pretty Mouth]

Does Table D2M of the 14th Ed actually quote a minimum size of bonding for TN-C-S.

14th edition doesn't give a minimum size for TN-C-S specifically, it's 6mm minimum for all earthing arrangements

 

[westward10]

Yes, it's 6mm, subject to consulting with the supplier

I suspected something like that the 15th Ed cites consulting with the supplier and with this info you have no idea whether 6.0 was actually correct in the first.

 

[MMecca]

I suspected something like that the 15th Ed cites consulting with the supplier and with this info you have no idea whether 6.0 was actually correct in the first.

I suppose there's no reason to suggest that it wasn't done properly in the first place.

 

[westward10]

I suppose there's no reason to suggest that it wasn't done properly in the first place.

You have no way of knowing. What you must do is follow BS7671 and not some random person from the NICEIC.

 

[Aaron b]

Has anyone here seen a bonding conductor thermally damaged? It sounds odd to me.