ring main question / explanation

[ban-all-sheds]

I don't understand for what reason you would need a 4mm ring final in a standard domestic house?

Because the installation method for the cable would de-rate 2.5mm² to less than 20A?

Or it could be because the designer was concerned not to overload a section of the circuit, but it seems bizarre that anybody who'd put that much thought in wouldn't just go with a 4mm² radial.

And what mcb would be protecting it.

32A, of course - there is no other option.

 

[JohnW2]

I don't understand for what reason you would need a 4mm ring final in a standard domestic house... And what mcb would be protecting it.

32A, of course - there is no other option.

I'm not so sure about that. If one chose to ignore the deemed-to-satisfy provisions of BS7671 and design from first principles, I imagine that if one did all the calculations (with appropriate assumptions about load distribution etc.) one could produce a 'from first principles' design justification for the standard 32A ring final using a cable with a CCC of at least 20A. That being the case, I presume one could undertake the same exercise and justify a 'rating' (MCB) greater than 32A for a 4mm² RFC - one might guess that, with most cable installation methods, 45A might well be justifiable by such calculations.

However, why on earth anyone would do it is another question!

Kind Regards, John.