Ring Final Circuit

[steve100]

I have a query about the Standard A1 Ring Circuit.

It is my understanding that a 2.5mm2 cable is sufficient, this is because the current carrying capacity in a worse case scenario is 20A and because they are run in parallel this gives a total current carrying capacity of 40A and so a 32A MCB will protect the cable. It is also my understand that the 20A figure is taken from Table F6 on page 150 of the green OSG, however there is a worse case scenario (reference method 103) which has a current carrying capacity of only 13.5A for a 2.5mm2 cable. So why is 20A used as the worse case, is it because section H2.3 on page 175 of the green OSG states that the current carry capacity for a A1 ring circuit mustn’t be less than 20A if a 2.5mm2 cable is used?

Hypothetically speaking, can I assume that if I was to increase the cable size to 6mm2 that this would meet the regulation in respect of installation method 103? I do appreciate that there would be other factors to consider but just looking at the logic.

Thanks

 

[telectrix]

think you're correct. could just increase to 4mm, though.

 

[steve100]

I did think that but but on page 176 it refers to 20A, so presumed that it was talking about the indavidual current carrying capacity so thought that would then have to be 23.5A from table F6 on page 150, to bring it above 20A, what do you think?

 

[telectrix]

ain't got the green osg, only the red one. it's only ref. 101 and 103 that takes 2.5mm below 20A. i see what you mean , though, with method 103.

 

[steve100]

Yeap I would have thought you could just go up to 4mm2 for method 101, but seems logical to have to up to 6mm2 for 103. The other bit (on page 175 green OSG) is under ring circuits so presume its the same for the red one, as do t have the red one?