c2 or is it

[James]

Assuming ref method C for the 4mm...

Can anyone tell me what regulation this doesn't comply with?

I know i am not quoting a reg for you, will be a little while before i am home to open the book.

however, i recall that only one spur is allowed from each point on a ring under the standard definition of an RFC

it is not about the current capacity of the radial part of the circuit so much as the capacity of the terminal at the socket where the spur is made off.

when you think about a ring circuit, any terminal on the ring is going to be subjected to a maximum of 16A on a 32A ring.
if you put a spur on that could carry......(hmmm, thinking)

as i am working this out, i am struggling to find a good technical reason why it is dangerous.

going to look at the book when i get back and see if this is just a case of (we have always done it like that, so if its not like that then it must be wrong)

 

[Julie.]

Appendix 15.

And if the 4mm^2 is downrated to less than 32A it would also breach 433.1 (and be a C2)

Unfortunately I can remember that it was regulations A35 and A40 - in the 14th edition! But you were allowed two outlets on a spur - A40 but not greater than the number of sockets on the ring -A35.

Sad I remember that, but it was a key question in the exams of the day!

 

[Pretty Mouth]

Thanks for the replies. I query this because I cannot find a regulation that it automatically breaks.

Appendix 15 isn't regulation, it's a design guide for ring finals. Sticking to it will ensure compliance, but deviating from it does not AFAIK automatically mean non compliance.

I can see how it increases the likelihood of overloading a leg if the spur point was located close to the board down one leg, so 433.1.204 would be broken. But I'm not sure this would be a problem if the spur point was close to the mid point of the ring. I assume the current would divide itself up between the 2 legs, just as it might if there was a cluster of sockets actually on the ring, situated around the mid point.

The terminals of the sockets must be able to take more than 20A, otherwise a 32A radial supplying sockets wouldn't be allowed.

There is nothing in App 15 or the OSG to say that more that one spur cannot be taken from the same point on the ring (although the OSG mentions limiting the total number of unfused spurs to no more than the number of sockets + fixed equipment on the ring). A similar situation could be created by, eg, spurring 2 double sockets off a double socket on a ring. AFAIK this would be compliant with the design guide, but would still create a heavily loaded point on the ring.

Just to be clear: I'm not looking for an excuse to install this way, I'm just interested in the details of our trade, particularly where ring finals are concerned