Does this look right?

[EFLImpudence]

Of course not, but the conductors are, electrically, in parallel, and the authors of BS7671 seem to have decided that it is acceptable to assume that the shorter conductor may be considered to carry not more than 75% of the total current.

Surely that would be non-compliant as the conductor may only be rated for 20A (62.5%).

Wasn't the original rule that it should be 66% and then Methods 100 and 102 'fiddled' to allow 2.5mm²?

 

[JohnW2]

But what is the probability of that happening, and for long enough to cause a problem?

Well, in my hands, the probability is close to zero, because I "know better". However, in my utility room there are a WM and TD, both within 'lead range' of that socket. They are actually supplied by a different circuit but .... !!

As I've said, if the load gets close enough to one end of the ring, the current in the 'other' leg becomes negligible, such that the cable of that other leg might just as well not be there. Your argument could therefore be just as easily used to 'justify' a 2.5mm² radial protected by a 32A OPD. In either case, it is unlikely that the CCC of the cable will be exceeded for a long enough period to "cause a problem", but .... !!

Kind Regards, John

 

[JohnW2]

Surely that would be non-compliant as the conductor may only be rated for 20A (62.5%).

Or 66.7% if its a 30A OPD.

Wasn't the original rule that it should be 66% and then Methods 100 and 102 'fiddled' to allow 2.5mm²?

That's what I've been told.

Kind Regards, John

 

[stillp]

Surely that would be non-compliant as the conductor may only be rated for 20A

But the circuit is rated for 30/32A.

 

[EFLImpudence]

Aculay 66.7% if its a 30A OPD.

Still less than 75%.

Does the 0.725 derating factor for 30A 3036s come into that?

 

[stillp]

Your argument could therefore be just as easily used to 'justify' a 2.5mm² radial protected by a 32A OPD.

Perhaps, but I wouldn't use it to justify that. I'm just asking for consideration of whether the exception "Accessories to BS1363..." to the general rule that the rating of the protective device shall not exceed the CCC of any of the conductors, is a valid reason to assume that only accessories to BS1363 are permitted in a ring final.

In either case, it is unlikely that the CCC of the cable will be exceeded for a long enough period to "cause a problem"

If it were not unlikely, then the circuit fails to conform to the final sentence of 433.1.103 (in the BGB) and therefore is not in accordance with BS7671.

 

[stillp]

That's what I've been told.

By whom?

 

[EFLImpudence]

But the circuit is rated for 30/32A.

It is.

So, the regs. are saying 24A on a 20A conductor is acceptable.

 

[stillp]

It is.

So, the regs. are saying 24A on a 20A conductor is acceptable.

Yup. In a RFC.

 

[EFLImpudence]

A Radial Final Circuit? Surely not.

 

[stillp]

 

[JohnW2]

Still less than 75%.

Indeed.

Does the 0.725 derating factor for 30A 3036s come into that?

Good question. One might expect that is should, but nothing said in 433.1.204 seems to suggest that such is the case in relation to that reg.

Kind Regards, John

 

[JohnW2]

Perhaps, but I wouldn't use it to justify that. I'm just asking for consideration of whether the exception "Accessories to BS1363..." to the general rule that the rating of the protective device shall not exceed the CCC of any of the conductors, is a valid reason to assume that only accessories to BS1363 are permitted in a ring final.

I don't realy understand what you are saying.

If it were not unlikely, then the circuit fails to conform to the final sentence of 433.1.103 (in the BGB) and therefore is not in accordance with BS7671.

I think you need to either tell us what that is in the BYB or quote the sentence in question! I thought you were probably talking about 433.1.1 (BYB), but now I wonder!

Kind Regards,

 

[JohnW2]

By whom?

People in this forum.

Kind Regards, John

 

[stillp]

I don't realy understand what you are saying.
I think you need to either tell us what that is in the BYB or quote the sentence in question! I thought you were probably talking about 433.1.1 (BYB), but now I wonder!

I only have the BGB here. "The sentence is: Such circuits are deemed to meet the requirements of Regulation 433.1.1 if the current-carrying capacity of the cable is not less than 20 A, and if, under the intended conditions of use, the load current in any part of the circuit is unlikely to exceed for long periods the current-carrying capacity of the cable."
(My emphasis)
Edited to correct a silly mistake.