Adding two spur sockets to Ring Final

[JohnW2]

Most ?

As I recently wrote, I fear that in the hands of a pedantic person, it could approach 'most' in a domestic installation, since there can't be many cables in such an environment which don't ever come within one-cable's-width of some other cable.

As I said, I would hope that some common sense might be applied.

Kind Regards, John

 

[EFLImpudence]

As I recently wrote, I fear that in the hands of a pedantic person, it could approach 'most' in a domestic installation, since there can't be many cables in such an environment which don't ever come within one-cable's-width of some other cable.

Not sure I understand what you mean.

My "most" indicated that most circuits - with domestic in mind - are method C.

 

[EFLImpudence]

As for the grouping:

Table 4C1
NOTE 9: "If, due to known operating conditions, a cable is expected to carry not more than 30 % of its grouped rating, it may be ignored for the purpose of obtaining the rating factor for the rest of the group.
For example, a group of N loaded cables would normally require a group rating factor of Cg applied to the tabulated It.
However, if M cables in the group carry loads which are not greater than 0.3 CgIt amperes the other cables can be sized by using the group rating factor corresponding to (N-M) cables"

would exclude most, if not all, of domestic cables.

 

[JohnW2]

Not sure I understand what you mean. My "most" indicated that most circuits - with domestic in mind - are method C.

I realise that, but was saying that someone being pedantic might think that most were "Method C with de-rating factors" (i.e. Iz < It). However, I realised that you would probably come back with ...

As for the grouping: Table 4C1 NOTE 9: "If, due to known operating conditions, a cable is expected to carry not more than 30 % of its grouped rating, it may be ignored for the purpose of obtaining the rating factor for the rest of the group....

... but my thinking was that some people ('the pedantic') may well argue that one can never be confident that that condition will be satisfied for any sockets circuit, that it certainly won't be satisfied for things like immersion and shower circuits, and quite possibly would not be satisfied by cooker circuits - seemingly only leaving things like lighting circuits for which one could be 'confident'. In any event, even if one believes that the condition is satisfied ...

... For example, a group of N loaded cables would normally require a group rating factor of Cg applied to the tabulated It. However, if M cables in the group carry loads which are not greater than 0.3 CgIt amperes the other cables can be sized by using the group rating factor corresponding to (N-M) cables...

... If I understand that correctly, it is merely reducing the number of cables that have to be consider as being 'grouped'. If that is the case, then if (N-M) is greater than 1, then some de-rating will still be required, albeit not as much as would be the case if none of the cables satisfied the "30%" criterion. Have I misunderstood?

Kind Regards, John

 

[EFLImpudence]

Looking at Table 4C1; there are various NOTES which allow the derating requirements to be lessened:

Apart from 'bunched' cables (so don't 'bunch' 'em) the maximum derating factors for even twenty cables are 0.70 or a higher factor - i.e. less derating.

As typical lights, immersion and cooker circuit cables are oversized by more than these factors in the first place and most socket circuits generally also lowly loaded, does that only leave the shower cable to consider for its very short time usage?
This can easily be satisfied by (quite small) spacing from adjacent cables or laying with lighting cables.



I have just realised there is no difference in the factor for over nine cables (for 'laying').

 

[JohnW2]

Looking at Table 4C1; there are various NOTES which allow the derating requirements to be lessened: Apart from 'bunched' cables (so don't 'bunch' 'em) the maximum derating factors for even twenty cables are 0.70 or a higher factor - i.e. less derating.

Indeed - it's "bunched" that is the problem - and whilst I agree that "don't bunch 'em" is the simple solution to that, I've seen many a CU in the vicinity of which at least some cables could probably be described as 'bunched' - and sometimes even under floors and in roof spaces, and I've even seen some where (presumably in the name of 'neatness') the 'bunching' has essentially been ensured by cable ties around a group of cables!

As typical lights, immersion and cooker circuit cables are oversized by more than these factors in the first place and most socket circuits generally also lowly loaded, does that only leave the shower cable to consider for its very short time usage? This can easily be satisfied by (quite small) spacing from adjacent cables or laying with lighting cables.

No real argument with that. Sockets circuits are perhaps the most potentially contentious, because of the unknown nature of future loading - which could theoretically be almost up to the limit of the cable (without de-rating). I think that this could potentially be a problem with ring finals since, if the installation was not Method C, a very small amount of de-rating could take the CCC of 2.5mm² T+E down to below the 'minimum permitted' of 20A - and, of course, if one can't use 2.5mm², it's hard to see what point there would be in even considering a ring final.

We're approaching this in different ways, but I think our bottom lines are the same. You are attempting to find ways of saying that there is unlikley to be a regulatory requirement for a problematic degree de-rating (because of grouping) in a domestic installation, whilst I am saying that, regardless of regulations (i.e. even if one was talking about a situation that, strictly speaking, was non-conformant with regs), I think that common sense still results in the same conclusion.

Kind Regards, John