Theorectical discussion on fusing down smaller cables

[JohnW2]

Possibly, but I would say that ", with or without unfused spurs," separated by commas means that the spurs are separate and not being considered.

As I said, I think that it is 'open to debate'

Yet, it only states that while showing 2.5mm² conductors - which could handle two single sockets.

Yes, but only if the CCC were 26A (i.e. Method C) - which is why I was wondering whether (per above) they were talking of a situation in which CCC was permitted to be as low as 20A (as it certainly is for the 'ring' cable).

Yes, but 1.5mm² has a CCC of 20A, anyway.

It does (for Method C) if you look at 4D5 - 4D2A says 19.5A

Kind Regards, John

 

[EFLImpudence]

Yes, but only if the CCC were 26A (i.e. Method C)

Method C is 27A - but 26 or 27 makes no difference anyway.

- which is why I was wondering whether (per above) they were talking of a situation in which CCC was permitted to be as low as 20A (as it certainly is for the 'ring' cable).

Then why show 2.5mm²?

It does (for Method C) if you look at 4D5 - 4D2A says 19.5A

Why look at 4D2A?
Anyway, I am sure you would not quibble about the half an amp when the load is 13A maximum.[/quote]

 

[JohnW2]

Method C is 27A - but 26 or 27 makes no difference anyway.

Apologies - I forget to insert the symbol - it was meant to say "≥26A".

Then why show 2.5mm²?

You're asking the wrong person

However, as one thought ... if one thinks, as I suggested one might, that what 433.1.204 says about cables applies to unfused spurs as well as the ring itself, then not only would the spur cable be permitted to have a CCC as low as 20A, but it would also be required (unless MICC) to have a CCC CSA of at least 2.5mm².

Why look at 4D2A? Anyway, I am sure you would not quibble about the half an amp when the load is 13A maximum.

Quite - as I said, 1.5mm² cable would obviously be more than adequate for one single socket - it was you who commented that it would (if Method C) "have a CCC of 20A, anyway" (even though 13A is all that is needed)!

Kind Regards, John
Edit: typo corrected

 

[EFLImpudence]

but it would also be required (unless MICC) to have a CCC of at least 2.5mm².

You mean CSA.
Yes - another anomoly.

Quite - as I said, 1.5mm² cable would obviously be more than adequate for one single socket - it was you who commented that it would (if Method C) "have a CCC of 20A, anyway" (even though 13A is all that is needed)!

Yes but you said:

as if 1.5mm² was also not allowed because its CCC was inadequate.

 

[JohnW2]

You mean CSA.

I did. Now corrected. Apologies.

Yes - another anomoly.

Indeed - and seemingly a particularly weird anomaly! It's hard to see why they felt the need to impose a minimum CSA as well as a minimum CCC - particularly given that, as we've been discussing, a cable with a lower CSA than their 'minimum' could (if one looks at the appropriate table in BS7671 ) satisfy the CCC requirement!

Yes but you said: .... as if 1.5mm² was also not allowed because its CCC was inadequate.

It was just a comment, with no intended 'implication'.

Kind Regards, John

 

[ericmark]

I think we have two different situations, one where in the control of an ordinary person, and second where in the control of a skilled or instructed person. With type tested distribution units (consumer units) we are to start with limited to a 125 amp supply, in real terms a 100 amp supply, and we have to consider items can be changed without talking to a skilled person. So when a hob is swapped from a 13A to 32A the owner may well simply upgrade a MCB/fuse/RCBO without reference to any paper trail, so if some where along the run an 6 mm² cable has a 2.5 mm² cable in some way spliced to it, then the owner may cause overload to the smaller cable.

However with an industrial premises one would hope documentation would highlight any such problems.

I would say the 3 meter rule is to allow one to supply a board from buss bars with a cable large enough for the board, rather than one large enough to take the whole supply to buss bars. Looking at regulation 433.2.2 in old 17th Edition. The

the position of the protective device has neither branch circuits nor outlets for connection of current-using equipment

leaves some questions, where a board has for example a 32A, 16A, 6A and 6A MCB's is that considered as limiting the input to 60A so it would be OK to feed it with cable able to carry 60A even if from a 100A fuse? Or would the main isolator need to be a 60A trip to ensure if a MCB is changed then you can't have an over load?

As skilled if we went to a board and saw the supply was not man enough we would highlight this, although a ordinary person can't put in a new circuit, as far as I am aware nothing stops them swapping the 6A MCB for a 10A MCB because they have fitted a load of 2" quartz lamps in the kitchen.

So consider a domestic premises similar to my own, clearly marked DNO fuse at 60 amp, the supply goes to a henley block and then to a consumer unit and fuse box, the consumer unit only supplies the old garage which has been turned into a granny flat, the fuse box supplies main house, and one could clearly use 10 mm² cables for both supplies rated we will say at 64A with installation method C.

With two showers, both electric, two cookers, and other loads it would be easy to exceed the 60A incoming supply and rupture the DNO fuse, they could easy replace the fuse with a 100A if the cable to the Consumer unit is 16 mm² or 25 mm² even if the house fuse box only has a 10 mm² supply. Specially since the fuse box hidden in the ceiling. Seeing a 10 way consumer unit one would not expect to find a 12 way fuse box hidden in the ceiling.

One does see Henley blocks feeding extras in old houses, often some thing rather small mother old house until re-wire the Henley block fed a fused isolator which in turn fed the battery charger for the stair lift, no more than 1 amp load. It also fed a fused isolator to the shower the two additions both looked the same until one inspected to see fuse size.

I didn't know which was which, and likely neither did my dad. And there was no paper trail, even when stair lift fitted by council social services.

In the house I am now in, the surveyor who did the house buyers report stated that the fuse box hidden in ceiling that feeds all of the main house was redundant, so we have a paperwork trail to show not used, even when in reality it feeds whole of the main house. My late sisters house was wired side to side, good idea really, as if either ring fails, one can maintain essential stuff like freezers without needing extension leads up or down stairs, and the loop impedance for the two rings is better, and the loading more even. However one does not expect a side to side split, so one can make errors with isolation. If it was a commercial premises I would say very good idea, but in a domestic one has to consider not under the control of a skilled person.

So in premises under the control of skilled persons if it follows BS7671 then no problem, but in premises in control of ordinary persons we should try were we can to do the normal so what the owner expects is actually what they find.

 

[JohnW2]

I think we have two different situations, one where in the control of an ordinary person, and second where in the control of a skilled or instructed person. ....

I'm not so sure about most of this - at least, about the common sense of most of the situation.

For a start, I have never been particularly clear as to what is meant by "under the control of" (skilled or instructed persons). People seem to feel that, say, an electrical installation in a residential property is "under the control of" the occupier (or maybe owner) of that property - but what does that actually mean?

What surely matters is that the installation is, and remains, compliant with BS7671, and that requires adequate competence, knowledge and understanding on the part of whoever designed constructed and tested the existing installation, and of anyone who subsequently works on the installation - neither/none of whom will usually be "in control of the installation" in a common-sense sense.

There are certainly some apparent anomalies. For example, why on earth can a single-phase installation ≤100A with a type-tested CU apparently not enjoy the 'dispensation' to have devices with lower-rated breaking capacity IF that installation happens to be "under the control" of a "skilled or instructed person"? In a sensible world, one would think that such a person could 'change hats' and say that they were also an "ordinary person" - but the BS7671 definition of an "ordinary person" appears to preclude that!

... So in premises under the control of skilled persons if it follows BS7671 then no problem, but in premises in control of ordinary persons we should try were we can to do the normal so what the owner expects is actually what they find.

As above, what matters is that an installation is, and remains, compliant with BS7671 (even if some aspects are 'uncommon'), and it is such a situation ('uncommon', but compliant) that we have been talking about in this thread. I certainly agree that anything 'uncommon' (as well, ideally, as everything else!) should be adequately documented, but I'm not convinced that that necessarily has much to do with who is "in control" of the installation, whatever that means. I would imagine that a pretty high proportion of commercial, and even many 'light industrial', installations relate to businesses that do not have a resident in-house electrician, so who is 'in control of' those installations?

Kind Regards, John