Multiple FCUs

[ban-all-sheds]

The big difference being that with three sockets on the unfused spur, it's easy for somebody to plug 3 x 2kW loads into those sockets resulting in the cable being overloaded. With three fixed loads hard-wired to FCU's, that can't happen. Yes, it contravenes the regs., but from the purely technical viewpoint it's no worse than having a double socket on the spur (considerably "better" in fact).

3 x 13 = __________?

Disconnecting three small loads from FCU's and wiring three 2kW loads in their place would also be an alteration.

Would you say that a BS 7671 certificate was required?

I consider that rather different than hard-wired loads connected to FCU's, since to change those involves making alterations to the fixed wiring.

Changing a fuse is making alterations to fixed wiring?

 

[JohnW2]

Disconnecting three small loads from FCU's and wiring three 2kW loads in their place would also be an alteration.

Would you say that a BS 7671 certificate was required?

That's a bureaucratic question, the answer to which could be either yes or no However, what I think is true is that anyone hard-wiring a 2kW load to an FCU which they happen to find must be competent to ascertain that the circuit is sutable for that purpose (it could, for example, be connected to a lighting circuit) - which might tend one towards a 'yes' answer.

... and, of course, a label saying '3A maximum fuse' might give them a clue, even if they were not fully competent!!

Kind Regards, John.

 

[ban-all-sheds]

This, as you know of old, is where we disagree. As far as I am concerned, Appendix 15 (and, indeed 433.1.5) merely indicate some ways in which one can satisfy the requirements of Chapters 41, 42 & 43 and Part 5 of BS7671 - but the designer remains free to find other ways of satisfying those requirements.

Indeed, but as soon as you deviate from deemed-to-satisfy stipulations or advice then you have to do a lot more design work, and you have to show that your design is OK. That will involve loading patterns, usage modelling etc.

IMO a label on an FCU does not count as good design, and I repeat the point I've now made, and you have studiously ignored, twice - if you have no need so pressing that it cannot be ignored to not have that 4th FCU, does not having one qualify as the best possible design?

433.1.5 say that accessories to BS 1363 may be supplied from a ring final circuit, but I see nothing about 'only accessories to BS 1363'

433.1.5 is a deemed-to-satisfy exception to the rest of 433.1.

It doesn't need to say "only accessories to BS 1363" because they are the only items identified in the exception.

In terms of the regs, you can't make that distinction

Don't try and make it then.

There are designs which are deemed to satisfy Ib ≤ In ≤ Iz.

Your 3 FCUs is not one of them, and does not become one just because you label the FCUs.

Again we disagree. I would regard replacing a hard-wired fixed load with one which had a much higher current demand as being an alteration.

Do you think that an EIC/MEWIC would be required?

... if you would regard replacing a 7.5kW showwr with a 11.5kW one as 'not an alteration' then presumably, to be consistent with your current argument, you would have to require that 7.5kW showers were installed with cable rated adequately to supply a 11.5 kW one.

No, because we're back to Ib ≤ In ≤ Iz.

If the circuit was already able to support an 11.5kW load then you could just install one. If not then the changes necessary would be an alteration.

Do you think it is a good design to make changing fuses in an FCU a notifiable activity?

 

[Paul_C]

3 x 13 = __________?

39, obviously. But what point are you trying to make? If the three loads connected as proposed by John are, say, 120W each, then it will be only 3 x 0.5A = 1.5A.

Would you say that a BS 7671 certificate was required?

For a new fixed appliance hard-wired to the installation, I would say that a minor works certificate could be justified. But whether a certificate is issued or not, it's not going to have any bearing on the technical issues.

I consider that rather different than hard-wired loads connected to FCU's, since to change those involves making alterations to the fixed wiring.

Changing a fuse is making alterations to fixed wiring?

No, but hard-wiring a new fixed appliance to an FCU is.

I know it could be argued that in terms of feeding a fixed appliance there's little difference between it being hard-wired via an FCU and being connected by way of a BS1363 socket installed specifically to feed that appliance. So long as the socket is used only for that purpose, then fine. But to my way of looking at it, the difference comes in that it's easy for anyone to simply unplug that fixed appliance in order to use the socket as a temporary supply for a heater or some other heavier load. To do the same with an FCU requires actually opening up accessories which form part of the fixed installation in order to connect a heavier load. To me, that takes it out of the category of being a simple "unplug that, plug in this" action to one which requires a deliberate alteration to part of the fixed installation, and that puts it more on a level with the 11kW-for-7kW shower replacement scenario.

 

[Paul_C]

Indeed, but as soon as you deviate from deemed-to-satisfy stipulations or advice then you have to do a lot more design work, and you have to show that your design is OK. That will involve loading patterns, usage modelling etc.

I would say that it's pretty clear in terms of being "at least as good as" something which is deemed to be satisfactory. Even if the loads connected to the three proposed FCU's were a full 3A each, that would still be only 9A on the spur. Even with just a single 13A socket on a spur, the connected load could be 13A. With a double socket, it could be higher.

IMO a label on an FCU does not count as good design

How about the warnings on 13A "strips" indicating that although there are four or five sockets the maximum connected load must not exceed 13 amps?

if you have no need so pressing that it cannot be ignored to not have that 4th FCU, does not having one qualify as the best possible design?

If you were aiming for the best possible design, would you be using rings with spurs in the first place, bearing in mind all the limitations they bring with them?

There are designs which are deemed to satisfy Ib ≤ In ≤ Iz.

Your 3 FCUs is not one of them, and does not become one just because you label the FCUs.

Is the maximum connected load less than the current rating deemed to be the maximum for the spur?

 

[JohnW2]

Indeed, but as soon as you deviate from deemed-to-satisfy stipulations or advice then you have to do a lot more design work, and you have to show that your design is OK. That will involve loading patterns, usage modelling etc.

It wouldn't involve much, in this case. 'loading patterns and usage modelling' would essentially consist of a statement of my considered opinion that there was no reasonable expectation that the total load would ever exceed 27A and, indeed, no real expectation that it would ever exceed 12A, even if future people undertook 'alterations'.

There are designs which are deemed to satisfy Ib ≤ In ≤ Iz.

I don't understand how 'deemed to satisfy' comes into it. Dependent on the values of those three quantities, a designed circuit either does or does not satisfy the inequality - there's no 'deeming' involved. Furthermore, that inequality can never be satisfied by a ring final circuits - since, regardles of Ib, In will not be less than Iz. That's why ring finals have to be given special dispensations.

If the circuit was already able to support an 11.5kW load then you could just install one. If not then the changes necessary would be an alteration.

Exactly. Similarly, in relation to the 3 FCUs, If the circuit was already able to support 3 x 13A loads, then you could just install them. If not then the changes necessary would be an alteration.
In both cases, a skilled person is needed to determine whether the existing circuit was able to support the proposed new hard-wired loads, and hence whether any alteraction to the circuit are required before the new loads can be safely connected. What's the difference?

Do you think it is a good design to make changing fuses in an FCU a notifiable activity?

I don't think 'design' is the right word, and I don't think that changing fuses, per se, should be notifiable, but, since it requires skill to determine whether the circuit can support the new load, I do indeed think that there would be a case for the changing of a hard-wired load for one of greater current requirement being notifiable (if anything is to be notifiable!) ....

...the point of mine which everyone keeps ignoring is that the existance of an FCU (or BS1363 socket) does not necessarily indicate that they are connected to circuits which are designed to carry a 13A load. It could (indeed, my 3 could, very reasonably) be fed from a lighting circuit.

Kind Regards, John.

 

[ban-all-sheds]

39, obviously. But what point are you trying to make? If the three loads connected as proposed by John are, say, 120W each, then it will be only 3 x 0.5A = 1.5A.

You can no more enforce a design current for that spur of 9A by putting labels on the FCUs than you could enforce a 9A current for 3 sockets by putting labels on them.

For a new fixed appliance hard-wired to the installation, I would say that a minor works certificate could be justified.

Interesting.

So in your opinion nobody not qualified to issue BS 7671 certificates could change their oven?

But whether a certificate is issued or not, it's not going to have any bearing on the technical issues.

It may have tremendous bearing on whether something is an alteration.

No, but hard-wiring a new fixed appliance to an FCU is.

See above re an oven.

To me, that takes it out of the category of being a simple "unplug that, plug in this" action to one which requires a deliberate alteration to part of the fixed installation, and that puts it more on a level with the 11kW-for-7kW shower replacement scenario.

Does replacing a shower require a BS 7671 certificate?

 

[EFLImpudence]

I thought a certificate should be issued for every job.

Are we confusing certification and notification again?

 

[Paul_C]

Similarly, in relation to the 3 FCUs, If the circuit was already able to support 3 x 13A loads, then you could just install them. If not then the changes necessary would be an alteration.
In both cases, a skilled person is needed to determine whether the existing circuit was able to support the proposed new hard-wired loads, and hence whether any alteraction to the circuit are required before the new loads can be safely connected. What's the difference?

None as far as I can see. In both cases we have a load (or loads) hard-wired to the circuit, the characteristics of that circuit being determined at the time it was installed as meeting the requirements of the connected load. In both cases, we're talking about somebody coming along at some future time and replacing that load with a different, heavier load, which will require the person doing that work to assess whether the existing feed to the point at which the load will be connected is suitable or not. I don't see how the fact that an FCU is involved in one case but not the other makes any difference to that.

...the point of mine which everyone keeps ignoring is that the existance of an FCU (or BS1363 socket) does not necessarily indicate that they are connected to circuits which are designed to carry a 13A load. It could (indeed, my 3 could, very reasonably) be fed from a lighting circuit.

Most certainly they could, although there is the point with such an arrangement that if somebody did connect a large load in place of your original small loads then the MCB or fuse for the lighting circuit should provide adequate protection for the cables, whereas the 30/32A protective device on a ring will not adequately protect the spur running to the FCU's.

 

[ban-all-sheds]

I would say that it's pretty clear in terms of being "at least as good as" something which is deemed to be satisfactory. Even if the loads connected to the three proposed FCU's were a full 3A each, that would still be only 9A on the spur. Even with just a single 13A socket on a spur, the connected load could be 13A. With a double socket, it could be higher.

And with 3 spurs it could be 39A.

It seems that you and John believe that you would have exercised reasonable skill and care in preventing the load on that spur cable becoming 39A by sticking labels on the FCUs.

I do not, any more than I would accept it with sockets on the spur, or indeed with sockets on a ring so close together and/or so close to the origin that a notice warning people not to use them all at once and/or not to use them for significant loads for long periods was needed.

How about the warnings on 13A "strips" indicating that although there are four or five sockets the maximum connected load must not exceed 13 amps?

Utterly pointless. I've never understood why they have them, because the load is limited to 13A by the fuse in the plug.

What if the labels on the FCUs fall off or fade? Are there recognised standards for their durability which would give them acceptable permanence?

Are there any other areas in BS 7671 where something non-compliant can be made so by affixing a notice? (Note that's not the same as something always requiring one.)

Imagine reading a post here like this.

"I've just moved into a new house, and I've found a spur from a ring main with 3 fused switches on it. They're all supplied by the same cable, it's not a case of 2 of them running from the 3rd.

Is this OK?"

What percentage of replies do you think would advise either putting them on the ring or rewiring it so that 2 are on the load side of the 3rd, or adding a 4th with all of them on the load side of that, and what percentage do you think would advise putting labels on the FCUs to prevent an overload?

Quite honestly the more I think about your idea that labelling counts as reasonable skill and care the more it beggars belief.

If you were aiming for the best possible design, would you be using rings with spurs in the first place, bearing in mind all the limitations they bring with them?

I probably wouldn't, not as a default.

But rings are explicitly allowed, and they do have characteristics which could make them a reasonable design.

For the life of me I cannot see why 3 FCUs with labels is reasonable when weighed against the alternative of 4 FCUs.

Is the maximum connected load less than the current rating deemed to be the maximum for the spur?

No more so than it would be with 3 labelled sockets.

 

[ban-all-sheds]

I thought a certificate should be issued for every job.

Are we confusing certification and notification again?

I'm not.

So if Mr Homeowner replaces his cooker, how does he go about completing this:

?

 

[Paul_C]

You can no more enforce a design current for that spur of 9A by putting labels on the FCUs than you could enforce a 9A current for 3 sockets by putting labels on them.

But you can enforce a design current of 9A maximum by connecting loads which cannot draw more than 9A in total, in the same way as you can enforce a 40A maximum current on a shower circuit by installing a shower which draws no more than 40A.

So in your opinion nobody not qualified to issue BS 7671 certificates could change their oven?

Not at all. I think it would be justified for somebody working within requirements of BS7671 to issue a certificate. But that isn't everybody.

It may have tremendous bearing on whether something is an alteration.

If you remove one part of a fixed installation and replace with another, then I'd say that's an alteration.

Does replacing a shower require a BS 7671 certificate?

As above for the oven.

 

[JohnW2]

What's the difference?

None as far as I can see. In both cases we have a load (or loads) hard-wired to the circuit, the characteristics of that circuit being determined at the time it was installed as meeting the requirements of the connected load. In both cases, we're talking about somebody coming along at some future time and replacing that load with a different, heavier load, which will require the person doing that work to assess whether the existing feed to the point at which the load will be connected is suitable or not. I don't see how the fact that an FCU is involved in one case but not the other makes any difference to that.

Exactly. I can understand the viewpoint and concerns of those who are not happy with my '3A FCUs' with their hard-wired (small) loads, but what I totally fail to understand is why they apparently don't have exactly the same concern about the shower circuits.

It could (indeed, my 3 could, very reasonably) be fed from a lighting circuit.

Most certainly they could, although there is the point with such an arrangement that if somebody did connect a large load in place of your original small loads then the MCB or fuse for the lighting circuit should provide adequate protection for the cables, whereas the 30/32A protective device on a ring will not adequately protect the spur running to the FCU's.

That's true, and I've already attempted to plug that one hole in the argument by asking what view people would have if that spur cable were 4mm² - but BAS then switches argument and starts worrying about the potential concentration of load at one point of the ring! I suppose I should have countered that by talking about a 4mm² spur at the mid-point of the ring - but I suspect that he would then find yet something else to be unhappy about

Kind Regards, John.

 

[Paul_C]

And with 3 spurs it could be 39A.

Only if you connect three 13A loads to them. That's not what John has proposed though.

It seems that you and John believe that you would have exercised reasonable skill and care in preventing the load on that spur cable becoming 39A by sticking labels on the FCUs.

I haven't commented on the labels, but even without them, I would say that you have exercised reasonable skill and care in preventing the load on the spur cable from becoming 39A by connecting only loads which total 3A, 6A, 9A, or whatever the figure happens to be.

Just the same as if you install a shower circuit which can supply 40A maximum, you can exercise reasonable skill and care in preventing the load from exceeding 40A by connecting an 8kW shower and not an 11kW one.

Are there any other areas in BS 7671 where something non-compliant can be made so by affixing a notice?

No, but then neither John nor I have argued that to be the case, at least I'm not trying to say that. I'm not looking at it from the point of view of whether it actually complies with the requirements of BS7671 or not, but of whether it actually results in a situation which is any worse than any number of other arrangements involving rings and spurs which BS7671 does allow. And I don't think it does.

Imagine reading a post here like this.

"I've just moved into a new house, and I've found a spur from a ring main with 3 fused switches on it. They're all supplied by the same cable, it's not a case of 2 of them running from the 3rd.

Is this OK?"

What percentage of replies do you think would advise either putting them on the ring or rewiring it so that 2 are on the load side of the 3rd, or adding a 4th with all of them on the load side of that, and what percentage do you think would advise putting labels on the FCUs to prevent an overload?

What percentage would ask what those units are feeding, or what the person intends to use them to feed?

If you were aiming for the best possible design, would you be using rings with spurs in the first place, bearing in mind all the limitations they bring with them?

I probably wouldn't, not as a default.

Well, we can agree on that one!

Is the maximum connected load less than the current rating deemed to be the maximum for the spur?

No more so than it would be with 3 labelled sockets.

But it's easy for any casual user to plug anything which can draw up to 13A into a BS1363 socket. It's not so easy to change two or three small hard-wired loads for heavier loads, since that requires changes to the fixed wiring.

So if Mr Homeowner replaces his cooker, how does he go about completing this:

Mr. Homeowner has no need to complete it, because he doesn't have any particular need to be working within the confines of BS7671.

 

[EFLImpudence]

So if Mr Homeowner replaces his cooker, how does he go about completing this

He could if he had tested the circuit.

However, you raised the certification as if it were a criterion to define certain work.