Multiple FCUs

[EFLImpudence]

Regarding fans off lighting circuits, I personally think that unless the manufacturers specifically request fusing down, it is satisfactory to wire this type of appliance direct.

Not you personally - it is. Unless the appliance comes with flex attached, there is no reason for the manufacturer to stipulate fusing.

With a standard kitchen/ bathroom fan arrangement, it is difficult to imagine how a significantly larger load could be substituted. However, in your cellar situation, potentially your spur off a spur arrangement could be used to supply (for example) a tumble drier and a washing machine.

That would be the new owner's mistake. If you saw three 3A FCUs in a row, might you not wonder?

Of course, the specific situation can alter how the designer considers selection and erection of equipment. As you suggest, some scenarios are unlikely (or far less likely) to be abused and in theory you could ignore certain regulations without fear of creating a dangerous installation.

Regulations are not being ignored. 2.5mm² cable - max 9A. (less than 13A socket, not to mention a double)..

However, the regs try (I have never said they have succeeded) to account for all scenarios and be idiot-proof and this is why they contain regulations that are, for various reasons, contentious.

That doesn't stop you working out an alternative that an idiot may not be able to do.

When I assess an installation for safety, I would not think, for example, that your RF circuit has three accessories spurred off each other, but they are only running low-current appliances so not a danger.

If you saw three 3A FCUs in a row, might you not wonder?

I would record it as a potential at risk situation.

Fair enough - potential.

I think everyone carrying out electrical work owes it to themselves to consider how that installation may be used now and in the future and take that into account in the design and installation process.

Not possible - no more spurs, then?

 

[JohnW2]

With a standard kitchen/ bathroom fan arrangement, it is difficult to imagine how a significantly larger load could be substituted. However, in your cellar situation, potentially your spur off a spur arrangement could be used to supply (for example) a tumble drier and a washing machine.

It's unlikely (as would be apparent if you saw the location) but, I agree, theoretically not impossible. I would personally think that what you're postulating is just about the most extreme scenario one could reasonably anticipate (without getting very contrived). Do you agree?

If so, I would suggest that such an (extreme) scenario is within the design parameters of what I was talking about. The short 2.5mm² spur cable from the ring is clipped direct, so you would have to postulate a total load on the FCUs greater than 27A for an appreciable period of time before the cable's CCC was exceeded. OK, so some joker might wire 3 tumble driers into my FCUs, but that's a ridiculously contrived suggestion, and I can't honestly think of any remotely likely way in which a persistent load >27A would be connected, can you?

Of course, the specific situation can alter how the designer considers selection and erection of equipment. As you suggest, some scenarios are unlikely (or far less likely) to be abused and in theory you could ignore certain regulations without fear of creating a dangerous installation.

Agreed - but see above I don't believe that I actually would be 'ignoring' (or violating) any regulations. What BS7671 says about ring final circuits, and, in particular, what is written in the ('informative') Appendix 15 about such circuits, merely relate to arrangements which are deemed to satisfy the underlying requirements of the regs - but a designer remains free to utilise other designs which satisfy those underlying requirements.

In the case under discussion, as far as I can see, the only issue is the possible oveloading of about 4 feet of 2.5mm² cable. However, as above, it is my opinion (and, dare I presume to suggest, probably also yours!) that it's incredibly unlikely that anyone would ever connect loads to that which drew more than 27A - so my opinion, as designer, would be that the design does satisfy the requirements of BS7671 as regards the current carrying capacity of its cables under all conditions that can be reasonably anticipated. Do you disagree?

I think everyone carrying out electrical work owes it to themselves to consider how that installation may be used now and in the future and take that into account in the design and installation process.

As above, I believe that I have taken into account all reasonable (and some pretty unreasonable!) possible future (mis)uses of the installation. Do you disagree?

Kind Regards, John.

 

[JohnW2]

When I assess an installation for safety, I would not think, for example, that your RF circuit has three accessories spurred off each other, but they are only running low-current appliances so not a danger.

If you saw three 3A FCUs in a row, might you not wonder?

...and particularly if, as I said would be the case, those three FCUs in a row were each clearly labelled "Maximum 3A Fuse"

Kind Regards, John.

 

[Paul_C]

However, in your cellar situation, potentially your spur off a spur arrangement could be used to supply (for example) a tumble drier and a washing machine.

It could. But the same argument could be made for dozens of other similar scenarios as to what somebody might do in the future. Presumably you don't consider a twin 13A socket on a unfused spur to be a problem, as it's recognized by BS7671. What if somebody comes along later and changes that twin socket for a triple, or just runs a foot of cable from that existing socket into a another twin socket, then plugs in a washing machine, dryer, and heater?

When I assess an installation for safety, I would not think, for example, that your RF circuit has three accessories spurred off each other, but they are only running low-current appliances so not a danger. I would record it as a potential at risk situation.

So would you also record a twin socket on an unfused spur in a similar way, because of what somebody might change at some future date? If not, why not?

 

[ban-all-sheds]

...and particularly if, as I said would be the case, those three FCUs in a row were each clearly labelled "Maximum 3A Fuse"

I thought you were going to have a 4th FCU, upstream of the other 3?

3 FCUs on one unfused spur contravenes the regulations, and cannot be brought into compliance by you putting labels on them, any more than 3 sockets on an unfused spur could be made OK by labels on them saying "Max load 3A".

 

[ban-all-sheds]

What if somebody comes along later and changes that twin socket for a triple,

3-gang sockets are fused.

or just runs a foot of cable from that existing socket into a another twin socket, then plugs in a washing machine, dryer, and heater?

That would be an alteration. All bets are off.

So would you also record a twin socket on an unfused spur in a similar way, because of what somebody might change at some future date?

No.

If not, why not?

Because that future change would be an alteration. All bets are off.

 

[ban-all-sheds]

I would not think, for example, that your RF circuit has three accessories spurred off each other,

Why not?

They clearly are.

but they are only running low-current appliances so not a danger.

Neither would 3 socket outlets with a bunch of phone/iPod/etc chargers plugged in.

I would record it as a potential at risk situation.

I'd give it a 2.

 

[JohnW2]

...and particularly if, as I said would be the case, those three FCUs in a row were each clearly labelled "Maximum 3A Fuse"

I thought you were going to have a 4th FCU, upstream of the other 3?

As I've said, I probably will - and that is a discussion stopper. However, what everyone is discussing is the situation which would exist if I did not have the upstream FCU.

3 FCUs on one unfused spur contravenes the regulations, and cannot be brought into compliance by you putting labels on them, any more than 3 sockets on an unfused spur could be made OK by labels on them saying "Max load 3A".

Well, that's you're opinion, but others seem to disagree. I don't think it's as 'obvious'a situation as you make out.

In other situations, people (I think maybe even including you) have argued that fixed loads cannot result in an overload situation (only 'faults'), and therefore that the final length of cable supplying it does not need to be able to carry more than the normal operating current of the fixed load. If one took that view in relation to a ring final circuit, the arrangement being discussed would presumably be OK. Indeed, if one took that argument to its logical conclusion, it would still be OK to wire the fixed load(s) directly into the ring without any intervening fuse at all.

In terms of 'the power of labelling', consider the following (albeit extreme) situation.....
Firstly, can we agree that there is a requirement for the 'design current' of a circuit not to exceed the In of the circuit's OPD?
Next, do you agree that you would be happy (and confident that you were compliant) by connecting fittings which included 12 B22 lampholders to a lighting circuit protected by a 6A MCB?
I would suggest that the reason for that compliance would be the labels on the light fittings indicating the maximum permissible lamp size. If the infamous 'meddler' came along with a dozen 150W lamps (and many people still have good stockpiles), the total current draw at 230V would be around 7.8A.

Kind Regards, John.

 

[JohnW2]

or just runs a foot of cable from that existing socket into a another twin socket, then plugs in a washing machine, dryer, and heater?

That would be an alteration. All bets are off.

If not, why not?

Because that future change would be an alteration. All bets are off.

That's been my point all along. I personally regard the re-wiring of a hard-wired fixed load (much higher than the previous load) as being 'an alteration'- in which case, just like you, I feel that 'all bets are off'.

What is your definition of 'an alteration' - since it clearly must be different from mine.

Kind Regards, John.

 

[JohnW2]

I forgot to ask ....

3 FCUs on one unfused spur contravenes the regulations....

What it 'contravenes' is the deemed-to-satisfy suggestions given in Appendix 15 of the regs - which, incidentally also do not mention having two FCUs on one spur (even though that's no different from a double socket). As often discussed, that doesn't mean that one cannot comply with the regs in ways not mentioned in that 'informative' Appendix.

so, the question .... What would you say if the 3 FCUs were supplied with a 4mm² spur from the ring?

Kind Regards, John.

 

[ban-all-sheds]

Well, that's you're opinion, but others seem to disagree. I don't think it's as 'obvious'a situation as you make out.

Appendix 15 may be only informative, but it is part of BS 7671, and it says what should be done.

I can think of no circumstances which would make your 3 FCUs not supplied from a 4th so imperative that you would not at least have contravened 134.1.1 by doing it.

I wouldn't consider doing it, and I would have no hesitation in giving it a 2 on a PIR.

In other situations, people (I think maybe even including you) have argued that fixed loads cannot result in an overload situation (only 'faults'), and therefore that the final length of cable supplying it does not need to be able to carry more than the normal operating current of the fixed load. If one took that view in relation to a ring final circuit, the arrangement being discussed would presumably be OK.

Indeed not, because 433.1.5 ring finals are a special case.

Indeed, if one took that argument to its logical conclusion, it would still be OK to wire the fixed load(s) directly into the ring without any intervening fuse at all.

No, because 433.1.5 ring finals may only supply BS 1363 accessories.

Until we had ring finals accessories were plugged into BS 546 sockets with no intervening fuse.

In terms of 'the power of labelling', consider the following (albeit extreme) situation.....
Firstly, can we agree that there is a requirement for the 'design current' of a circuit not to exceed the In of the circuit's OPD?

Yes.

Next, do you agree that you would be happy (and confident that you were compliant) by connecting fittings which included 12 B22 lampholders to a lighting circuit protected by a 6A MCB?

Yes.

And 12 DSOs on a ring final circuit with 2.5mm² T/E protected by a 32A one.

I would suggest that the reason for that compliance would be the labels on the light fittings indicating the maximum permissible lamp size.

Any such labels would be because of the ability of the lampholder to withstand the heat from the lamp.

If the infamous 'meddler' came along with a dozen 150W lamps (and many people still have good stockpiles), the total current draw at 230V would be around 7.8A.

Or a dozen 275W Philips Photofloods giving a load of 13.8A.

The point here is not Ib vs In, it is Iz vs In.

With the lighting circuit the cable could not be damaged by someone ignoring the labels and breaking the design rules. With your 3 FCUs it could be.

What is your definition of 'an alteration' - since it clearly must be different from mine.

Subject to a disclaimer that I've not given this exhaustive thought, I'll say "anything which requires a certificate to be issued".

So adding a socket or an FCU is an alteration.

Replacing a fixed appliance is not an alteration.

Changing a fuse in an FCU is not an alteration.

 

[Paul_C]

3 FCUs on one unfused spur contravenes the regulations, and cannot be brought into compliance by you putting labels on them, any more than 3 sockets on an unfused spur could be made OK by labels on them saying "Max load 3A".

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).

What if somebody comes along later and changes that twin socket for a triple,

3-gang sockets are fused.

Fair point - Although I think some of the older ones weren't.

or just runs a foot of cable from that existing socket into a another twin socket, then plugs in a washing machine, dryer, and heater?

That would be an alteration. All bets are off.

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

So would you also record a twin socket on an unfused spur in a similar way, because of what somebody might change at some future date?

No.

If not, why not?

Because that future change would be an alteration. All bets are off.

Ditto.

but they are only running low-current appliances so not a danger.

Neither would 3 socket outlets with a bunch of phone/iPod/etc chargers plugged in.

Again, with three sockets it's easy for anyone with absolutely no technical expertise whatsoever to come along later and plug three much heavier loads in. I consider that rather different than hard-wired loads connected to FCU's, since to change those involves making alterations to the fixed wiring. At that point, there comes an expectation that somebody making such changes to the actual wiring will understand the possible ramifications of those changes.

I personally regard the re-wiring of a hard-wired fixed load (much higher than the previous load) as being 'an alteration'

As do I. Hence it falls into the same category as all those other possible scenarios of what somebody might change in the future, like the replacement of a shower with a more powerful model, etc.

 

[ban-all-sheds]

What it 'contravenes' is the deemed-to-satisfy suggestions given in Appendix 15 of the regs - which, incidentally also do not mention having two FCUs on one spur (even though that's no different from a double socket).

It is different, just as two single sockets would be, and for the same reason.

As often discussed, that doesn't mean that one cannot comply with the regs in ways not mentioned in that 'informative' Appendix.

Indeed not, but there would be a lot more design effort needed.

so, the question .... What would you say if the 3 FCUs were supplied with a 4mm² spur from the ring?

I would say this:

1) Would concentrating that much load on one point of the ring meet the deemed-to-satisfy provisions in 433.1.5 and Appendix 15 1(i)? (BTW - I'd ask the same Q about socket outlets right next to each other on a ring.)

2) Do you have a compelling reason to do that rather than have the 4th FCU, and if not why does not having the 4th FCU qualify as a good design?

 

[JohnW2]

Appendix 15 may be only informative, but it is part of BS 7671, and it says what should be done.

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.

Appendix No, because 433.1.5 ring finals may only supply BS 1363 accessories.

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'

[The point here is not Ib vs In, it is Iz vs In.

In terms of the regs, you can't make that distinction, since the regs require both those conditions to be satisfied. I agree that the cable will not be at risk provided that Iz>In, regardless of Ib, but that does not alter the fact that the regs do require Ib ≤ In.

Replacing a fixed appliance is not an alteration.

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. If not, we're back again to the shower situation...

... 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.

Kind Regards, John

 

[JohnW2]

so, the question .... What would you say if the 3 FCUs were supplied with a 4mm² spur from the ring?

I would say this:
1) Would concentrating that much load on one point of the ring meet the deemed-to-satisfy provisions in 433.1.5 and Appendix 15 1(i)? (BTW - I'd ask the same Q about socket outlets right next to each other on a ring.)?

This is a new consideration (we've previously been discussing only potential risks to the spur cable0 - but what you say is the point. Although you may ask the question, the strongest that Appendix 15 says about positioning of sockets (each of which could have spurs) is that 'exceeding the cable's CC for long periods of time ... can generally be achieved by .... locating socket outlets to achieve reasonable sharing...' - so, particularly if the designer judged it unlikely that large loads would be plugged into all of them, there is nothing explicit in the regs to prohibit side-by-side sockets - which are, of course, quite common - particularly in kitchens, workshops and 'computer rooms' etc.

Indeed, as we've discussed often, a double socket represents a type of 'side-by-side' situation, given that the vast majority of Joe Publics have no reason to think that they can't plug two 13A loads into the outlets. Whatever, if one accepts that side-by-side sockets (with spurs) aren't explicitly prohibited, a 4mm² feed to three CUs would not really be any different.

Kind Regards, John.