Multiple FCUs

[JohnW2]

It would comply if it was only 2 fcu's (no worse or better than a twin s/o)but as you have 3 it does not comply.

That surely is open to debate? It can easily be argued that 3 FCUs with 3A fuses is potentially a lot 'better' than a double socket into which two plugs with 13A fuses (and associated loads) could be plugged. After all, 9 is a lot lower than 26, and even appreciably lower than 13.

Kind Regards, John.

 

[JohnW2]

If the the load only requires 3A and cable sizes is suitable, why the need for any additional FCU.
I would just install some switches.

Switches from where - the output side of a single FCU? If so, the fuse in that FCU would have to be increased to 10A or 13A, and I doubt that the cables of the loads would have enough CCC for that to be compliant, I could, I suppose, upgrade the loads' cables to address that, if necessary. However, as I've discussed before, I'm a great believer in, wherever possible, protecting loads (not just their cables) with the lowest rating fuse possible - and I would then have <3A loads protected by a 10A or 13A fuse.

Kind Regards, John.

 

[westie101]

Electrically as a design proposal it seems to present no problems, However can you guarantee over the life of the installation that no one would connect three larger 13A loads to the FCUs

Whilst you are using it you of course can, but, as with all of us, you will not be there forever.
As has been suggested, feed the 3 3A FCUs from a 13A one

 

[holmslaw]

..

 

[JohnW2]

Electrically as a design proposal it seems to present no problems, However can you guarantee over the life of the installation that no one would connect three larger 13A loads to the FCUs. Whilst you are using it you of course can, but, as with all of us, you will not be there forever.

As I just wrote, that was the main issue I thought people would raise, but (apart from me) you are the first person to have mentioned it.

However, I have to wonder how far one is meant to take these arguments based on 'what someone may do in the future'. Don't forget that, with what I was proposing, this 'someone' would not only have to change the fuses in the FCUs but would also have to re-wire new fixed loads. Regs cannot cater for all the silly things that people might do in the future - should any cables less than 10mm² be banned in installations in case some joker comes along and connects a shower or cooker to the circuit at some time in the future? As you say, someone could change the FCU's fuse from 3A to 13A and load the circuit accordingly. By the same token, someone could change the 6A MCB of a lighting circuit to a 32A one and add a few 13A sockets to that circuit.

...so, how far are we meant to go?

As has been suggested, feed the 3 3A FCUs from a 13A one

That is certainly a solution as far as the regs are concerned, since BS7671 doesn't care what you connect to a spur after it's been through a 13A fuse. However, the 'discrimination/selectivity police' might have an issue with what you're suggesting!

Kind Regards, John.

 

[JohnW2]

And 3 x 13 is 39Amps, almost twice the rating of the 2.5 cable.

Sure, but as I just wrote to westie, that would require that the FCU fuses were changed to 13A and three new fixed loads were re-wired into them. As I asked him, how far are we expected to go in idiot-proofing?

Kind Regards, John.

 

[westie101]

I am no fan of the concept of taking things to the ultimate and have serious doubts about the interpretation of some parts of the (non statutory) regs I see here.
I do however believe in the concept of thinking designs through, you are the designer and should be able to show mitigation for your design.
In other words to use the argument that someone would have to not comply with the (non statutory) regs to justify your design doesn't really work.

I would suggest you need to show, that the connected load could not be increased in other ways.

 

[JohnW2]

I am no fan of the concept of taking things to the ultimate and have serious doubts about the interpretation of some parts of the (non statutory) regs I see here. I do however believe in the concept of thinking designs through, you are the designer and should be able to show mitigation for your design. In other words to use the argument that someone would have to not comply with the (non statutory) regs to justify your design doesn't really work. I would suggest you need to show, that the connected load could not be increased in other ways.

It's obviously impossible to "show that the connected load could not be increased", since it obviously could be increased by someone who had such a desire, just as a 6A MCB could be 'upgraded' to a 32A one!

My main argument would be that the load could only be increased by changing a hard-wired fixed load, and that if one can't assume that a person doing that would not ensure that the circuit supplying it was adequate for that change, I don't know what one could assume. I would say this was very analgous to showers - should we never be allowed to install a (currently perfectly adequate) 6mm² shower circuit, because someone might subsequently install a higher powered shower and 'upgrade' the MCB without also upgrading the wiring?

Out of deference to the views of those here (which, needless to say, come as no great surprise!), I'll probably 'give in' and feed the 3A FCUs from a 13A one - it only menas one extra FCU and a few extra inches of cable. However, I am in some senses 'annoyed' by having to do this, in relation to what seems to me to be a reasonable design in engineering terms, and which only goes wrong if someone subsequently decided to install different hard-wired loads without first checking that the circuit was suitable! Ah, well!

Kind Regards, John

 

[westie101]

[HINT] Just looking at an extension lead, written in it with nice trnsfer type lettering is the following:-
Max current fully wound 8A (1920W)
Max current fully unwound 13A (3120W)

[/HINT]

I have seen FCU's with max permitted load marked on them!

 

[JohnW2]

[HINT] Just looking at an extension lead, written in it with nice trnsfer type lettering is the following:-
Max current fully wound 8A (1920W)
Max current fully unwound 13A (3120W)[/HINT]
I have seen FCU's with max permitted load marked on them!

Oh, maybe you have misjudged me I have always intended that the FCUs will be very clearly marked with their maximum permitted loads and fuse size (I get a lot of domestic grief because of my 'obsessive labelling' of all thinsg electrical, when I feel that is necesasry for idiot-proofing!). Are you suggest that is enough to make my proposal acceptable?

Kind Regards, John.

 

[westie101]

I don't see why it shouldn't

 

[JohnW2]

I don't see why it shouldn't

Well, as I said, that's what I had always intended, but I can feel a hoard of dissenters rushing towards us

I can just imagine them asking, for example, whether we think it would be acceptable to have 2 double sockets (4 outlets) (rather than the BS7671-permitted one double socket) on an unfused spur of a ring final circuit provided that we wrote "max 3A fuse in any inserted plug" on each of the sockets

Kind Regards, John.

 

[ban-all-sheds]

Why not install an FCU with a 13A fuse and have all the 3A ones on the load side?

 

[JohnW2]

Why not install an FCU with a 13A fuse and have all the 3A ones on the load side?

Out of deference to the views of those here (which, needless to say, come as no great surprise!), I'll probably 'give in' and feed the 3A FCUs from a 13A one - it only menas one extra FCU and a few extra inches of cable.

Kind Regards, John.

 

[ban-all-sheds]