Wiring multiple sockets on fused spur - what is the electrician calculating?

1) The cable going into the FCU is larger than that coming out (ignoring the fact that it's 2.5 going in, as in my setup)
The fuse is there to protect the cable.
13A fuse will protect a 1.5mm Cable, so diagram is correct.
You have used 2.5mm after the fuse so you are extra safe (not less safe).


2) That comment about the number of sockets being dependent on the load characteristics - I mean, it shouldn't make any difference, should it?
It makes a difference in that the Wiring Regulations and the user do not want a fuse blowing all the time because of bad circuit design.

In your case, 2 or 4 or 8 or 16 or 32 or 64 sockets in your garage will not blow the fuse because you do not have enough items to plug in and switch on at the same time to blow the fuse. YOU are likely to only use a few sockets at a time.
And so you have perfectly accounted for your diversity requirements in your situation.

In say a computer server room, you might consider how many sockets are connected off the FCU/ fuse, because in this scenario ALL the available sockets are likely to all be used. And having too many available sockets, of which all are in use, may cause the fuse to blow (Noting again that fuse will blow before damage to cable). So in this situation you might account for the diversity requirements by having (say) 4 sockets connected to the FCU.

SFK
 
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in a garage, it's would be obvious where the cables go.

But if it's under floor boards, then I see your point.

We should stick with Ring ;) *runs
 
in a garage, it's would be obvious where the cables go.

But if it's under floor boards, then I see your point.

We should stick with Ring ;) *runs
 
The fuse is there to protect the cable.
13A fuse will protect a 1.5mm Cable, so diagram is correct.
You have used 2.5mm after the fuse so you are extra safe (not less safe).

That's true but he is concerned that the FCU has a cable before it at all (rather than wired directly into the ring), and my point has been that there is no difference in the load before and after, so it should make no difference.

Yet this example shows such a difference. In this example, why is it not 1.5mm before the FCU too?

2) It makes a difference in that the Wiring Regulations and the user do not want a fuse blowing all the time because of bad circuit design.
In your case, 2 or 4 or 8 or 16 or 32 plugs in your garage will not blow the fuse because you do not have enough items to plug in and switch on at the same time to blow the fuse. YOU are likely to only use a few plugs at a time.
You have perfectly accounted for your diversity requirements in your situation.
SFK

Okay thanks that is clear.
 
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Regarding the standard 32A/2.5mm² ring circuits.

What is the possible load on, say, 10 double sockets on a ring in relation to the 32A MCB?
What will happen if you do overload it?

Does anyone think a ring circuit should be limited to three single sockets or one double and one single socket (although even that might be too much)?
 
"there is no difference in the load before and after"and so "why is it not 1.5mm before the FCU too"

They show 2.5mm2 before the FCU simply because the ring it is connected to is 2.5mm and it is easier to wire from the ring to the FCU using the same wire.

The 1.5mm2 after the FCU is rated to sufficiently be protected by the fuse,

But (just as you have done), you have upgraded from the above spec to connect your sockets to the FCU with 2.5mm.

Your personal decision to upgrade after the FCU does not mean you have to upgrade the thickness of the wire before the FCU.

In your case you meet the spec before the FCU, and you are better than the spec after the FCU. So your electriction should be satisfied.

And, as a reminder the fuse remains the intentionally designed weak point to protect the cable.

SFK
 
Thanks. I wonder if it's diagrams like this that have him confused.

He's still convinced that the cable needs to be rated to at least double the fuse rating, anyway o_O
 
Thanks JohnW2 for digging that out. But actually looking at it I feel like it might back up the electrician's case, because: 1) The cable going into the FCU is larger than that coming out (ignoring the fact that it's 2.5 going in, as in my setup)
Does that not make things even 'worse'. The 'normal' thing to do (which I imagine is what you are doing) would be to wire the sockets in 2.5mm² cable. However, that diagram is indicating that if there is a 13A fuse upstream, one only needs to use 1.5mm².
2) That comment about the number of sockets being dependent on the load characteristics - I mean, it shouldn't make any difference, should it?
As I said, that statement appears to be ridiculous (well, it certainly would be if the sockets were wire with 2.5mm² cable) - since, by analogy, it would mean than no 32A ring or radial sockets circuit could have more than one double socket (or two single ones)!
Could he be onto something?
In my opinion, for what it's worth, no!

Kind Regards, John
 
Justin
In response to "He's still convinced that the cable needs to be rated to at least double the fuse rating".

1) Cable is NOT Current rated, as this rating would change depending on its location and temperature

2) Ignoring all the other factors discussed above:
- The spec diagram above shows 13A fuse protecting 1.5mm2 cable.
- You are using 2.5mm2 cable.
So you have (almost) double rated your cable by using 2.5mm2 cable over the spec 1.5mm cable.

SFK
 
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Thanks. I wonder if it's diagrams like this that have him confused.
I'm sorry to say it, but he just doesn't understand the basics.
He's still convinced that the cable needs to be rated to at least double the fuse rating, anyway o_O
That's a good example of him 'not understanding the basics'. Cable current-carrying-capacities take into account the fact that it requires a lot more current than 13A to blow a 13A fuse.. If a cable is 'rated' ('current carrying capacity') to be able to carry, say, 13A when installed in a certain way and protected by a fuse, that means that it is deemed to be able to carry 26A (or whatever) without coming to harm.

Kind Regards, John
 
Just to inform Justin - although this is not shown in any diagrams either.

If the ring circuit is protected by a 32A MCB, then had you used 4mm² (which is much nicer to work with than 2.5mm²) from the supplying socket to any number of garage sockets then you would not need the FCU at all.

However, were the circuit protected by a 30A rewireable fuse, then this would not be compliant because rewireable fuses have a fusing factor of 2, meaning that the current-carrying-capacity of the cable must be downrated by a factor of 0.725 - 37A for 4mm²; 37 x 0.725 = 26.8A; less than 32A.
That is the 1.45 FF of the MCB divided by 2.

The 1.45 of the MCB means that its rating times 1.45 (32 x 1.45 = 46.4A) will cause it to trip in 1 hour without damage to the cable
 
Just to inform Justin ... However, were the circuit protected by a 30A rewireable fuse, then this would not be compliant because rewireable fuses have a fusing factor of 2, meaning that the current-carrying-capacity of the cable must be downrated by a factor of 0.725 - 37A for 4mm²; 37 x 0.725 = 26.8A; less than 32A. ... That is the 1.45 FF of the MCB divided by 2. ... The 1.45 of the MCB means that its rating times 1.45 (32 x 1.45 = 46.4A) will cause it to trip in 1 hour without damage to the cable
... and to further inform Justin, in the case of the BS1362 fuses (in FCUs and plugs), there is conflicting information out there about their fusing factor (although all seem to agree that it is less than 2), but the following seems to suggest that no 'downrating' (relative to the tabulated CCCs which assume an MCB with FF=1.45) may be needed with such a fuse ...

upload_2019-10-4_17-39-17.png


Kind Regards, John
 
Thought about this? .....
... nothing wrong with that (and, in 'future-proofing' terms even better) - but, if 13A total is all the OP wants in his garage then, despite the 'electrician', nor is there anything wrong with the his proposal.

Kind Regards, John
 
... nothing wrong with that (and, in 'future-proofing' terms even better) - but, if 13A total is all the OP wants in his garage then, despite the 'electrician', nor is there anything wrong with the his proposal.

Kind Regards, John
Absolutely.

I see nothing wrong in the slightest with attempt 1. In fact, Assuming the JB's were proposed for a reason such as saving a lot of cable, I think it is a very adequate design.
 

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