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

I think atempt 3 may not be practical for
DIY.

Another cable thru wall.
Another joint in backbox

AndyPRK said:

I think atempt 3 may not be practical for
DIY.

Another cable thru wall.
Another joint in backbox

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I haven't seen the workmanship but his written work and common sense seems pretty good.
Knowing the calibre of a lot of new build work, chances are the existing hole may be big enough, or alternatively removing the existing cable, running a bigger drill through the existing hole and running 2 cables may be a fairly easy project. Especially if we are talking about thermal block or studwork. The achillies heel may be the available space in the backbox of the kitchen socket, this is dependant on: The depth of the box, how deep the box is in the wall and the thickness of the existing socket. To date it has never beaten me but sometimes very tricky.

EFLImpudence said:

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

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JohnW2 said:

... 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 ...

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This is all very informative, thanks.

So to be clear, the CCC tables are explicitly calculated assuming a fusing factor of 1.45, and that is stated in the BS7671 guidelines? I had not seen this because unfortunately I don't have them, I have seen the CCC table repeated on various sites but without the assumptions. Clearly I should get hold of the actual guidelines.

It seems clear that my electrician isn't aware of that either.

So even if the FCU does actually have a fusing factor of 2 (rather than 1.45 as seems to be implied by the list JohnW2 posted) that would mean that the 27A CCC for my 2.5mm cable (clipped direct) would only be downrated to 19.575A, which is still fine for my 13A fuse.

SUNRAY said:

Thought about this?

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Yes the main reason this would be challenging for me is that its not easy to access the other side of the wall where the cable comes into the kitchen (the cabling was evidently installed before the plastering and kitchen units were put in).

Furthermore I'd be hesitant to extend the ring myself - when I started this saga I thought I'd take the most straightforward approach to adding the sockets I wanted to the garage, and I thought current-limiting the spur would be that. Of course even that has turned out, for various reasons, to be a lot more bother than I had hoped!

Justin0 said:

So to be clear, the CCC tables are explicitly calculated assuming a fusing factor of 1.45, and that is stated in the BS7671 guidelines?

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Not explicitly stated, but implicit. It is stated that the CCCs tabulated in BS7671 apply directly (in conjunction with any 'downrating factors) in the case of an MCB (FF=1.45) and that if one has a BS3036 fuse (FF=2), then one has to apply a 'correction factor' of 0.725 (i.e. 1.45/2).

Justin0 said:

It seems clear that my electrician isn't aware of that either.

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Seemingly far from the only thing he is unaware of!

Justin0 said:

So even if the FCU does actually have a fusing factor of 2 (rather than 1.45 as seems to be implied by the list JohnW2 posted) that would mean that the 27A CCC for my 2.5mm cable (clipped direct) would only be downrated to 19.575A, which is still fine for my 13A fuse.

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Exactly. However, BS7671 only seems to view the world as BS3036 fuses on one hand, and 'everything else' (hence all presumably deemed to have a FF of 1.45) on the other hand.

Kind Regards, John

Justin0 said:

Yes the main reason this would be challenging for me is that its not easy to access the other side of the wall where the cable comes into the kitchen (the cabling was evidently installed before the plastering and kitchen units were put in).

Furthermore I'd be hesitant to extend the ring myself - when I started this saga I thought I'd take the most straightforward approach to adding the sockets I wanted to the garage, and I thought current-limiting the spur would be that. Of course even that has turned out, for various reasons, to be a lot more bother than I had hoped!

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Very fair comments and this confirms my previous assessment of you attitude.
Somehow I had it in my mind the spur came straight through the wall behind the kitchen socket.