Spurs off spurs...

[Jonathan]

Hi,

After lurking for many months here, and absorbing as much knowledge as I can, it's time for me to ask a question!

Some background, first of all.

I am currently in the process of wiring my kitchen ring.

I've provided two lengths of oval conduit (buried in the plaster) going up to each backing box in the wall from the underfloor void, on the basis that it will be much easier to access cables in the future should any maintenance etc. be required. (There wasn't enough depth for round conduit, without ripping out kitchen units and replastering the entire wall)

I'm wiring the ring using 4mm^2 cable, as due to the installation method (in conduit in a thermally insulated wall) I belive that

2.5mm^2 cable will not quite take the expected load and I'd like to use a 32A MCB for this circuit. (It will feed a washing machine, drier, microwave, kettle, toaster etc.

So far so good.

However. Being as good at forward planning as I am, the spec has changed since the wall has been plastered and tiled :) The upshot of this is that the boiler now has its own feed from the CU (mainly for ease of future UPS/generator installation), and I am left with a double socket with only a single piece of oval conduit feeding it. (The original plan was for the ring to go up to the boiler FCU, across to this double socket and then down).

I'd still like to have a double socket in this position. I tried pushing two 4mm^2 cables through the single piece of conduit, but can only get one through. So, I will simply connect this socket as a spur from elsewhere on the ring - that should work fine.

(Now we finally get to the question! :-))

The kitchen extractor fan is positioned approx 1.5m above this 'problem' double socket, and has conduit going down to the double socket's backing box - I originally intended to feed the fan FCU as a spur from the socket.

Now that this socket is itself a spur, I know that I'm not meant to take another spur off it. Of course, all the conduit is buried in the wall, which has been plastered etc...

Am I right in thinking that the reason for this "no spurs off spurs" rule is to protect the cable leading to the first spur?

If this is the case, then presumably a 3A kitchen fan would not cause a problem? The spur to the socket would be fed via a 4mm^2 cable, and from there it would be 2.5mm^2 to the fan's FCU (fitted with 3A fuse).

Is this "fan off a spur" a design decision that could reasonably be justified, or would you advise against connecting the fan in this way?

Comments always appreciated!! :)

Thanks,

Jonathan

-- No email replies, sorry, this mailbox is not monitored.

[John Rumm]

Sounds ok so far...

This bit has be a little confused however... if you really do mean a "ring" (i.e. a cable run that starts at the CU runs to each socket in turn before returning to the CU) unless you are covering a huge distance (where voltage drop may become an issue) then I can't see the need for

4mm sq cable. 2.5mm sq is rated at 20A even when burried in insulation. Hence the parallel run of cable you get by virtue of the ring has 40A capacity - plenty to support use of a 32A MCB.

Yes that is right. If you want more than one socket (single or double) you would connect them via a FCU. That way the cable to the first socket is not at risk.

I can't see anyone getting too worked up over that - you would not be creating a situation that could potentially overload the cable.

[Jonathan]

Thanks for the reply John.

You could well be correct on this point, but I'll try and explain my reasoning:

- The washing machine, tumble drier, microwave and possibly kettle are all close together at one end of the ring. To avoid this (unbalanced) load potentially being over 20A and thus overloading the cable, I planned on running at least the length from the CU up to those sockets in 4mm sq.

- From Table 4.7 (

)

2.5mm sq cable is rated at 18.5A when in conduit in thermal insulation. There's a handy online calculator at and this tells me that, if using a 32A MCB to protect a ring wired in 2.5mm sq cable enclosed in insulating material, the overcurrent protection device has a nominal current higher than the corrected nominal rating of the cable and hence is not suitable.

I decided it would be much simpler to just wire the ring using 4mm sq cable, rather than try to calculate exact capacities based on the precise lengths of cable that are in conduit in the walls, and the lengths that simply run under the floor etc..!

Does that make sense at all? :)

Agreed, I wouldn't consider connecting more than one socket in this way.

Cheers. I'll probably mark the FCU anyway, with a label saying "3A MAX" or similar to avoid the situation of a future owner trying to fit some device that draws more than that.. although I can't really imagine what would fit in place of a kitchen fan, that would require such large amounts of current! :-)

Thanks for your advice,

Jonathan

[John Rumm]

Yup fair enough, as you say the load will be split betweent the two cables in the ring in proportion to their resistance (i.e. length) to the position of the load. So if you have most of the load clustered near to one end of the ring you may have a potential problem.

My Electricians guide book as an errata that corrects that table to 20A

- hence why I used that figure (I was going to go with 18.5 from memory to start with). It might be the TLC calculator need updating (or there is an errata needed for the errata in my book!)

yup.

[Martin Angove]

It's not really an error AIUI, more like a "re-grading". They realised that everyone was using 2.5mm for rings, often without doing the calculations, according to the existing tables this was occasionally "iffy". They apparently did some tests and discovered that "flat twin and earth" is better rated than they previously thought (or perhaps they just used some TippEx and a biro) and so they used "Ammendment 1" (right at the back of the full regulations book) to uprate the cable to 20A in conduit in an insulated wall. The notes given in the ammendment help explain. The same table is reproduced in the On Site Guide, but I don't have a copy to hand to tell you where right now... 4F? 5F? It was F I think.

HOWEVER... according to this from the original post:

it is possible that the cable is not actually installed in an insulated wall, unless of course the plaster is some special mix which has insulating properties. Installation methods 6 and 15 (in conduit / not in conduit, in an insulated wall) refer to the case where the cable only contacts a "thermally conductive" material on one side and is otherwise surrounded by insulation. This might be the case in a stud wall or timber-frame construction where the cable is clipped to the back of the plasterboard, and the whole is stuffed with insulation - see the notes for table 4A1 and the ammendment to 4A1

In the usual case where the cable (or conduit) is clipped to brickwork and then plastered over, even if this is an outside wall where the cavity is insulated, installation method 1 (effectively) applies - and

2.5mm2 cable is rated up to 27A.

Of course, there is no reason other than cost and ease of installation

*not* to use 4mm in a ring if wished, and you can be assured that the circuit is more than capable of being run at 32A.

Hwyl!

M.