Electrics in a flat

[Kevin Webb]

My apologies in advance for a very long post. I have a number of questions and your help would be much appreciated. My flat has a small plastic cupboard just inside the from door containing the elecric meter. This also contains the original pressed steel fuse box from the 60's with 3 fuses, one for the cooker, one for the ring and one for lights. The flat is wired in micc. There is a chunky plastic junction device after the main fuse that goes to the original fuse box and to a newer single fusebox that has been added for an electric shower. I have removed various other later additions to the circuits as they were very badly done. I'd now like to do the following: Add an addional double socket in each bedroom as a spur from a socket on the ring that has no other spur on it. I would use standard 2.5 T&E. The earth arrives at the original socket via the MICC casing, and is conducted to the socket via the backbox and socket screws. As suggested on this group I have added an earth cable from the backbox to the socket's earth. As the backboxes had no earth terminal I have bolted the cable to the box or used one of the extra tags on the box for socket screws with a 3.5mm bolt and small washer (each double back box has 6 tags for socket screws in total). I would have replaced the backboxes but I didn't want to disturb the micc. I would now expect to take my earth for the new sockets from the earth connector on the original socket. Does this sound OK in principal? Anything to watch out for? I'd also like to remove the extra fusebox for the shower and replace it with a RCD 'shower' unit that accommodates 2 MCBs. One MCB will be for the shower, the second would be to add a ring for the kitchen. The kitchen now has only 2 sockets since I removed some that had been added and wired in flex from the cooker switch. I want to add more sockets and sockets spurred from switched fcu's for washing machine etc. This shower consumer unit will just fit in the gap left by the shower fuse. I'd like to replace the original fuse box as well but that would mean disturbing all the micc cabling fixed to it via brass glands; I also doubt whether the wires from the micc would reach the terminals in a larger consumer unit. If I do this much work then am I liable to rewire the whole flat? I really don't want to do a complete rewire because routing the cables in this flat would be a nightmare. The cooker will also be moved and I need to move the cooker switch/socket. This could be an opportunity to lose the existing massive switch/socket unit and replace with a small 45a isolator. However, is there any reason why I should not rip out the existing micc for the cooker and replace with 6mm T&E? This would be a very short run of about 3m from the fuse box to the new isolator. This would mean that I have a mix of MICC and T&E at the original fuse box. Is that a problem?

TIA for any help. KW

[Owain]

"Kevin Webb" wrote | I'd like to replace the original fuse box as well but that would | mean disturbing all the micc cabling fixed to it via brass glands; | I also doubt whether the wires from the micc would reach the | terminals in a larger consumer unit. If I do this much work then | am I liable to rewire the whole flat? I really don't want to | do a complete rewire because routing the cables in this flat | would be a nightmare.

I know it can look naff in domestic environment but (I'm guessing you have concrete floors and ceilings) have you considered leaving the lighting circuit on MICC, and wiring completely new power, cooker, shower (and immersion heater?) circuits in skirting trunking? This would easily connect to a new CU.

The lighting MICC could be left in place, assuming that the existing lighting provision is generally adequate, terminated into a steel adaptable box wherever the MICC emerges from the wall, and then taken to the new CU nearby in PVC T&E.

This would enable a gradual provision of new circuits and then when you're ready to change over you just lose the old CU and its power circuits, move the lighting across, and plaster in the old MICC boxes in the wall.

Owain

[Kevin Webb]

You are correct, floor and ceiling are concrete. In addition the internal doorways are floor to ceiling with glass panels above the doors. These make cabling across the doorways very difficult/untidy. So the main route for cabling would have to be round the periphery of the flat and the ring would have to turn round and fllow its route back to the consumer unit when it reached the exterior door. I can see the advantages of your suggestion. Skirting trunking would greatly simplify the cabling for the ring main. But the cost of replacing all of the skirting with trunking would be very high - unless you know where to source the trunking at a reasonable price. Does anyone have any other comments on the proposals in my original post, please? KW

[N. Thornton]

Another approach is to put cabling in the bottom corner where floor meets wall, or the top where coving is often used, and run coving or skirting in situ. Best not to use ready made coving for this.

How do you run coving or skirting in situ? A rounded (or decoratviely cut) piece of wood or metal is run along the wall to mould applied cement. This could be used at the bottom to give you normal sized skirting with wires buried in it. Dont forget to take account of the concrete's thermal insulation effect of the wire, which increases the necessary cable sizes. The surface finish can be made smooth afterwards by wiping it over with plaster/filler/artex etc. (Dont try and pattern the artex.)

I've seen this method used twice, and it works well, as long as the rounded corner style goes ok with the property. And while youre at it, its worth running any extra wires you might posibly need in future, such as extra mains cores, phone, ethernet, or a multicore multipurpose low v wire.

If you dont like this approach there are several others. One is to raise the carpet by 5mm or so, with gaps in the raising material used for running your wires. Metal plates over the wires protect it from damage. Another might be to run the wiring externally, and only bring it in at socket points. Another might be to run micc wire visibly but arrange it to fit in with a decorative scheme, such as run it along the top of the skirting and treat it like part of the skirting, or perhaps run 2 micc wires at 1.5" and 3" above and paint both wires and skirting the same colour, etc. Another is to run it where you like and do an artistic paint job with it so it stands out. Another is to remove skirting, run the wires as low down as poss, and replace skirting directly above wiring. Filling over the wiring is then also an option. There are various options if youre creative.

Is it still permitted to use wooden trunking? If so, all you need are some strips of wood to assemble trunking skirting in situ, with one or more of the pieces being decorative moulding. If not you can use pvc trunking with same depth skirting sitting directly above it, so it looks like part of the skirting.

If your chosen look needs just one single wire to look ok you can always put sockets on radial circuits.

A switched lighting socket circuit as well is handy. If youre running concrete this is your one chance to fit the wires easily.

Regards, NT

[Owain]

"Kevin Webb" wrote | You are correct, floor and ceiling are concrete. In addition the | internal doorways are floor to ceiling with glass panels above | the doors.

Lovely :-)

| These make cabling across the doorways very difficult/untidy. | So the main route for cabling would have to be round the periphery | of the flat and the ring would have to turn round and fllow its | route back to the consumer unit when it reached the exterior door.

You don't have to use a ring, you could use radials, you just have to watch the floor area limits. Then you can run round the perimiter, tee off as you like along the internal walls, without worrying about 'more spurs than sockets on the ring' or completing the ring. AFAICR a single 4mm (for a 32A radial) takes less space in trunking than 2x2.5mm for a 32A ring-and-back-again.

| I can see the advantages of your suggestion. Skirting trunking would | greatly simplify the cabling for the ring main. But the cost of | replacing all of the skirting with trunking would be very high - | unless you know where to source the trunking at a reasonable price.

It is, I agree, not cheap. But if it tidies not just mains but phone, computer, tv, intercom, etc etc away, might be worth it. And it's the sort of thing that is high mark-up so if you have a reasoably sized order for the whole job you can ask for discount from the wholesaler.

Starline is £35 per 3 metres from TLC. Mini-trunking on top of the skirting would be a lot cheaper, but less sightly.

Another idea, if you have the headroom and can shorten the doors without them falling apart, is battens and chipboard to make a false floor. As well as providing a lot more freedom for cables and pipes this could be warmer and more resilient underfoot.

Owain

[stefek.zaba]

If price is an issue, making up one's own wooden "trunking" with small battens with a ply/hardboard cover and a lick-o-paint is a lower-cost option. You'd need to derate cables if you end up with several close together ("bunching"), and separate 240V cables from lower-voltage ones (network, phone, speaker and so on); and aesthetics-wise you wouldn't want this boxing-in to stand prouder of the wall than the existing skirting; but with nice attention to detail, you could make it look quite OK...

HTH - Stefek

[Andy Wade]

I'd be inclined to do insulation resistance and earth fault loop impedance tests on all the MICC circuits first - i.e. do yourself a periodic inspection report before going any further. If all is well, and the circuits aren't likely to be overloaded, then leave them alone - MICC is a very high quality wiring system. OTOH if any pot seals have failed and moisture has got in - the insulation readings will make this very clear - then perhaps the time has come for a comprehensive re-wire.

For your new CU, consider fitting a larger-than-2-way split-load unit to provide some spare ways and allow the gradual addition of new wiring as round tuits permit. For the N'th time an electric shower does not normally[*] need or benefit from RCD protection - supplementary bonding in the bathroom is the principal safety measure - and, as the OSG says, "30 mA RCDs installed to provide protection to socket-outlets likely to feed portable equipment outdoors should protect only those sockets." IOW your new kitchen ring should be RCD protected and the shower probably not.

[*] Assuming TN earthing. Refer to Table 7.1 in the OSG for details of max. circuit length /sans RCD/ as a function of cable size, overcurrent device and type of earthing.

[Kevin Webb]

Thanks for the many replies and suggestions. There are a number of good ideas here as to how I could rewire the mains with minimum pain. Does the fact that all the ideas relate to rewiring mean that I would be wrong to extend the existing wiring as I proposed in my original post?

KW.

[Kevin Webb]

Thanks. I will take your advice and get the system tested before I add to the existing set-up. Unfortunately I can only just fit a 2-way unit in the available space until I eventually remove the old fusebox. As I want to RCD-protect the new kitchen ring the shower will get the RCD protection by default. Presumably I could have a main switch, MCB for the shower and a dual purpose unit for the kitchen ring (MCBO?) but would I gain anything from that?

KW.

[N. Thornton]

Why are some places wired in micc? A lot of expense for what gain? I can understand it for fire alarm curcuits, but domestic sockets?

Regards, NT

[Owain]

"N. Thornton" wrote | Why are some places wired in micc? A lot of expense for what gain? | I can understand it for fire alarm curcuits, but domestic sockets?

In this case (floors and ceilings all concrete) the wiring was probably installed with the boxes against the shuttering and then the concrete poured around it. This means the wiring method has to withstand liquid concrete. Usually steel or PVC conduit is used, but steel conduit takes a lot of workmanship, and PVC probably not available at the time. MICC is very easy to run (especially hidden, where appearance does not matter), the effort is at the terminations, and there don't seem to have been an excess of them on this installation.

Also submains were often wired in MICC, so in a flats type installation the labour would already be working with MICC anyway and no extra skills would be required.

Owain

[Kevin Webb]

In this case the micc wiring appears to have been done after the structure was erected and before the (fairly thin) plastering was done. The common areas (stairways of the block) are surface-wired over brick and the cabling is buried in plaster via the shortest possible routes within the flats. I don't know what IEE standards would have required in the 60s but someone seems to have believed that using micc within the flats would meet those requirements and be cost-effective in these down-market flats.

KW

[Andy Wade]

RCBO (yes you could).

Not really.