Advice on wiring concrete flat with conduits please

[happyhero]

Hi, can someone advise me please, I have been asked to rewire one of those all concrete purpose built flats with conduits in the concrete from when they were built. I've not done one before, so I am thinking that I will be running in singles possibly using the old cables as draw wires, are these places really easy because of the existing conduits or really awkward to do, I have had conflicting information from others.

In theory I would expect them to be really easy but in practice I suspect there may be problems. I had to cut the ceiling for someone in one of these places a while back and used a diamond blade in a small 4 1/2" grinder, I was amazed how hard it was to cut, full of hard stones etc.

Any info/advice appreciated.

 

[sparkwright]

This will depend on the type of conduit used/age of the building.

If it's in very narrow metal black 'grip' tube (1930ish) where the elbows are fitted with screws then it's impossible to re-use it.

If it's more recent and is half inch to 20 mm conduit (metal or plastic) then it should be fairly easy to do. If metal tube it's quite likely there is no separate cpc, and the earth is achieved from the conduit. Would be an idea to include a separate cpc.

If plastic tube there is always the possibility of a crushed or broken join in the conduit where it may have got stood on before the floor was screeded.

Many rewires of this nature need extra sockets, so you may not have conduit and socket boxes where you need them.

Keep all old screws from the accessory boxes, particularly the ceiling rose besa boxes - they are probably imperial and you may find it a struggle to replace them.

You may also find you need to run other cables that the building never had before, like 10 mm earth to water/gas or smoke detectors.

If new kitchens are to be fitted the existing conduits may have to be adapted/removed.

Is there a requirement to change the height of the sockets and switches?

The bedroom ceiling roses may be near the windows.

The switch boxes may be of a different size or shape if before 1950s.

The accessory boxes on the walls may be round conduit boxes if quite old.

 

[flameport]

In many cases, it will be possible to pull new wires in using the old ones.
Generally you will want to pull out most of the old wire first, leaving one which is then used as a draw wire.

The conduit will be the CPC if metal, however a separate CPC should be drawn in with the new wires. This won't usually be a problem as the older wires will probably have much thicker insulation than the new.

The only issues will occur where excessive corrosion has occurred, typically in bathroom ceilings or outside balconies.

I had to cut the ceiling for someone in one of these places a while back and used a diamond blade in a small 4 1/2" grinder, I was amazed how hard it was to cut, full of hard stones etc.

Concrete slab ceilings and floors are never cut, as they are structural components. If the conduits can't be reused, then surface fix the new cables and overboard the ceilings.
Same applies to floors.

 

[Taylortwocities]

I know I always seem to be a Grumpy Old Spark but my advice on this is as follows:

If you are not fully committed to do this job don't just walk away from it, run.

If, for some reason, you feel that you still want to do it then make sure it is on a time and materials basis. If you do a fixed price for a job like this you will lose money.

Any thoughts of using the existing conductors as draw wires are pipe dreams. It is not going to happen. The conductors will be wound round each other, and you will not get a good pull past joint boxes that have been used as through routes. Many of those boxes will be buried in the concrete and will not be accessible..

 

[holmslaw]

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[holmslaw]

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[bernardgreen]

Perhaps you could explain how the original wiring was installed through these buried inaccessible boxes.

Assembled as a loom in conduit and put in place before the concrete is poured into the framing ( moulds ).

I have seen that method in an apartment block being built in Spain and in several factories around Europe.

 

[Taylortwocities]

Thank you, Bernard!
Perhaps things are done in a different way - if you live in a grass hut.

 

[bernardgreen]

My pleasure.

Never buy an apartment in Spain if you want to own it for longer than the life time of ( cheap ) PVC cable.

On recollection "loom in conduit" had to be the way an Ex MoD block of flats had been wired. The flats had no visible conduit in the cast in place concrete ceilings floors. There had to be hidden junction boxes on both lighting and power circuits as few if any of the switches / sockets had more the than one wire per terminal. Factory assembled looms enabled rapid on site installation for what were at the time intended as temporary " for the duration " (of the war) accomodation units. Quite likely the method was then carried on into the rebuilding of housing.

 

[holmslaw]

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[holmslaw]

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[Alarm]

Either way the place needs a rewire.
OP price it with surface conduit, let them worry about covering it.
lan it right and in the future they CAN pull thru if needed.

 

[bernardgreen]

I have seen that method in an apartment block being built in Spain and in several factories around Europe.

But you've never seen it used in the uk, have you?

As I posted I am certain it was the way some MoD buildings were constructed. The flats I referred to were in London E11. I discovered it when asked to add some power sockets for a family friend who lived there. I declined to do it. Later when replacing a lamp switch I found only two wires to the lamp roses and only two wires to the switches. No visible trace of looped lives or neutrals.

 

[sparkwright]

Apart from the the ancient narrow conduit where you have to wire it as you screw the elbows on, a proper conduit system should have no inaccessible inspection boxes.

Round conduit should be installed so it can be rewired - that's the point of it.

There may be hidden joins in the conduit and the wiring where alterations have subsequently been done, badly.

If the conduit has been done properly, and years ago it probably would have been, there should be no problems drawing in new wires.

I recommend getting a helper to assist you in drawing in the wires.

 

[sparkwright]

Either way the place needs a rewire.
OP price it with surface conduit, let them worry about covering it.
lan it right and in the future they CAN pull thru if needed.

What kind of advice is that?

If the conduit can be reused, and if it's in a proper conduit system it can, why consider fitting and charging for surface mounted conduit everywhere? In a flat.

No only would the client be paying for all the new conduit and fittings, they'd have to get someone to cover it all up, according to you. And I don't think a builder would be able to do this as the all the door frames, skirting boards and everything would have to be drastically altered.

The builder would just say, 'Get the electrician back to do it properly'.

Rightly so.