Protecting cables in walls/hiding conduit.

[jason61c]

A couple of rooms in my house are about ready for plaster. I've done a wiring plan, although I need to work out some routes etc.

I'd like to plaster some rooms before wiring. Its lime plaster and about 15mm thick total. Some older plaster that I don't need to remove is about 40mm thick

I was thinking about placing backboxes in position and running 20mm oval conduit to them so I can plaster the walls.

Would this be too tight for a 2.5mm T+E ring or a 4mm radial? It would also offer mechanical protection.

Any better ideas?

 

[JohnW2]

I was thinking about placing backboxes in position and running 20mm oval conduit to them so I can plaster the walls. ... Would this be too tight for a 2.5mm T+E ring or a 4mm radial? It would also offer mechanical protection.

Are you talking about installing the cables in the conduit before you plaster, or just plastering in the conduits and hoping that you'll be able to get cables through them subsequently?

Do I take it that you are familiar with the regulations regarding the routing of buried cables (which are not altered by having the cables in plastic conduit)?

Kind \Regards, John

 

[jason61c]

I was thinking about placing backboxes in position and running 20mm oval conduit to them so I can plaster the walls. ... Would this be too tight for a 2.5mm T+E ring or a 4mm radial? It would also offer mechanical protection.

Are you talking about installing the cables in the conduit before you plaster, or just plastering in the conduits and hoping that you'll be able to get cables through them subsequently?

Do I take it that you are familiar with the regulations regarding the routing of buried cables (which are not altered by having the cables in plastic conduit)?

Kind \Regards, John

Hi John

I'm hoping to plaster the conduits in place and cable at a later date. I need to do it this way as until I've pulled other parts of the house apart theres a few cable routes left to sort out... I'd cover the conduit with a metal capping if needs be.

I also need to decide on the best way to bury/protect so I can finish my circuit calculations.

 

[jason61c]

I was thinking about placing backboxes in position and running 20mm oval conduit to them so I can plaster the walls. ... Would this be too tight for a 2.5mm T+E ring or a 4mm radial? It would also offer mechanical protection.

Are you talking about installing the cables in the conduit before you plaster, or just plastering in the conduits and hoping that you'll be able to get cables through them subsequently?

Do I take it that you are familiar with the regulations regarding the routing of buried cables (which are not altered by having the cables in plastic conduit)?

Kind \Regards, John

Hi John

I'm hoping to plaster the conduits in place and cable at a later date. I need to do it this way as until I've pulled other parts of the house apart theres a few cable routes left to sort out... I'd cover the conduit with a metal capping if needs be.

I also need to decide on the best way to bury/protect so I can finish my circuit calculations. All cable routes will be vertical

 

[ericmark]

Pushing a length of twin and earth down straight conduit can be done but it would require access to it to ensure the cables are feed in smoothly and likely some Yellow 77 lubricant as well.

Even then often the conduit will move slightly and burst the plaster.

Singles in steel conduit go in easy but twin and earth can be a real pain even with steel conduit it seem to have far more friction than singles.

The problem today is the rules on metal in the wall and earthing so steel conduit needs to be earthed.

I have many times tried to replace twin and earth in capping and oval conduit and some times it works but 60% of the time one ends up having to patch the plaster.

As long as there is no paper on the wall and the plaster is good then coving up the channel where cables have been laid is not too hard. However if the wall have been papered then forget it.

 

[jason61c]

There won't be any paper. I find the rules slightly confusing, sometimes I read it as PVC conduit will be fine, other times I read it as the PVC would have to have some protection.

I've just found some 25mm oval which in theory means with a touch of lube the cables should pass quite easily.

 

[JohnW2]

Hi John, I'm hoping to plaster the conduits in place and cable at a later date. I need to do it this way as until I've pulled other parts of the house apart theres a few cable routes left to sort out... I'd cover the conduit with a metal capping if needs be.

Eric has given you some useful pointers. Covering plastic conduit with metal capping would serve no useful purpose.

I also need to decide on the best way to bury/protect so I can finish my circuit calculations. All cable routes will be vertical

Even if vertical, they have to remain in the 'safe zones' created by accessories (switches, sockets etc.), and will need to be RCD-protected.

Kind Regards, John

 

[ericmark]

In theroy conduit should be installed empty and wires pulled in after. In practice often the wires are feed into conduit first. If you look at the Guide the number of cables put into a conduit is quite low and there is always plenty of room left after the conduit is considered full.

If you take a length of plastic oval conduit on the ground and try feeding 2 x 2.5mm twin and earth as one would for a ring they seem to go in quite easy. But stand on a set of steps and try and feed in where the twin and earth has to be bent into a curve then straightened as it is fed in and it is really something else and often results in unnecessary expletives to be spoken by the electrician.

 

[JohnW2]

There won't be any paper. I find the rules slightly confusing, sometimes I read it as PVC conduit will be fine, other times I read it as the PVC would have to have some protection.

PVC is effectively 'no protection', and covering it with metal capping doesn't alter that. If the cable is not within safe zones and/or is not RCD protected, the only solution would be conventional ('heavy') metal conduit.

Kind Regards, John

 

[jason61c]

There won't be any paper. I find the rules slightly confusing, sometimes I read it as PVC conduit will be fine, other times I read it as the PVC would have to have some protection.

PVC is effectively 'no protection', and covering it with metal capping doesn't alter that. If the cable is not within safe zones and/or is not RCD protected, the only solution would be conventional ('heavy') metal conduit.

Kind Regards, John

The whole system is going to be on RCBO's, with RCD protection would straight conduit be ok or am I not reading it properly?

Thanks for your useful input by the way.

 

[ericmark]

Years ago the plasterers trowel would often damage cables before they were made live. Putting metal capping over the cables did two things. One it protected the cables from the plasterers trowel and two it gave a solid base for the plasterer to work on. However it also resulted in metal in the walls so at a latter date a nail or screw could cause the whole bit of tin to become live and not only that nail or screw became live but the fault could also be transmitted to other items which touched the metal capping. So in 2008 the rules were changed and if any metal is used in a wall then had to be earthed. As a result in the main we moved to plastic capping which tends to be hard to plaster over. There is nothing which says we must use capping and if the plasterers trowel can damage the cable it can also damage the capping and cable and really all the capping does is make it harder to plaster.

The oval conduit does tend to keep the pairs of wires together and does reduce the flexing while the plasterer is trying to cover them. There are of course other cables like ali-tube which can be dressed down the wall and are both easy to plaster over and also do not require RCD protection. But also they cost more. For a special like the supply to a freezer I would consider Ali-tube but RCD's have come on leaps and bounds and the debate seems never ending as to if to use RBCO, Ali-tube, or group many circuits on one RCD. The latter means your more likely to be aware should the power fail but the RCBO is less likely to fail in the first place. And really if a freezer is faulty then having RCD protection is good.

Planning is the hardest part of a re-wire and it is not helped as not all RCD's are equal some trip easier than others even though they have the same rating.

The radial needs to be shorter than a ring to stay within the volt drop but it does allow one to wire a room at a time each room having it's own radial. Testing radials is also easier. But 20A RCBO's bump up the price and so not easy to say which is best.

 

[JohnW2]

The whole system is going to be on RCBO's, with RCD protection would straight conduit be ok or am I not reading it properly? Thanks for your useful input by the way.

You're welcome.

Provided that the cable is routed in the prescribed 'safe zones' and has RCD (or RCBO) protection, then that is fine, regardless of whether the cable is, or is not, in plastic conduit.

Kind Regards, John

 

[JohnW2]

There are of course other cables like ali-tube which can be dressed down the wall and are both easy to plaster over and also do not require RCD protection. But also they cost more.

You always mention that when these discussions arise, but have you ever seen an electrician use ali-tube (or similar) cable in a domestic setting? AFAICS, the only real benefit is in terms of not 'requiring' RCD protection, but with the increasing uniquity of RCDs (and the requirement for RCD protection of virtually all sockets circuits, and some others) there seems to be little point. ... or am I missing something?

Kind Regards, John

 

[ericmark]

Reading the regulations is not easy. I will stand corrected on this but if you ran the cable down the corner of a room and at socket hight (assuming at least one socket on the wall) went horizontal to next corner that would comply as corners are safe zones.

However it is not what we expect and some one did it in my house and I ended up sawing through the horizontal cable with a hack saw blade when installing the water supply for the fridge. (This was actually two way lighting)

Following standard installation methods is the way to go as in the future people know what to expect. You can buy trunking designed to take sockets at any point so you can easy move them all very well in the office but not really what you want in the house.

I in my first house had power track on the kitchen wall so I could plug in anywhere along the track it seemed great but having to have the special plugs became a real pain so this house all 13A sockets.

 

[ericmark]

There are of course other cables like ali-tube which can be dressed down the wall and are both easy to plaster over and also do not require RCD protection. But also they cost more.

You always mention that when these discussions arise, but have you ever seen an electrician use ali-tube (or similar) cable in a domestic setting? AFAICS, the only real benefit is in terms of not 'requiring' RCD protection, but with the increasing uniquity of RCDs (and the requirement for RCD protection of virtually all sockets circuits, and some others) there seems to be little point. ... or am I missing something?

Kind Regards, John

I agree the idea is sound but the cost of Ali-tube and the problems of buying short lengths mean in real terms more trouble than it's worth. I did say freezer which as long as marked freezer only does not need RCD protection.