Cables in insulation

[SALL2009]

I have 2 power cables for mains sockets running through insulation in a stud partition wall. Only around 6 inches of the cables are in the insulation. The insulation type is wool (rock wool – typical yellow stuff)

My question is, do the cables need to be covered with anything or not?

 

[ban-all-sheds]

Are they surrounded by insulation, or are they touching the plasterboard?

How thick is the insulation

Who installed the cables, and what does the Electrical Installation Certificate say about that circuit?

What size cables are they?

What is the rating of the fuse/MCB?

Is it a radial circuit or a ring?

 

[SALL2009]

Are they surrounded by insulation, or are they touching the plasterboard?

How thick is the insulation

Who installed the cables, and what does the Electrical Installation Certificate say about that circuit?

What size cables are they?

What is the rating of the fuse/MCB?

Is it a radial circuit or a ring?

I'll try to answer what I can.

I haven't boarded one side yet, so they can be touching plaster board at one side.

Insulation is 100mm, but I can reduce it to 50mm in that area.

The house has been recently rewired, but not sure who did it. (I have just bought it.

I’m not an electrician so don't know details of the circuit. But its one cable coming into the wall socket and then one going out (spur) to a wall socket in another room.

All sockets of the house are on one fuse.

The cables were there from the start. It used to be a brick wall, which I have replaced with a stud wall.

 

[STI]

Hi when you say do they need to be covered do you mean with some form of protection or do you mean with the insulation. Do the cables run vertical and or horizontal to the socket

 

[SALL2009]

Hi when you say do they need to be covered do you mean with some form of protection or do you mean with the insulation. Do the cables run vertical and or horizontal to the socket

What I meant was, do they need to be covered with something else to protect them from insulation.

Cables come from under the floor boards and then run vertically up to the socket. The run between floor board and the socket is around 6 inches.

 

[STI]

On the assumption we are talking standard cable ie grey or white sheath covering the 3 other wires then i'd say no. If the wires run vertical directly to the socket outlet they are in a safe zone.

Strictly speaking if the cable is touching the inner wall surface then the current carrying capacity of the 2.5mm (I assume) cable drops to 21A and if it isnt touching and just suspended in the insulation then it drops to 13.5A.

But and here i would bow to people with more knowledge if you are talking about a run of 6" i am not sure how much you need to apply the derating.

 

[PrenticeBoyofDerry]

SALL2009,
From what you have said, I assume you have a socket circuit that runs through thermal insulation 150mm up to socket then returns 150mm down.
If the cable is totally surround for this length, a derating factor of 51% will be applied as the run is more than 200mm through the insulation.
So if the circuit cable size is 2.5mm and T&E the capacity will be reduced from 27amps to less than 14amps, if it is 4.0mm T&E 37amps will be reduced to less than 19amps.
So you have problem, to remedy this the cable must not be totally surround or it can be run in conduit which will have a derating factor but not as large.
27amps will become 20amps and 37amps will be reduced to 26amps.
But the next thing we will need to know is the size breaker on this circuit and the size cable?

 

[SALL2009]

Untitled

• SALL2009
• 3 Oct 2010
• 1

Here is the picture of the cable. think it is 2.5mm . As I looked at it again to take picture. The run of the cable in the wall is only 4 inches up and then back down. The picture is the back view. I have boarded the socket side.

I will look at the size of the breaker on the consumer unit later today and post.

 

[SALL2009]

Untitled

• SALL2009
• 3 Oct 2010
• 1

Here is the picture of the consumer unit. The big switch with the test switch says 80A 30mA. I'm not sure if this is the breaker size for all three circuits on the right.

The middle one is all the sockets of the house. The other two (electric shower and stair lift) are no longer in use and are terminated under the floor boards.

I hope that all the info needed.

Do the cables need to be in conduit? If so, can I run both cables through one conduit?

 

[SALL2009]

any thoughts?

 

[skenk]

any thoughts?

you need to follow relevant instructions above to make sure cable is not derated to any less than 20A, which is the minimum for cable on a 32A ring

One 32A socket circuit for a house is bad design so you should probably have run a new circuit instead of extending existing

If it was rewired 'recently' it probably should have a separate socket circuit for the kitchen

Live cables should be disconnected and made safe at both ends not just terminated under the floor

 

[flameport]

The solution to the original question is simple.

Push the cables against the plasterboard so they are in contact with it, and shove a load of plaster or cement over the cables so they are completely covered.
They are therefore installed as method C, no derating required, and any remaining space can be filled with as much insulation as you want.

 

[PrenticeBoyofDerry]

The only other thing to do, if not upgrading cable size, would be to split the ring in to two radial circuits protected by 20A breakers. With the the thermal insulated runs installed in conduit.

 

[SALL2009]

any thoughts?

Live cables should be disconnected and made safe at both ends not just terminated under the floor

This is what I plan to do.

Turn the breaker off at the consumer unit.
Attach a junction box to the loose end of the cable and secure the lid of the junction box.

Isn't that making it safe?

 

[SALL2009]

As for the original question, I will place another piece of plaster board next to the cable and then put insulation in. So the cable will have plaster board on both sides.