Insulation and plumbing

[Jupiter01]

The old chestnut I suspect...
I have twin and earth cables (2.5mm and 1.5mm) clipped to my floor joists and the roof joists. I am planning to put rockwool in both. What is the definitive position with covering cables with insulation?

For instance, I am planning to have around 300mm of rockwool in my loft. There will be cables amongst this according to my current plans.

Thanks in advance.

 

[flameport]

If cables are covered with insulation, the maximum current they can carry is reduced significantly.
Whether this is a problem or not depends on the cable size, protective device and the load the circuit is designed for.

 

[Jupiter01]

Hi,

Does it also present a fire hazard?
I think my ring mains are minimal in length and number of sockets, relatively speking but nonetheless, can I take any particular precuations? i.e. leave some clearance, loop cables abve insulation, etc.

Thanks for your help.

 

[sparkwright]

Well, the experts who install loft insulation professionally for a living just seem to lay the insulation over and against all electric cables.

When insulation first made it's appearance, it was so thin that usually it avoided cables, as typically they were clipped near the top of the joist.

Nowadays, as you have already indicated, the insulation is so high it will be in contact with just about every cable.

I think we decided in the past that lighting cable is typically large enough to cope with loft insulation anyway.

Other cables could technically be a problem, but as you say, how are you supposed to seperate these from the insulation, without a load of ag?

Where does the 'plumbing' bit in the title come into this? Mistake I reckon.

 

[Jupiter01]

Yea mistake on the title sorry . I doesn't let me edit it unless I'm missing something?

So, I'm concluding that its not a fire hazard?

 

[endecotp]

Why are you concluding that it's not a fire hazard?

As flameport said above, we need to know cable sizes and fuse or MCB ratings for the circuits. Someone might then work out how much insulation is safe for you.

 

[Jupiter01]

I thought it doesn't matter since all the insulation companies are comfortable with piling on the insulation...
The cables under the insulation will either be the 1.5mm lighting cable or the 2.5mm ring main cables. There are two ring mains upstairs, I would estimate that there will be aroud 15 metres of ring main cable under the insulation.

The breakers can be whatever rating you advise as they aren't in yet. These cables are for new cirucits that are being laid. I wil try to keep cables above the insulation wherever posisble but i know that this wil be unavoidable in many situations.

 

[stillp]

I thought it doesn't matter since all the insulation companies are comfortable with piling on the insulation...

That's because they're insulation companies, not electricians!

 

[sparkwright]

You never said it was new wiring just installed.

In that case, provision could have been made to avoid this problem.

A larger cable size could have been used, there's calculations for this, often results in going up a size.

 

[stillp]

These cables are for new cirucits that are being laid.

You might need to wire them in 4mm².

 

[EFLImpudence]

I thought it doesn't matter since all the insulation companies are comfortable with piling on the insulation...

They don't care; it's not their house.

They fill the wall cavities with insulation with no regard for what may already be there.

The cables under the insulation will either be the 1.5mm lighting cable or the 2.5mm ring main cables.

As above the lighting will be alright as you are fitting hugely oversized cable for it.

There are two ring mains upstairs, I would estimate that there will be aroud 15 metres of ring main cable under the insulation.

The same cannot be said of the ring final circuit.

The breakers can be whatever rating you advise as they aren't in yet. These cables are for new cirucits that are being laid. I wil try to keep cables above the insulation wherever posisble but i know that this wil be unavoidable in many situations.

Yes, install it on battens above the insulation.

 

[endecotp]

Well if it's not in yet, then you just need to design it properly! (Your first post didn't make that clear, though - you implied that the wiring was already present.)
Work out the load, observe the installtion method, and then look up the appropriate cable size in your on-site guide.

 

[Jupiter01]

Sorry for not being clear. These are new circuits but some of them have already had cables laid down and clipped to joists....
I will try to raise these above the insulation and of course put any new ones above the insulation too. I like the suggestion of battes above insulation

 

[endecotp]

Get yourself a copy of the IEE on-site guide. It is full of tables telling you things like the max current a cable of a certain size can carry under some depth of insulation.

 

[ban-all-sheds]

Jupiter - if you are installing new circuits why don't you already have a copy of the OSG and the Regulations?

I am planning to put rockwool in both.

Has this come as a surprise to you?