insulation over wires or not... Again?

[martlewis]

Hi,

Some of you may remember a few weeks ago I asked the same question and the topic turned into an argument between bickering members and was taken completely off topic. Though there were a few helpful posts my question was never really answered so I thought I'd give the forums another chance. In the past I've had really good advice and it's a shame if a good forum is going down the pan.

Anyway... I have a wood built shed that was wired 2.5mm t&e ring, 1.5mm t&e lighting circuit and a dedicated 16A supply for a table saw. As an after thought I'm now going to insulate the shed with loft insulation and plasterboard. This means the cables will be trapped between the wooden outside walls of the shed and the insulation in direct contact with it.

Is this acceptable as I've heard in the past you should not lay cables in a loft in direct contact with insulation?

 

[JohnD]

Cables have a "rated current" that they can take. This varies by cable size and method of installation.

For example, a 2.5mm PVC and Copper flate T&E clipped direct to a wall or joist, at ordinary 30 degrees C room temperature max and not insulated, is rated to carry 27Amps single phase. If it is installed directly in an insulated wall it is rated at 21Amps. If it is in conduit in an insulated wall it is 20Amps. This is so that the slight heating of the cable, by electricity passing through it, can escape fast enough to prevent it overheating. For 1.5mm T&E the figures are 20, 15 and 14.5A. These apply if the circuit is protected by a cartidge fuse or a B-type MCB as is usual in domestic installations, which I expect you have. A cable which is protected by a rewirable fuse is derated to 0.725 of the rating.

For cables which are partially buried in insulation, on one side or all round, are derated again according to the length of cable in the insulation. If it is more that 500mm you halve the rating. This is common in lofts.

Cables of different materials, or at different room temperatures, are rated differently.

It is worth remembering that lighting circuits (at 6A) in loft insulation start out well below their rating, so when you do the calculation they are often OK. However, immersion heater and electric shower cables are often run close to their rating and need to be increased to a larger size.

There are additional rating factors for multiple cables within a singlr conduit that usually don't apply in domestic installations, but are worth knowing about so you can avoid them. It is good practice laying cables so that they are not buried in insulation.

The derating calculation, if you are not used to it, is best performed at a desk with a handbook and paper so that you apply all the various factors without missing any.

 

[ricicle]

You will need to de-rate your protective device for the ring circuit from 32A to 20A as the cable will be totally surrounded by insulating/non - heat conducting material for more than 50mm.The cable (2.5) needs to be de-rated by 0.5 in this instance of its rating if it were clipped direct (27A x 0.5 = 13.5A)
A cable for a ring circuit needs to have a current carrying capacity of no less than 0.67 times the rating of the protective device.

32A x 0.67 = 21.44A ok for a clipped direct 2.5mm t+e (or in trunking )
20A x 0.67 = 13.4A ok for above application

 

[JohnD]

p.s. and this is not to do with insulation de-rating - you must not lay PVC cables in contact with expanded polystyrene insulation - this is because the plasticisers added to the PVC to make it flexible are absorbed by the polystyrene so that the PVC becomes brittle and may crack or break.
This is not usually a problem in lofts as fibreglass is common. There are polystryre beads in some cavity wall, though, and the slabs are handy in a shed apart from this problem.

 

[martlewis]

Would it then be better to unclip the cables from the walls an attach them on the surface of the plasterboard instead of in the wall?

 

[JohnD]

Yes, then they will not be buried in insulation. For neatness, you might consider mini-trunking. Put them at a height where they are unlikely to be knocked.

The de-rating for 2.5mm is then only about 1Amp for 2.5mm and about 0.5Amp for 1.5mm (sorry haven't got exact figure to hand) for the conduit/trunking on the surface of a wall.

 

[ricicle]

JohnD
2.5 clipped direct 27A, in trunking on a wall 23A so still ok as it is greater than the 21.44 req by 433-02-04

 

[martlewis]

No problem, the cables run about 5 ft up the wall anyway, just thought about making it neater by hiding cables.

btw, the insulation is the fiberglass type.

 

[securespark]

it's a shame if a good forum is going down the pan.

Part of what makes a good forum is that there are many people posting there. This is bound to flag up differences of opinion, especially if those posting are not giving best advice.

 

[JohnD]

Oh no it isn't.

 

[davelx]

Oh no it isn't.

Well, since I'm about to dive into a month of pantomime....


Oh Yes It Is!