Loft Insulation and 1mm Cable

[Nordberg]

Does reference method 4 on page 120 OSG mean that the cables in the loft must be in conduit or are they OK clipped direct to the sides of the joist as this will ensure that they are not completely surrounded by thermal insulation?

Assuming that wood is classed as "Thermally Conductive"? Page 117 OSG.

 

[Damocles]

Wood is quite a good insulator. Never lived in a log cabin? I think I remember once looking up the conductivity of wood and fibreglass blanket and they were quite similar.

 

[Nordberg]

I dont live in a log cabin either, but if anybody can clarify the first question i would really appreciate this?

 

[ban-all-sheds]

OOI - if it's a 6A lighting circuit, why does it matter?

 

[mapj1]

If the cable is being run at its limits, then you would be wise to consider that nailed to wood as being insulated on one side. However, most wires in the lighting circuit are nowhere near being run at maximum load, and don't even get warm to the touch, let alone approach the 70 degrees the BSI consider the upper limit for continuous operation. The other thing is the use of insulation like styropor and other foams that contain plasticisers that may attack the cable over the decades. In which case short lengths of conduit or capping slid over the cable to protect it are recommended.
In the idea; world run cables out of the way of the insulation, bit for lights don't lose sleep if you cant.
(10KW Showers on thr other hand, require serious thought)

 

[Nordberg]

So, just to recap then

If my 1mm lighting cable is clipped to the side of the joist in the loft space and then covered with loft insulation, it is then considered to be completely surrounded by thermally insulating material and should therefore be derated by a factor of 0.5?

The design current is a little under 2.2amps and would be protected by a 6 amp type b MCB. But if it is considered to be completely surrounded by thermal insulation then the current capacity of the cable will be less than the MCB.

Please spell it out in plain english as I seem to be having problems interpreting the brown books.

Cheers

 

[mapj1]

OK, the 1mm Tand E in free space would be good for ~ 14A, but even fully insulated, its good for ~7A, but you are only going to draw 2.2A so it will be fine. In any case, even if the load were unlimited, the 6A MCB would be fine.
Should be Ok.

 

[ban-all-sheds]

The design current is a little under 2.2amps and would be protected by a 6 amp type b MCB. But if it is considered to be completely surrounded by thermal insulation then the current capacity of the cable will be less than the MCB.

Do you not have any idea of cable capacities?

Please spell it out in plain english as I seem to be having problems interpreting the brown books.

No disrespect, but if this is the case then perhaps you shouldn't be doing electrical work...

 

[Nordberg]

Well pardon me for breathing!!!!

Whats the point in having a useful tool such as this when you cant get straight answers from certain people?

I passed 16th edition in December 04 but this dosnt make me an expert by anybodies definition.

I find the brown books a little hard to interpret in some areas and dont have much practical experience.

If thats alright with you?

 

[ban-all-sheds]

I find the brown books a little hard to interpret in some areas and dont have much practical experience.

If thats alright with you?

Err - no, actually, it's not alright, not when it comes to something as simple as looking up the capacity of 1mm² T/E in Table 4D5A, also reproduced on p126 of the OSG.....

 

[Nordberg]

Alright grumpy, im catching up slowly.