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Cables and Insulation


daiking

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Why do electricians never allow enough slack on loft cables so they can be raised above 300mm of insulation. Its not as if 10”-12” of fluff is the cheapest and quickest way to meet a government requirement, oh wait…

 

I’m not too fussed about the lights, low load, yadda, yadda. However the electric sockets for this room are distributed via the loft not under the floor. I’m not sure if it’s a radial or a ring main circuit. One of the cables has actually been fed through a hole in the ceiling joists. This seems like complete *******ery to me, when all the other cables are laid above the joists. As well as the unnecessary damage to the joists it makes it difficult to lay the insulation as there’s a 50mm gap between the cable and the ceiling. I either squeeze the 100mm loft roll under it and lay 200mm above it or just put the full 300mm over it leaving a small air pocket under the cable.

 

As I understand it, the cables should be in contact with a surface like the ceiling or a joist OR they should be open to the air so as not to overheat. Although you would normally consider a bedroom to be a low electrical load room – I must consider the use of more than one high load device: hair dryer, straighteners, curlers, rollers, iron etc.

 

I’ve slapped 100mm of roll over it for the moment as it was bloody cold without it but I wonder what is ALARP before I put the other 200mm over it?

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8 hours ago, daiking said:

Why do electricians never allow enough slack on loft cables so they can be raised above 300mm of insulation. Its not as if 10”-12” of fluff is the cheapest and quickest way to meet a government requirement, oh wait…

1. Cos they could not predict the future - of insulation.

2. The client didn't want to pay for copper they didn't need although the cable supplier would be very happy.

3. They worked out that across all their jobs the resulting voltage drop would be wasteful, requiring more power stations then heretofore even if it made little difference in this one job. (Green before their time so perhaps they could have predicted the future!)

4. Nailing cables onto sky hooks not taught on their ONC / NVQ.

5. The boss worked out that across his lifetime the resulting increase in costs associated with the cable clips would mean he / she would not be able to: get new car / retire early / take up with a toy boy / girl - dump the wife / husband and still have something in the bank....

I could go on - perhaps somebody will.

 

HOWEVER this does not mean I don't feel your pain - empathy is everything you know.

 

 

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3 minutes ago, ProDave said:

So you are saying this is a new build, wired like that and no insulation fitted?

 

Best compromise, unclip the cables, lay them flat on the plasterboard and insulate over?
 

 

The transverse cable is through a hole in each joist so cannot be laid on the ceiling.

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14 hours ago, ProDave said:

So you are saying this is a new build, wired like that and no insulation fitted?

 

+1 to that question. Daft electrician?

 

15 hours ago, daiking said:

is under insulation ok?

 

I found this study awhile back..

 

http://electrical.theiet.org/wiring-matters/15/current-ratings.cfm?type=pdf

 

Basically it says cables under insulation might need to be derated by reducing the circuit breaker value. Otherwise the wire might have to be fatter than the standard 2.5mm^2.

 

6) Where the cable of a ring final circuit protected by a 32A fuse or circuit-breaker is installed fixed to a joist and with more than 100mm of thermal insulation the installation designer may have to consider using a cable larger than 2.5mm2.

 

The exception was some lighting cables where they say..

 

Quote

5) Where a lighting circuit, protected by a 6A or 10A overcurrent protective device, is fixed to a joist under thermal insulation, a 1mm2 or 1.5mm2 flat twin-and-earth cable can be used providing it is not subject to other derating factors

.

 

 

Edited by Temp
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3 minutes ago, Temp said:

 

+1 to that question. Daft electrician?

 

 

I found this study awhile back..

 

http://electrical.theiet.org/wiring-matters/15/current-ratings.cfm?type=pdf

 

Basically it says cables under insulation might need to be derated by reducing the circuit breaker value. Otherwise the wire might have to be fatter than the standard 2.5mm^2.

 

 

 

 

The exception was some lighting cables where they say..

 

.

 

 

 

No offence to electrician's here but they're all daft. 

 

That's what the book says but in the real world, what should I do? Over? Under? Putting the cables in contact with something as a heat sink (spare UFH spreader plate)? Chop and add a length with JBs either end?

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15 minutes ago, Temp said:

If it's just one or two rings I would lift the cables above the insulation and put JBs at each end. The JB's should be "accessible".

 

To be honest I'm not sure if its ring or radial or whether that would make a difference. It is only this one cable that is trapped by the joist that I am really concerned about. 

 

Also aware about accessible cable connections and MF for inaccessible. If I did lengthen the cable, I'd probably run the cable along the purlin way above the insulation. As such, although its the loft, I understand I could use standard screw terminal JBs as it is accessible. 

 

This or this would be fine?

 

http://www.screwfix.com/p/chocbox/54936

http://www.screwfix.com/p/debox-in-line-connector-box/8692h

Edited by daiking
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