wiring when boarding loft

[markjones]

Hi,

i have an old 1880's terraced house and want to board the loft. At the moment there is some old looking insulation (I think I will get some new stuff).

Anyway, the electric wires go under this insulation at the moment. I know this may be dangerous. Is it ok if the wires go over the insulation?

Also, some wires run on top of the joists. Is it better if they run on the sides of joists so I can lay the board over them?

Any help would be great!

 

[masona]

Cut out the slot on the ceiling joist for wire to run across (not too deep approx 10mm)and remember to mark it where it is before screwing the boards down.If you don't want to cut the slot out,you can add 2"x3/4"on top the ceiling joists and butt up to the cable wires on both sides.Obviously more work this way depending how many wires going across each joist or how many slots to cut out.As for the wires under the insulation,I can't see why it can be dangerous.( Breezer's might be able to correct me on this).

 

[markjones]

Great. thanks for the advice.

When I had the survey done it said that wires under insulation can be dangerous as they could heat up and cause a fire. I can't see it myself but want to get it right if I'm going to screw boards down.

Cheers

 

[steveb]

The Surveyor is partly correct. If you install cables in thermal insulation you need to derate the cable. eg if a cable is capable of carrying 10amps you have to derate the cable when designing the circuit and only load the cable with say 5amps. Im not sure of the exact derating factors but the above should hopefully give you an idea of why your surveyors is highlighting this item.

What you need to do is determine the loading on the circuits to be installed in insulation, the size of cable on that circuit. I'm sure if you obtain these details somebody on this forum will be able to assist in determining if your cable is suitable.

The problem with installing in insulation is that the cable is not able to discipate any heat.

 

[MANDATE]

There must be thousands and thousands of houses where insulation is above the fixed wires in the loft,however I can appreciate heat wiil disperse better on top.However I wouldn't class it as dangerous.
Yes it would be better to run wires on the side of joists and if you have to cross a joist try and make it outside the boarded area,failing that I would rather cut a slot in the board than in the joist, that way anyone would know exactly where it was. A slot in the joist be should covered with a metal plate to provide some safety against nails etc

 

[steveb]

Yes Mandate my house is included in those 1000's. When I looked at the loading compared with cable size there was sufficient capacity in the cable to allow for derating. This probably applies to many dwellings.

If the cable was fully loaded then there is a potential problem with over heating.

 

[masona]

Surely,boarding out the loft not going to help if the cables is underneath!!

 

[steveb]

I agree, but I'm not sure if further derating of cable is necessary in this particular example. I would have thought that if insulation derating value is applied then this will be sufficient