flush light fittings and insulation

[aidf72]

Hi,
I need to get someone in to change lights in zone1-2 in the bathroom. I've deliberately delayed from laying new roof insulation to make it easier for whoever does it. 2 questions. First, how do you guys deal with cross-layed insualtion when asked to do these sorts of things? Second, given that all the literature says make sure wires arent covered by the insulation, are flush-fittings that push up into the roof space safe?
Thanks,
Aid.

 

[bobapcoed]

I installed them with existing insulation, and just removed it from around each fitting, in order for the heat to dissipate. It's not a problem covering the cables.

 

[bernardgreen]

It's not a problem covering the cables.

When the cable is covered with insulation its safe maximum current carrying rating has to be reduced.

This is because the cable warms up slightly when carrying current and this heat has to be disapated.

So covering the cables if they are already running at maximum rating IS a problem

 

[dingbat]

I got called to a number of jobs in the last couple of years where the bathroom lights had suddenly stopped working. After a few of these my first question became, "how recently did you top up your loft insulation?".

The secondary side of the ELV lighting set-up will be carrying twenty times the current on the primary side. Thermally insulate these cables and you may well melt the cable insulation and short out the transformers. It is an issue.

 

[aidf72]

So would it be best to cut out a portion of insulation to match the fitting size, and leave that area open and accessible?
Since the rafters are now buried under an 170mm additional insulation, securing cables against them would mean they were buried, so should l let cables just lie on the top of the insulation?

 

[bobapcoed]

Basing on my experience, and last time I looked at Ohms Law, 100 watts of LV lighting. at 12 V

100=12xI, therefore I= 100/12=8.5 Amps, well within the rating of 1.5mm cable which last time I looked was about 21Amps, although it was a long time ago!

 

[bernardgreen]

Basing on my experience, and last time I looked at Ohms Law, 100 watts of LV lighting. at 12 V

100=12xI, therefore I= 100/12=8.5 Amps, well within the rating of 1.5mm cable which last time I looked was about 21Amps, although it was a long time ago!

And a volt drop of 36 mV per Amp per metre according to the tables

equates to 306mV ( 0.3 volts) per metre at 8.5 amps

8.5 amps and 0.3 volts is 2.55 watts per metre.

Bury that in thermal insulation and it gets very warm.

The inside may be hot.

Warm PVC gets brittle much faster than cool PVC

 

[bobapcoed]

I make it .26 volt drop (8.5x306). Transformer and fittings I couldn't agree more, but the cable, depending on size and run, should be such that it isn't a problem. If it's a long run, or a large no of fittings, then up the cable size.

 

[bernardgreen]

I make it .26 volt drop (8.5x306). .

And a volt drop of 36 mV per Amp per metre according to the tables

8.5 times 36 equals 306 so the volt drop over one metre passing 8.5 amps is 306 milliVolts or 0.3 volts

 

[Spark123]

Given 4% of 12v is 0.48v doesn't give much of a run!

 

[bernardgreen]

Given 4% of 12v is 0.48v doesn't give much of a run!

The reason why some halogens are now supplied by modules with a sense wire back from the lamp to measure the volt drop and compensate for it by increasing the output voltage.

The worry though is the heating of the cable.

 

[Spark123]

Like some of the switch mode power supplies? How do they get around having lamps at varying lengths along the cable?

 

[bernardgreen]

Like some of the switch mode power supplies? How do they get around having lamps at varying lengths along the cable?

They don't. They rely on the cabling being equal to all lamps and they sample from one of them.

 

[aidf72]

soooo, was that a "Yes, cut away a section of insulation so the light fitting is uncovered" or "No, don't worry about the insulation" ?

 

[dingbat]

That was a 'don't install halogen downlights, they are nothing but trouble in a faddish package'.

If you must, you need to clear the insulation around the transformers, the secondary cabling and the lights. Then, reinstate the insulation in order to comply with Part L of the Building Regulations. You can do all this by supporting the insulation a few inches above all your wiring gubbins by building a structure on top of the ceiling joists for the purpose. You can buy purpose made galvanised sheet, fold-to-fit enclosures or you can use timber and plasterboard. Either way it's a fag and makes the cost of the downlighters themselves an insignificant part of the total cost. (I charge £50+ per point for installation of the 'orrible things!)