Downlights in bathroom with dimmer switch - Wattage?

[trev77]

Hi all, and what a great forum this is. We're doing up our 2.3 x 1.8 metre bathroom, and from reading various posts on here, this is what I think I'm doing:

1. Replace exisiting ceiling rose with a 4-terminal 5A junction box.

2. From this, run 1.5 mm cables to 4 separate 60VA transformers, which are in turn connected to 4 x 50W ELV halogen downlights, rated IP65.

3. In the loft, construct little boxes out of plasterboard to go round the lights.

4. Replace the lightswitch on the landing outside the bathroom with a dimmer switch.

My questions:

1. Should I a) run the 1.5mm cable from the JB to each transformer, or b) run cable from the JB to the nearest transformer, and from that transformer to the next transformer, etc?

2. When connecting the 1.5mm cable to the transformers, do I need JBs or will choc boxes be OK? In either case, I presume they should be fixed to joists or battens between joists?

3. The loft has chipboard T & G flooring - will the downlights get too hot if that flooring is replaced over them? How else could I allow airflow around the lights?

4. Would a 250W dimmer switch be man enough for the job, or should I get a 400W dimmer?

Sorry for all the questions, but hey, that's what a forum is for, right?

TIA

Trev

 

[didthathurt]

1. Replace exisiting ceiling rose with a 4-terminal 5A junction box.

Do I take it you intend to site the JB in the roof void and not in the bathroom?

In the loft, construct little boxes out of plasterboard to go round the lights.

If this is a straight forward roof void that should be OK - leave an air gap and ensure it's well ventilated too.

Should I a) run the 1.5mm cable from the JB to each transformer, or b) run cable from the JB to the nearest transformer, and from that transformer to the next transformer, etc?
When connecting the 1.5mm cable to the transformers, do I need JBs or will choc boxes be OK? In either case, I presume they should be fixed to joists or battens between joists?

1mm would do for this. Unless this is a set of lights designed to be linked another way, you can run a cable from fitting to fitting as long as you connect in parallel. I would use terminal blocks encased in this sort of thing for protection
http://www.screwfix.com/app/sfd/cat/pro.jsp?cId=A331811&ts=80760&id=46538
then you can just pop them up through the same hole the light will fit in to.

3. The loft has chipboard T & G flooring - will the downlights get too hot if that flooring is replaced over them? How else could I allow airflow around the lights?

Heat dissapation for the light and transformer must always be considered. This depends on how much space there is below the T & G. Loft spaces tend to have shallower joists that floors due to the load bearing nature of a floor. If it looks too shallow put a box that proitrudes thro the T+G (elsewhere the suggestion of clay flowerpots has beend made, I quite like that one). Other than that, I suppose you could create ventilation ducts below the T+G but ...

Would a 250W dimmer switch be man enough for the job?

Yes, but make sure the lights themselves are suitable for dimmer switch operation.

 

[Hairymary]

Just remember that these type of recessed lights are not energy efficient because
a) lamps have poor luminous efficacy ( ie they give off a lot of heat)
b) the nature of spotlights means that you have to install more than is ordinarily required to achieve a degree of reasonable illumination.
c) You have to remove the insulation above the ceiling to allow the lights to cool
d) you breach the ceiling plasterboard and need to 'house them' in a flowerpot or plasterboard box.
e) You increase the infiltration rate of outside air by cutting holes in the ceiling.

All of which add up to increased energy usage and higher running (lighting and heating) costs. You have probably gathered that I dont like 'em.

 

[didthathurt]

Hear Hear!!

 

[trev77]

Thanks for your replies.

Do I take it you intend to site the JB in the roof void and not in the bathroom?

Yes

you can run a cable from fitting to fitting as long as you connect in parallel

What does "connect in parallel" mean? In terms of the actual wiring, rather than generalisations about "in series" and "in parallel". I think there should be a useful picture in the FAQ, but it's not available.....the URL is http://imagehost.darkernet.co.uk/i/Series-Parallel.jpg but it just shows as a red cross here.

This depends on how much space there is below the T & G.

There is a space of 3 inches

Thanks for your input.

Trev

 

[didthathurt]

I wouldn't consider only 3" enough of a void to give the downlighters adequate ventilation or to keep the T&G safely clear of the sort of heat source that a downlighter is. BIGDave, on another thread, kindly placed this link
http://www.tlc-direct.co.uk/Main_In...ire_and_Smoke/Fire_Cover_Downlight/index.html
to a source of downlighter covers. If you look at the dimensions of the covers you'll see that the minimum space they occupy is 6" above the ceiling. Though you don't strictly need such covers, it does give you an idea of the sort of space you need around them. Any less and, at the very least, the transformers will pack in remarkably quickly.

To connect in parallel means that, at each junction, you use a 3 terminal block. You connect the Brown, Blue and Earth terminals of the inbound supply into the same connection as the Brown, Blue and Earth of the cable that runs to the next light. Into the other side of the terminal block you connect the Brown and Blue and Earth (if there is one) that runs to the downlighter in that position and put chocbox around the lot.

Its called in parallel because, if you picture the live and neutral wires as two parallel lines, like a railway track, then you are, figuratively speaking, hanging the light between the 2 so only the power used by that light runs through that light, the power being used by the rest runs directly to them through the main cable.

Also, I can't remember if I mentioned that ELV downlighters and bathrooms both make this notifiable work so, if you DIY you must inform your local Building Control Officer who will arrange to have the installation inspected for Building Regs purposes. Sorry, that's just the way it ius.

 

[trev77]

Thanks again, that makes connecting in parallel much clearer to me. Bit of a bind about the depth of the space being too small for adequate ventilation - i guess i'll have to cut through the chipboard where necessary to let the air in. Luckily the loft is only used for storage, so it shouldn't be too much of an issue.

Yes, I know about Part P and all that......mutter mutter Nanny State mutter mutter

Cheers

Trev

 

[Steve]

I think there should be a useful picture in the FAQ, but it's not available.....the URL is http://imagehost.darkernet.co.uk/i/Series-Parallel.jpg but it just shows as a red cross here.

darkernet shut down a few months ago. we lost a lot of pictures.

 

[trev77]

Just had a word with my friendly local electrical supplier, and he suggested wiring the cable directly into each of the transformers, rather that using choc-boxes or junction boxes at each of the 4 downlights....presumably this depends on whether there is enough room in the transformer to fit 2 cables where there is currently only 1 cable coming out of the transformer.

Is this common practice in the industry, and is it safe?

Also, what would I do with the earth from the T & E cables?