Low voltage lighting - too many transformers!

Postby matchmade » Sat Jan 26, 2008 11:06 am

I'm specifying the re-wiring of a house where there will be 21 downlighters, most of them on the downstairs lighting circuit. As the ceiling space is generally inaccessible, to get low-voltage downlighters (lower electricity bills, less carbon footprint blah blah blah), I have to have a transformer for each downlighter. This will cost an extra £8 per light, or £168 + VAT even with a trade discount.

In my bathroom, I can have one transformer per three downlighters, so it will be one-third the additional cost.

I'm reluctant to use the low-voltage lights because the extra cost will completely destroy the economic logic, plus any carbon saving is immediately made pointless by all the extra copper, other components and the carbon cost of manufacturing the transformer.

My question is: why can't I just have one larger transformer at the fusebox so the whole ring is taken down to 12V? I can then use a step-*up* transformer on the few lights on the downstairs ring that aren't downlighters.
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Simply Build It

Postby 333rocky333 » Sat Jan 26, 2008 3:43 pm

How would you make this 12 volt ring
As each lamp could possibly have a 50va lamp fitted
If there was 15 fittings there is potential of over 60amps running via that cable
The size would be over 10mm I guess
As yet not widely used, if wired as a ring it would reduce size of the cable.

Designers apparently are starting to use the ring format for mains lights ,to reduce cable sizing on large buildings
But would assume proper calcs required to work out cable sizes

Also how could you ensure integrity of ring
How would you connect each light to the ring ,Each one would be very highly fused from the source .

I once see a 12 v downlighter wired to a single output 500 va the small lampwires shorted out , the short circuit current melted 4metres of 1mm flex before blowing out the side of the cable

Unless designed correct ther seems high fire risk

WHY not wire one or more multi output large trans for the 12v.
This set up is used in shops and the tran is sometimes 4metres from the light

Wire 240v lights seperate

Trans come from 20 va to over 500 va suitable for low volt lights ,
provided each output is protected seperate within the tran and the correct thick 2core l/v cable used should be ok.

A single output large tran can be used,it would need to connect to a splitter unit to protect each light and wiring

However soft start tran recomended for the larger ones as the start surge trips mcb's sometimes.
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Postby ericmark » Sat Jan 26, 2008 3:52 pm

Cable size and volt drop. 12 volt needs 20 times the current than 240 volt simple 240 divided by 12 = 20. Yes I know we have 230 volts but makes it easy. Every tungsten lamp takes more current when cold adding on to that every transformer takes extra current on switch on so the more switches the better but multi-transformers on the same switch is no real advantage. Using electronic switch mode power supplies both reduces the inrush on start and compensates for volt drop or surges and are made to fit through the whole the down lighter goes into but they have a min as well as max amps so have to be better matched to circuit. On the other hand the 230 volt bulbs GU10 type have no control gear so can be changed from 1 to 75 watt with LED and Cold Cathode plus tungsten all fitting the same unit. There is also a GZ10 this is to do with heat being reflected or passing through the light reflector. The former bulbs fit both holders the latter only fit the special type. Down lighters designed only for discharge lamps are required to be fitted to so many of the lights in a house. And because that is design the builder does not have to calculate the current if the fittings are changed. 21 down lighters could be 21 watt on the other hand they could be 1575 watt and that's 6.8 amp and normally only 6 amp mcb's are fitted. How a down lighter can reduce a carbon foot print unless LED or Cold Cathode I don't know and I have not seen 12 volt versions of these lamps. My question is why do you want 12 volt except in bathroom? Far more important to select fittings that when they are used with the slightly longer LED and Cold Cathode lamps don't look unsightly. As to Carbon foot print only when wearing wellies! The rubber is mixed with carbon to stop static build up and sparks but don't think anything else does that so I have just stopped wearing wellies!!!!!

Postby sparx » Sat Jan 26, 2008 9:22 pm

Hi, first of all how do you save electricity by using LV lights? they use more than mains ones! ie 50w mains is 50W, 50w LV is 50W plus losses in transformer! the operating voltage is not relevent.
In answer to your ? you could use one transformer in theory but, it would be HUGE, as would cables from it to lights due to voltdrop.
EG say 12 lights @ 50W each = 600W @ 230 v = 2.6A so a 1.5mm2 cable would be plenty, however 600W @ 12v = 50A so would need to be wired in 10mm2 at least! a bit of a game to connect!
this is reason when buying triple down lights on 1 trans. the link wires are so short. The only place I personally will fit LV downlighters is in a bathroom if pushed by customer, Hate the damn things with a passion due to so many probs. with them, fortunately soon will only be able to fit LED or CFL types which are horrible and maybe just maybe folks will go back to proper surface fittings... some hopes,
regards SPARX
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Postby matchmade » Sat Feb 16, 2008 5:58 pm

Thanks to all who replied.

Sparx: sorry, stupid of me to think the voltage mattered on energy useage - so why does anyone use LV lighting at all?

By surface fittings do you mean so-called "lumiaires" in bathrooms? What are your preferences then for reception rooms? Downlighters have become ubiquitous in kitchens and are now often in reception rooms now, for feature lighting and as an alternative to pendants or surface-mounted fittings.
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Postby sparx » Sat Feb 16, 2008 8:18 pm

Hi, sorry personal preferences should not come in to it,
I am old fashioned enough to use pendants,
regards SPARX
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Postby ericmark » Sat Feb 16, 2008 8:53 pm

In theory the brighter quartz lights are more efficient than tungsten but by such a small amount unless they were very carefully set up one is unlikely to get any gain. but once one gets into colour correction and light temperature it becomes really complex and add to that the fact we can see more in orange light than other colours it starts to get very murky. As to saving energy that's another question as if heat is required then cold light does not save energy only in hot countries where they used air conditioning to remove the heat or outside does cold light come into it's own. Each energy system has its best use of course the gas lights we had in the old caravan would keep the caravan nice and warm in the evening without the need to use a fire but when we got new caravan with loads more insulation and electric lights we needed the fire on in the evening. In a kitchen where we have so much heat already then florescence fittings must be best as cool yet loads of light no way would you want spots there. But in living room or bed room spots to read with makes loads of sense giving local heat as well as light so allowing lower temperature for room as a whole. Transformers however do not project the heat to where it is required so except for safety can't see the point in extra low voltage lighting. I agree with sparx and I also use in the main pendants for lighting except for reading lamps for use in bed, kitchen, and landing the latter has emergency lighting so one can see to go down the stairs if there is a power cut.

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