Excuse my ignorance - I've never run DC cable before!
I'm installing some of these...
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ways of grouping them are possible, using various transformers (e.g. one 0-250W, a few 0-50W and 0-100W).
I've done some quick calcs of the required cable size, and it seems stupidly large.
What size cable do people use in practice? How do people usually group the lights? Big groups / small groups? Daisy chained cable or star wired? Also, what _type_ of cable? What core colours for after transformer? (On site guide allows almost anything for ELV).
One transformer per light. The cost is minimal. If you group several lights on a 'large' transformer and that goes fsulty (it does happen!) you've lost several lights until you get a replacement.
I have severa lv lights, in the kitchen and also scattered around the outside of the bungalow, set up in the soffits. Each one has its own transformer, typically 50-70w for a 35 w bulb. Not had any problems at all.
It's the transformer that limits you: our kitchen ceiling has two rows of 3 with two 60W transformers. That was the electrician being unnecessarily stingy, as it is quite difficult to tell the lamps apart, and when a 30W one got into one of the holders, the set cycled on and off with the overheat (presumed) cut out, and I had the floor above up, before I realised...
Thus, when I did the bathroom I allowed for some particularly light obsessed individual being blind enough to put in a 50W lamp, and so allowed one 100W transformer for each pair - though still only need 20W for most purposes, but one brighter one (splash proof) over the shower to help wake me up!
Incidentally, we have a couple of round in-cabinet lights as well. Ours are 'brassed' and have frosted glass and take little 10W lamps. Make sure you 'try before you buy' cos ours are rubbish: the 'metal' being so fragile that it distorts and soon cracks the glass when you try to unscrew it to change the 'bulb'. You end up with a handful of broken glass.
ps. 'TW' above may be being a bit over cautious in allowing only one lamp per tf : so long as we've kept within the wattage limits (which means leaving a label somewhere to tell any user the maximum you've allowed for) we've had no failures at all in quite a few years now (sshhh!).
Wound trafos need to be close to their loading at all times, but are usually only used on suspended wire systems nowadays,electronic trafos don`t like the long lines and can act like an aerial.
Its voltage drop in the cable thats the catch, any more than 0.5V is very noticeable and means your cables are getting warm. Hence thick cable at any run length and putting a trafo per light usually easier.
and you are not about to either - these LV lights are AC ;-)
Indeed - if you run several lights from one cable the current can be high, and they are very sensitive to voltage drop (in the sense that the difference will be visible)
Personally I usually star wire them in groups. Typically in 1.0mm T&E[1], keeping each cable in the 1 - 3m sort of length range.
With six lights I would probably go for two 150W transformers. If possible positioning the thing such that it can be extracted later if required.
Colours are irrelevant in this case - although don't use the earth wire (I normally sleeve it and fold it back out of the way)
[1] Its cheap, good for up at least 10A (and probably more) in these installations, and I always have some in stock.
The last time I fitted LV lamps I just used a cheap transformer for each lamp and ran mains. They are £3 and it means that a transformer fault doesn't leave you in the dark.
Though I remember using P =3D I^2 * R to calculate the power dissipated in cables as heat - i.e. higher voltage =3D more efficient and lower voltage =3D huge cables or lots of electrical energy turned into heat on the way!
I assume this means the connector block between the transformer and the wires needs to be very hefty?
I found this on-line calculator that seems useful (though from what you've said 1% voltage drop at 12V is already far too much)...
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'm putting lights on top of (and underneath) kitchen cupboards. This means they're physically grouped (2 left of window, 5 right of window,
9 across the back wall), and with transformers on top of the cupboards they'll be easily accessible, with suitable wires run down in the wall to the lights below.
I was going to run one wire down per run of units, but I see now it makes more sense to run one wire down per light! (Even one per pair of lights would be pushing for 1.5mm2 according to that calculator?!). Not quite so neat, but normal people don't peer under the bottom of cupboards anyway.
On the the plus side, 12V lamps are more efficient in terms of the lumens produced per watt, so even a slight increase in losses in the transformer and wiring are offset by being able to get away with fewer total watts of lamps.
Usually with a 150W transformer, there is enough terminal capacity to take three sets of 1mm^2 wire. Hence no need for any connector block.
However, even if you did use a connector, then at 150W your max current would be in the order of 12.5A so not that excessive.
In percentage terms 1% would be fine (i.e. 0.12V). However a drop of 1V which would make no difference to a 240V system, is a significantly larger percentage on a 12V one.
For lighting above the cabinets it might be worth considering the mini fluro link lights. The make good reflective lamps with a white ceiling and cost a good deal less to run, and produce less waste heat.
I usually use these:
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you do a lap of a kitchen above and below the units you can still gobble up a couple of hundred watts, but less than using halogen. I normally switch the upper and lower sets separately. You can obviously combine these with some halogen task lights or in cupboard lights for glass fronted display units etc.
You can find typical wire resistances here:
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for example a pair of 20W lamps would be drawing 40/12 = 3.3A
on 1mm^2 cable 2m long that would drop 3.3 x (2 x 18.10) = 119 mV total, or around 1%. So that would be fine.
With lamps that could be up to 50W, then one cable per lamp would be ok for shortish distances. If you need longer runs (say 5m or more) then you could go up to 1.5mm^2.
Thanks for all the useful advice John (and others).
I don't have any 1mm twin and earth at present. I wonder if I can re- use all the old red+black 1mm I'm ripping out when re-wring? ELV doesn't have to pass part-P, does it? I can sheath it brown and blue if they're that bothered.
David Robinson wibbled on Thursday 24 June 2010 18:33
Line/+ve - Brown,black,red,orange,yellow,violet,grey,white,pink or turquoise Neutral/-ve - Blue
if you want to be pedantic (App 11, Onsite Guide).
But, it really doesn't matter. Persoanlly, I think it's more useful to write on the cable with "Sharpie" at regular intervals "SELV, 12V" or whatever, to avoid confusion...
as to cable, T+E (I'd go for 1.5mm2 if you have a 250W transformer as the current carrying capacity is closer to the potential short circuit current (unless the PSU has a short circuit "knee" curve.
But many would use flex for some/all of the SELV, particularly the final drops. Heat resistant is advisable if the fittings might get hot. I used silicone cable, with the proviso it's not as tough as PVC but my installation positions are well guarded.
SELV doesn't have much of a shock risk, but the fire risk is still very real
- so consider it in those terms.
Technically yes or at least under some conditions like special locations) (which might change).
Thanks Tim - that's exactly what I was looking at - but wasn't sure whether that part was legally binding. It's not like 12V electrical work is notifiable AFAICT.
I suppose it could confuse someone where it comes out of the wall. Once fitted to transformer + lights you'd need to be a bit of an idiot not to realise what it was. Though most of us can be idiots outside our areas of expertise, and each revision of the regs aims to keep more people out of the Darwin awards.
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