Question about Low Voltage Downlights

I am putting in Halogen downlighters into my house, 4 low voltage spots

12v 20 Watts.

What I dont understand is that the transformer I have bought says, that "the output leads must not exceed 2m in length", Im confused to why they have stated only 2 meters?

It looks like im going to need 3 or possible 4 meters in total to get to all the spots from the transformer. Does anybody know why I should not exceed 2meters for the output leads, what is likely to happen if I do ??

Thxs

Reply to
shawn
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shawn brought next idea :

They mean the total length from transformer to the furthest lamp, not the total length of cable to each run out to the lamps.

This would be OK:-

T____________________L T____________________L T____________________L T____________________L < Max 2 metre >

This would not:-

T____________________L______________L_____________L < Total legnth 3 metres >

What happens is the that due to the low voltage the resistance of the cable itself becomes much more of a problem.

Reply to
Harry Bloomfield

(a) low voltage cables are very susceptible to voltage drop (b) cables can act as radio antennae and the limited length might be necessary to keep radio-frequency emissions within limits

Dim lights and buzzy noises on the telly.

Can you use two transformers and keep output leads within length?

Owain

Reply to
Owain

thxs for that, if I would of done the bottom option ie total length of cable 3 metres with 4 lights on it would it of been unsafe is that what you mean by resistance?

Also what thickness of cable should I use if I went with top option ie transformer to 1 light but no more then 2 metres.. Ive got 0.75m 2 core

240v 6 amp 1440 watts should that be safe and sufficient please?
Reply to
shawn

Basically you may find some bulbs are dimmer than you expect.

As the others have mentioned, each meter of wire gives you some voltage drop, and since you are starting with only 12V it will have a much bigger visual impact than the same drop on a 240V bulb.

The is also another potential problem that can occur with long wires and some modern electronic transformers that can result in much higher apparent resistances being presented by the cable that you would otherwise expect from simple voltage drop calculations. (To explain why this happens does alas get rather deep and complex very fast! If you want to know more then google on this group as it has been discussed recently).

Reply to
John Rumm

shawn submitted this idea :

The the cable length and its resistance can have two effects...

  1. Voltage drop such that your lights can become dimmer the higher the resistance.
  2. The cable itself heating up.

In this case it is the dimming effect which is the problem needing to be overcome.

12v 20w = a little under 2amps which is well within the cables rating, so voltage drop is the only problem to be considered. If the transformer is mounted centrally within the four lamps and the most distant is less than 2 meters, then you can use the 0.75mm cable assuming each light is wired separately back to the transformer.

If any of the lights must be more distant than 2m then that would need to be wired in 1.0mm.

Reply to
Harry Bloomfield

Harry

You say that if the transformer is centrally posiitoned and a cable

0.75 taken to each of the lights it should be ok, as long as its less then 2 mtrs.

Probably a really scilly question but my transformer only has two connectors from it, and it doesnt even show which is live or neutral how would you wire / connect each of these cables from the lights back to the transformer. I would have like have 8 wires coming back from all of the lights, and only have 2 on the transformer. Is there some kind of connector I can buy and watt amp conector would I need, I would assume 4 - 20 watt light at 12 volts would mean 6.6 amp or better to be safe since they would all need to be connected?

Reply to
shawn

Hopefully last question re ceiling rose wiring

The rose appears to have been identified as loop in wiring circuit, ie two circuit cables plus a switch cable.

I was going to originally wire just 1 transformer. 4 x 20 watt 12 v downlights to it on a 4 metre cable. But with all the advice im thinking that I should get two transformers and wire that to the ceiling rose so that none of the cable lengths are more then 2 metres., would I also need to upgrade the ceiling rose as I dont know how many amps that is capable of supporting too?

Somebody suggested not wiring into a ceiling rose, but instead replacing the ceiling rose with a junction box, but I cannot see any junctions boxes that look anything like the wiring I have in the ceiling rose ie loop in wiring?

I appreciate any advice thxs

Reply to
shawn

Use this with a connector block inside, I don't use the plastic, but the porcelain type; but your choice. There are cable grips for the cable entries.

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Reply to
John McLean

John

What are you suggesting I use the connector block for, a replacement for the ceiling rose or for the cables back to the transformer?

Reply to
shawn

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?_dyncharset=UTF-8&howMany=5&searchText=chocbox&x=10&y=10>>BTW, B and Q sell these chocboxes, (they're in the screwfix group). Jaymack

Reply to
John McLean

Replacement for the rose, it's small enough to be pushed through from below, but I would recommend these anywhere where such joints/terminal blocks are required. Jaymack

Reply to
John McLean

Polarity is irrelevent on the LV side.

You could use a choc block connecting strip which are available from B&Q, but is your transformer designed for several lights? The ones with wires comming out are usualy for just one lamp. I used a transformer for each light.

Reply to
marvelus

Its OK just wire the mains side of the transformer into where the pendant wire is currently connected.

Electricaly the ceiling rose is just a 4 terminal junction box really, they look different but will have the same electrical connectivity.

Whichever you decide of your 2 methods you are going to need easy access from the floor above to the transformer and junction box above where the current ceiling rose is, as the transformer will one day need replacing and some of your wiring may become loose in the junction box and also in the connecting block. Maybe put a fifth light where the ceiling rose is now which gives you access to all the gubbins from below the ceiling.

If you dont feel happy about anything maybe use an electrician.

Reply to
marvelus

You say "Its OK just wire the mains side of the transformer into where the pendant wire is currently connected. "

We think we would have over 8 amps with one of the jobs we are going to do ie 5 X 20 Watt downlighters 12V 8.3 Amps total current for the circuit. So im guessing we should not really wire that into the standard ceiling rose where the pendant wire is currently conected?

Reply to
shawn

The mains side won't be drawing 8 amps. There's a transformer between the mains and LV side, so;

V x A (mains) = V x A (LV) (Approximately & good enough for this ...)

240 x ? = 12 x 8

? = 12 x 8/240

? = 0.4 Amps

Plug it into the ceiling rose.

Reply to
Huge

You're calculating the current on the 12V side of the transformer, the current on the 230V and primary side of the transformer is approximately

0.5A Jaymack
Reply to
John McLean

Thanks everybody, youve all been very helpful. If you need any help with any IT / PC problems im the man

Reply to
shawn

That inc mac/Linux probs?

Reply to
gort

shawn explained :

The amperage at mains voltage is tiny compared to that at 12v, so for this purpose it can be ignored. For the same wattage of lamp, every time you halve the voltage, the current has to double.

Depending upon the type - the transformers may need to be matched to the lamp load. Obviously the rating much be at least up to to load, but sometimes it has to exactly the same neither too high nor too low.

So basically there would be no problem with the current at the rose, however... It looks better if the rose is moved so it is hidden above the ceiling or even better use a four or six (two spares) terminal lighting joint box instead.

In the rose will be :-

  1. Live (red)

  1. Neutral (black)

  2. Earth (Green/yellow)

  1. Switch wire - This if wired correctly should be one of a pair of reds in one outer cable, but the two could well be red + black, black being commonly used as the switch wire - though it ought to be sleaved red or taped red.

All the above assumes the pre-2006 colours have been used.

Reply to
Harry Bloomfield

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