Wiring LV lights

[Dave]

I have a quick question regarding wiring LV transformers...

I'm changing the lighting in our bathroom from a single fixed 240v light to 4 LV downlights. I'm thinking 35w as I'm not overly sure if

50w will be too much or cause a fire risk(?).

I've had a look at some of the transformers on the screwfix website and was wondering if I should be buying a transformer per light or buy

2 transformer and run 2 lights per transformer?

These are the transformers I was looking at:

[Lobster]

Plus 4 x 35W will be a little less horrendous to run as well, costwise.

Without getting into the pros (are there any?) and cons of this form of lighting (and we do have them home because SWMBO likes them. Nuff said...). The transformer cost per light is almost the same regardless of whether you chose 1 big one or 4 small ones, so you're really much better off going for 1 per light. Easier for fault-finding for a start, and indeed if one does fail, you don't lose the whole bathroom to darkness.

David

[Dave]

Thanks David. This makes sense.

Can you give me an idea as to how would be best to wire the four transformers?

Cheers

Dave

[Dave Plowman (News)]

I'd go for one per light and get the type that will fit through the downlighter hole - thus if it fails it will be easy to identify and access for replacement.

[Roger Mills]

The particular transformer in your reference only has a range of 20-60 VA so you *can't* run 2 x35w lamps off it 'cos that requires 70 VA.

One of these per lamp would be ok, or you could use a bigger transformer for multiple lamps. It's best not to run the transformers flat out, but at the same time they won't work at all at below the minimum stated load.

The room which my wife uses as an office as 6 x 35w downlighters in two groups of three, with each group running off a 150w transformer.

Try to keep the wires between the transformers and lamps as short as possible, and use thick wire. Forget the usual current rating vs cable size used for mains because voltage drop is much more critical with LV. Wiring the lamps in 2.5mm^2 cable should be sufficient.

Also, give some thought to maintenance. How easy will it be to change a transformer if it fails? If there's a floor above, you need to be able to reach the transformer through a lamp hole, with enough slack cable on it to get at the connections.

If this is an upstairs bathroom with an attic above, you need a means of keeping the loft insulation away from the lamps - otherwise they will overheat and constitute a fire risk. Some people use something akin to upturned terracotta flowerpots.

[The Natural Philosopher]

You choice. I considered all of this, and decided that installation was easier on a one per light basis. The cost difference was pretty small.

[Dave]

So just for clarity, if I were to take down the existing light and place a junction box in the loft (as can be see in here:

I just need to feed 4 supplies from the switched live staight to each transformer?

Dave

[Roger Mills]

Basically yes. You'll need the neutrals as well, of course! These transformers don't usually have an earth connection.

You can use 1mm or 1.5mm cable on the mains side of the transformer, but

2.5mm is better on the LV side.

[Dave]

Thanks Roger.

Cheers

Dave

[John Rumm]

Yup, or you could daisy chain the switched feed to more than one transformer if that makes it easier to wire.

[Dave]

Yes, I think daisy chaining the switched feed may be easier.

Looks like I have a job for the weekend!

Thanks

Dave

[Owain]

But the earth wire in the cable must still be used, as the cables require cpc.

Owain

[The Medway Handyman]

I've often wondered what the benefits are. To my mind adding a transformer is adding a potencial problem. I must have chamged half a dozen last year.

[Roger Mills]

A lot more visible light per watt of input power and much longer bulb life. Both stemming from the fact that a 12v bulb has a very short fat filament compared with that of a mains bulb with the same wattage. That means that: a) it's more robust b) it can be run at a higher temperature - producing shorter wavelength (visible) light rather than infra-red.

Compare the light from a 60w car headlamp with that from a mains 60w bulb!

[Roger Mills]

And be connected to what?

[Owain]

s/problem/earning opportunity

Owain

[Owain]

The cpc at the supply end, and nothing at the double insulated transformer end.

Owain

[John Rumm]

b2) The light creates brighter specular reflection points on polished or metallic surfaces, making them more attractive for certain types of display lighting

c) 240V is undesirable in some locations, where SELV is preferable.

d) You can get dichroic bulbs in 12V which produce far less front projection of heat, and give an attractive colour spill to the rear which can look effective in display ligating situations.

[John Rumm]

A combination of the two often works well - say you have four transformers, you could have two connections from a 4 terminal junction box where the switched live originates, then each wire daisy chained to two transformers. That saves trying to get too many wires in any one set of terminals.

[Roger Mills]

What does that achieve?