Wiring 12v SELV bathroom downlighters.

[Mike2007]

Can't quite find the answer in a search so I am asking the boys on the DIYnot team!
Wiring 6, 50w downlighters in bathroom (IP65) to replace a single pendant fitting. I have heard that a 12v circuit through a transformer would have a higher current pull than a 240v lamp. Of course there are more lamps to this install, but is the new wire run adding a great more strain, what csa should it be?
Bathroom is 3m square approx and the lamps would spread out proportionately across the ceiling.
Cheers
Mike

 

[securespark]

First of all, do your transformer instructions recommend extending the output cables (assuming they have them)?

Basically, the further you go, the more volt drop is an issue. With a 12V supply, each 50W lamp is pulling 4.17A.

Have you got one transformer or several? If you've only got one, is it possible to replace it with a local one for each fitting? Then you don't need extended cable runs on the secondary.... The advantage here is if the tranny goes down the pan, only one fitting is affected.

 

[Steve]

To elaborate, 6 x 50w is 300 watts.

300 watts at 12 volts is 25 amps. You need rather thick cable for this (recommend 4mm²)

300 watts at 230 volts is 1.3 amps.

It is therefore recommended that you keep the low voltage side of the wiring as short as possible due to voltage drop and current loading. It is also recommended that you use a small transformer for each light, rather than one large one. When the single Tx fails (they do) you will loose all the lights until you can replace it. Here the benefit of multiple Tx's is obvious They are also easier to change. Single lamp Tx's can fit through the downlight hole.

 

[Mike2007]

Wow, it makes sense but much more complicated than I thought!
I wonder if I could take the 12v lights back and replace for 240v mains lamps. I guess that 240v with the right IP rating would suffice in this bathroom, otherwise 12v is tricky to install?
Would that mean that if the supply was 400v the wires could decrease in gauge?
How come vehicles (12V) don't need chunky wiring?

 

[Mike2007]

Just add another question to your response Crafty?
You are recommending a 4mm 2 core cable between each transformer and the fitting to reduce cable load. All these transformers are coming from an existing 1.5mm light cable??

 

[breezer]

have a look at the cble going to the actual starter on a car then tell me cars dont need "chunky wiring"

you are right about the cbale size and increasing the voltage, why do you suppose overhead poerlines are the size they are? its becuse the voltage is so high, if they were all at 240v the cable diamneter would be astronomical for the distance

4mm cable between each light if you have 1 transformer for all lights (i am sorry but for the life of me i cant see 4mm cable mentioned by crafty, must be my age)

if you have 1 transformer / light (good way to go) then the mains side can be 1mm

 

[Steve]

Many wires in cars are small because they control a power relay under the bonnet. The control in the car switches a small current which activates a relay, which switches the large load. This saves running big cables ino the dashboard. In my car for example, When i turn the headlight switch to dipped beam, i hear the relay click behind the glove box. When i activate full beam, i hear another click.

Back to your lights, you'd need 4mm cable if you had one output from that transformer and daisychained the lights. On a star configuration, you'd need 4mm supplying the star centre and then 1mm from there to each individual light. You have to consider the loads on each cable.