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wiring a low voltage lighting transformer

Postby greendog » Thu Jan 21, 2010 3:27 pm

I am replacing a ceiling light with low-voltage spots and need to connect the wires to the transformer terminals. i have checked the project pages but they don't mention this situation:

The transformer has 4 terminal, 2x L and 2x N like: L N N L

I have two cables coming out of the ceiling with old colouring, i.e one red, black, earth in each cable. I believe that makes this a spur? I know which one is the switch cable and have now labelled the black of that red.

I am confused as to what to do because there is no switch/line terminal for the switch cable as usually in a light fitting.

I was thinking of doing this: connect black to N, both reds to one L terminal and black from switch to other L; terminate earth as transformer doesn't require it. Would this do it? or what else should I do?

thanks for your help
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Simply Build It

Postby atl » Thu Jan 21, 2010 7:23 pm

You need a 4 terminal10amp joint box connect the 2 reds [permenent live] together into 1st terminal, the black switch wire with red tape into the 2nd terminal, the other black cable [neutral] into the 3rd terminal & the earths [green/yellow] into the 4th terminal connect a short peice of 3 core 1mm flex between the joint box & transformer the brown to connect to terminal 2 switch wire & L in transformer, the blue into terminal 3 neutral & N in tansformer, the green /yellow to terminal 4 earth & cut off in transformer.
Hope this is of help
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Postby greendog » Fri Jan 22, 2010 4:26 pm

hm, I didn't realise I had to put anything between supply cable and transformer. Does it have to be one of those round junction boxes used in the lighting circuit and does it have to be fused? or could i use a chocbox with strip connectors ( never used a chocbox, read about it in the projects, looks straightforward though). I guess the transformer has some kind of fuse inbuilt?

thanks for your help so far
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Postby ericmark » Sat Jan 23, 2010 1:59 am

Well in theory choc block does not have the protection required and it should be enclosed in something. I have seen small plastic boxes designed for job. Also anything which can’t be accessed for maintenance needs to be no maintenance type i.e. little springs not screws. However many do use choc blocks.

Normally there is no need to fuse. The standard ceiling rose is rated at 5/6A and so 5A fuse or 6A MCB normally protects all. The standard BA22d bulb holder is only rated at 2A but the bulb its self has built in fuse or should have.

Often you need to connect the line cables separate to lamp/inverter/transformer where ceiling roses are replace with other equipment.

For interest the phase wire we call line. Because both phase and neutral are called live is seems the L in plug tops means line not live. I still slip up and call line wires live and I would hope everyone understands anyway. Also low voltage is below 1000vac and above 50vac so normal house supply is low voltage. 12v lights are extra low volts and since separated by transformer/inverter is referred to as SELV (separated extra low voltage). Although the block which drops the voltage to 12 has a transformer inside normally the units have some regulation and work on high frequency so are normally inverters which are smaller and lighter than old transformers.
Not that it matters we all know what you mean.

Because the room is often limited I have seen many botch jobs to join the two cables line and switch together often a single choc block or crimp. However the problem arises when for some reason the floor boards are lifted and some one sticks in their hand to “Feel for” and they touch the wires and get a shock. Consider this may be you some time in the future so do ensure there is nothing anyone could touch if they feel around from above.
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Postby greendog » Sat Jan 23, 2010 6:39 pm

thanks ericmark

I think I maybe didn't describe what I thought was called a chocbox very well. It's a small plastic box in which you can place a strip connector and the box- once all connections have been made - snaps shut and has a small screw to secure the lid as well. So no chance of somebody accidentally touching exposed wires, but I am still unsure of the fuse issue as with the strip connector there obviously is none.

If, as I understand you, the bulb is/has a fuse function and presumably the transformer/inverter thing has one and possibly the switch too? (do switches have fuses :? ) do I need to add another one by using a junction box or is the "strip connector enclosure box" (chocbox) safe?

thank you for your patience !
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