What cable for DC 12V 20W kitchen lights?

David Robinson wibbled on Friday 25 June 2010 09:46

Check again - I'm pretty sure it is in special locations and kitchens. But it is stupid, so personally I'd ignore it - but just for the record like...

Depends on the light fitting. It is rated to 70C max. Some do get seriously hot out the back, some don't.

Kitchens are mentioned IIRC in Part P (the document).

Cheers

Tim

Reply to
Tim Watts
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That surely applies to any circuit?

It's the current wot does this...

Reply to
Dave Plowman (News)

Something you have to factor in when designing or extending the LV wiring side is the type of transformer used and how this relates to the cable type/size required.

If you are using a conventional heavy magnetic transformer, then you can use the standard voltage drop figures in the wiring regs to work out what size cable you need, although as you said, you need a much lower voltage drop on the 12V side than would be acceptable on the mains side. (For any sizable runs, minimum size is more likely going to be dictated by voltage drop than by current carrying capacity.)

If you try doing this with an electronic transformer, you will be very disappointed with the result. Problem is these generate AC output at somewhere between 20-60kHz, and not 50Hz. At that frequency, a physical property called "skin effect" has come into play, which means the electricity only travels along the outer skin of conductors, so the bulk of the conductor material is not usable, and the effective cross sectional area of the wire is very much less than the actual cross- sectional area. This became very obvious to me when I swapped a toriodal transformer for an electronic one, and light output dropped to probably only 1/3rd of what it had been. I had to upgrade the wire from 2.5mm to 6mm to get the light output back as it was.

I had a room with a central pendent with 3 x 60W and a pair of wall lights each with 1 x 60W, and wanted to convert this all to 12V. I decided to use 35W 12V capsules to replace all the 60W mains lamps. The transformers are in a metal case in the room above, directly above one of the wall lights, and for that light, I take the 1mm² cable directly into the transformer. For the other wall light, at the top of the run up the wall, I convert it to 2.5mm² to run across under the floor and up into the transformer. For the central pendent (3 x 35W), I initially also used a run of 2.5mm² but the voltage drop was too high, so I changed it to a run of 6mm², and that's fine.

In my case, I wanted to use mains cabling so that I could remove the

12V lighting and go back to mains at some point in the future without having to replace the cabling. If this had not been the case, I might have considered other types of cabling suitable for LV high frequency, such as flat stranded speaker wire (higher surface area to volume ratio, so less skin effect) or silver plated stranded wire (silver being a good conductor works very well plated onto copper for high frequencies).
Reply to
Andrew Gabriel

But with LV lighting the currents stack up quick, as NT says a single

20W lamp no great shakes, few 50W on same circuit at nearly 4A a piece, current climbs quick.

The voltage is mosttly harmless , but need to keep respect for the current and its heating ability, with the cable and making sure connectors are tight. Have seen choc bloc melt down because of loose screws with LV lights.

Cheers Adam

=A0 London SW

Reply to
Adam Aglionby

As does adding things willy-nilly to any circuit. If you can't do the maths it's best left to someone who can. After all, you must have chosen the transformer to run all these low voltage lights in parallel? And not by accident - a 50 watt toroidal costs around a tenner, but a 250 watt one nearer 50 quid. Electronic may not cost quite as much in proportion, but high current ones are still much more expensive. And

Perhaps the chock block was inadequately rated too?

Low volt lighting circuits don't generally carry any more current than a ring or cooker, etc. So it's not like they're using currents the average person hasn't come across before. If they choose to treat them as unimportant, workmanship wise. since they're low voltage, that's up to them.

All electricity can be dangerous in various ways. I know of someone who was injured by a PP3 battery...

Reply to
Dave Plowman (News)

would be good to put that on the wiki somewhere.

NT

Reply to
NT

Back in the day, guess mid 80s one of the companies doing the new fangled LV lights , replacing PAR38 for retail display, ran a roadshow for installers.

One demo was in the car park, 12V lamp on 240V , graphically demonstrated need for trafo and cost of wiring mistake.

Other one remember was 250W torrodial and loop of bell wire racing the

25A fuse, bell wire went first with some smoke in protestation.

The tightening was inadequate on a substanial block, seen it happen several times in last 20 odd years.

Its low voltage but high current, people including electricians just see the LV bit, the current even on a single 50W lamp will happily set something on fire.

C`mon theres got to be a story ;-) ?

Cheers Adam

=A0 London SW

Reply to
Adam Aglionby

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