As a part of my bathroom works for my parents, I insisted on jointing and redirecting all cables for the lights from the belows outside the room as the bathroom was to be tiled and I wanted all joints and ELV Tx's accessible.

However, I noticed that no.3 ELV downlighters were fed off no.2 ELV Tx's. Cables were 0.75. No problem I thought. That's usual.

Now I'm here having too much tme to think and I thought: hang on those MR11's I'm sure are 35w.

Secondary side Ampage:

I=P/U
=35/12
=2.91 x 3
= 8.75A

That's not including other additional calcultions like losses and what not.

Now these lights have never been used or left on a great deal. But how comes this has never caused an issue? Are cables that underrated by the boys in white lab coats?

Also, since my time, I have always been told: a Tx for each light (as I have done for my newly installed lights), has it ever been customary to install lights in groups on one TX?

Edited: 19 June 2012 at 10:53 PM by MrOther

''Also, since my time, I have always been told: a Tx for each light, has it ever been customary to install lights in groups on one TX?''

Yes, in the '80's big iron cored transformers, 200w, 400w were common, output fuses on each of the multiple outputs, wired to lots of fittings. A special stranded 2.0mm 2 core cable was available for the connections. Weighed a ton and was a floorboards up job to get to it.
Transformer went, and the whole lot was in darkness.

The situation you describe with the flex is ok, the killer comes when the runs are long and the heat gets excessive.
Im sure there is still an explanation pn the TLC website for sizing 12v runs.
Its just easier nowadays to run the 'mains' to each light for the transformers rather than distribute it at 12v.

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Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry

As Pete says, large transformers were very common in the eighties.

I remember putting two 300W Tx's in for a barn conversion. But there was no way they were going under the floor. I mounted them into fire rated Legrand steel cabinet mounted above the floor in a corner.
Looking back now, it was an unnecessary expense. Better to use GU10s with LED lights and fire rated enclosures, that's until something new appears on the block.

Legh

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Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."

I found a calculator that's bout it. Well as it's my house I'll just have to downrate the lamps.

It does say:

"Due to the possibility of voltage drop, the correct choice of cable is vital. Undervolting by more than 0.6v reduces the efficiency, affects the colour of the light and may affect the lamp life."

However, in my scenerio each lamps is feed in radials from the TX so surely it's still 35w per leg?

You used to get 105 watt transformers able to supply 2 lamps, whats the transformer rating?
No problem with multiple lamps off 1 transformer just as others say, loose the transformer loose all the lamps

The DIY sheds used to specialise in such kits with either 3x35W or 5x20W lamps fed off one 105W transformer - all pre-wired. Previous occupant of this house used such for the bathroom - all open backed put into a false dropped wooden ceiling. No IP rating ever thought about but then what do you expect from B&Q.

Mind you never had a problem with them - transformer runs very cool, lamps have never blown in 10 years and no signs of rot in the ceiling.

Who needs BS7671 or B Regs?

Yea 105w most certainly an old B&Q kit.

How sensitive are downlighters to volt drop?

TLC see says that a slight fall by 0.6v can start can make massive detrimental effects (lighting shade /colourand lamp life) one of my runs is 4mish from TX to lamp. (TX needed to be remote for easy access.) Had to install it on the quick and thought it'd be okay but now I'm self-doubting (which is the normal for me)

I can't believe this is strictly true considering how many DIYers install downlights.

@Fm - re: my lost thread and your post: - do you per chance work for yourself? If so it may be a good learning experience for me to PM you the details of my bathroom install, the spec and how it all panned out. I thought my inexperience of housebashing was a major contribution however we had a very good, well recommened and well known carpeter come in to do a "small" job that grew on him and totally bewildered him.

Thanks all for your replies, as always.

That's 5%.

Point taken.