Output wiring from SELV transformer - T+E OK?

[Hallsy]

Looking at some SELV transformers I have for ELV downlighters it mentions using a special cable, presumably one made by the manufacturer or any other cable suitable by the regs. The outlets look like they are designed for 2-core flex by the size, but can normal 1.5mm T+E be used?

I had to do some concealed wiring a while back and I planned to use a transformer for each lamp, but now I'm using two transformers for 5 lamps - this would mean using the T+E I've already run straight out of the transformers rather than feeding them.

I could change the wiring if I had to but it would be hassle - I perosanlly can't see why T+E would be a prob as long as the earth is cut back at each end?

Also it mentions not crossing the incomer and outgoing - I presume this is for isolation/interference reasons between LV & ELV? I can't be 100% sure that the cables won't cross in at least one place - is this a problem in practice?

 

[Hallsy]

Anyone??

I looked up the cable the manufacturer mentioned and as I thought it's a 0.75 twin flex.

Once I start cutting the holes for the spots today I'll get an idea of whther I can swap my T+E for flex or not, but even if I can't I don't see 1.5 T+E being a problem as long as it fits in the terminals OK.

Just that I'd clipped the T+E all around the joists neatly, etc and would be a shame not to use it!! Guess I should have thought about using 2 transformers instead of 5 in the first place!!

 

[Qedelec]

I could change the wiring if I had to but it would be hassle - I perosanlly can't see why T+E would be a prob as long as the earth is cut back at each end?

NEVER, ever cut back cpc's. If it is not required sleeve it and terminate in a terminal block. Otherwise use the correct cable.

 

[Shaunoffshotgun]

Excuse ignorance but why should you never cut back cpc ?

Is it in case something is fitted in the future that needs earthing ?

 

[JohnD]

A sign of the old hand is that he's been reduced to impotent rage once too often, by trying to fit things where the old cable was cut off just that little bit too short.

In a few years time QED might find himself in your house trying to fit new lamps for the new owner.

 

[Qedelec]

Yes
Its very bad practice.
By terminating it you are making sure it is available for the next Leprechaun to use.

 

[Hallsy]

Fair enough, but by cutting it back you are ensuring it won't be connected to an LV earth or touch a metal part that is earthed, I believe I'm correct in saying that an SELV transformer is isolated from the LV side and so shouldn;t have ANY contact with an earth on the LV side.

Surely cutting it back you ensure it won;t be misused.

I can see what you're saying as for futurte use, but if someone wants to change the lighting and re-use existing cable they should check out what sort of circuit is being used anyhow.

But, back to original question - T+E is OK yes, but it's prefered for me to put the cpc either end in a block.

 

[securespark]

This isn't a huge long piece of cable, then? You have to think of Volt Drop, especially in SELV where a 50W lamp = 4.17A.

TLC sell special flat twin cable for SELV connections.

 

[Hallsy]

Not really, one transformer will have two runs one at about 20cm the other at say 50cm, the other transforemr will have 3, two at about 50cm and one at possibly 2m.

Volt drop would be less on bigger cable though right, so why would that mean that using 1.5 T+E (29 mV/A/m) as opposed to 0.75 flex be a problem?

I think I'm going to use flex anyway as I can just about access the cabling and it will make termination easier.

 

[philrosenberg]

***Sorry posted in wrong place. Moved to new thread***

 

[Steve]

but if someone wants to change the lighting and re-use existing cable they should check out what sort of circuit is being used anyhow.

have you read some of the posts on here?