Cables to connect halogen downlighters.

[judder]

Hello everyone. When wiring up six (50 Watts each) halogen downlighters, is the type of cable used important? Can you use flex? Can you use 2mm^2 cable?

I guess it's all to do with current limits i.e. 6 x 50 W draws about 1.3 Amps. I don't know the limit on flex, but 1.3 Amps sounds ok; is it?

Thanks in advance.

 

[Taylortwocities]

You should bone up on your basic ohms law


P= I x V

So a single 50W lamp will run at 50watts / 12volts = 4.2 amps.
Six of those will pull 25amps.

So you got to look at volt drop as well as cable size.

Use more transformers closer to the lamps rather that string them out - otherwise the lamps at the end will be dimmer than the rest.

PS Did you mean 2mm² ?

 

[judder]

Perhaps I should have mentioned that the lamps in question operate at mains voltage (~230 V), whence a power of 300 Watts (6 x 50) corresponds to 300/230=1.3 Amps drawn (in the absence of resistance in the cables). Obviously the resistance in the cables will have the effect of reducing the 230 V. I've just found a table regarding the current limits for flex (//www.diynot.com/pages/el/el003.php) thank you to diynot for that.

Given that the thinnest flex can take 3 Amps, is there any reason or regulation which prevents it from being used to wire up 6 x 50 W downlighters at mains voltage?

By the way, I did mean 2 mm squared, but don't know how to superscript a 2 in the forum text. Sorry.

 

[Taylortwocities]

Ah yes, I made the wrong assumption. Been running 12v downlights all week

What protective device do you have in that circuit?

1.5mm² flex would be OK if it has a 6a mcb,
You'll need 3 core cable, do not forget that you must extend the earth right thru the cable run to the end. The lights may not need it bu the cable does under fault conditions.


PS to get the ² sign, put your fingre on the alt key and hold it down.
Type 0178 on the number key
release the alt key and voila!

 

[judder]

Thanks very much for the reply. So there's no rule preventing the use of flex, provided it can take the current?

I didn't realise the earthing requirement for the cable. The lights themselves have no earth connection. Why does the cable require an earth line running from the junction box to the end of the lamp 6 and how should an earth be continued when there is no connection point (for earth) at the downlights themselves?

Thanks for typing tip too!

 

[Taylortwocities]

That third conductor in cables is called the Circuit Protective Conductor or CPC (Earth to the layman.)

But the word Earth does not properly describe the conductor. BAsically it provides an earth path in the event of a fault.

The integrity of the earth (fault) path is checked and tested at the end of each circuit to ensure that the fuse will actulaay blow in the event of a fault.

You need to use 3 core cable for this reason. Wiring regs stipulate there must be one and I would mark it as a deviation if i found it in an installation.

 

[judder]

Thank you. That's fair enough and very clear. However, as a 3-core cable runs from light one light two, and then a second three core cable runs from light two to light three, etc, what is the simplest, acceptable way to connect the CPCs?

By the way, this is all very interesting and helpful.

 

[Taylortwocities]

Better qual lamps have an earth terminal, for the purpose.
Alternatively, use a piece of terminal strip.

 

[judder]

Thanks very much. One more question though, if the cable is required to have a continuous earth, why doesn't the flex leading to an individual light bulb (i.e the bit you see dangling from the ceiling - I'm not referring to downlights here) need to be earthed? I realise that such a length of flex is very short, e.g. 20 cm; surely that can't be the reason though.

 

[VanSolo]

I take it you are talking about lighting pendants here. A cpc is not required as it has a non-metallic lampholder and a lamp (bulb) does not require an earth.

 

[judder]

Yes, I am talking about pendants, but could not think of the word. Thank you for your reply. Pendant fittings are non-metallic so don't require an earth - fair enough, but what about the cable itself (which in the case of joining downlighters, or indeed the lighting ring itself, must be earthed).

My halogen downlighters are metallic, but do not have an earth terminal, yet the cable leading to them is required to be earthed. This is all very confusing for my poor little brain.

 

[VanSolo]

I think you may be getting confused between the LV (230 volts) and ELV (12 volts) sides of the circuit. The LV side requires a cpc (earth) but the ELV side after the PSU (transformer) does not.

 

[judder]

Thanks for your reply, but I don't think so. The downlighters in question are mains voltage. Everything I'm considering is for ~230V.

 

[VanSolo]

Oops didn't read the post correctly
If the lights have no earth terminal or earth fly lead they must be double insulated. Will have a square within a square symbol on them.

 

[Taylortwocities]

True, but the interconnecting cables (230v) must be 3-core with the earth(cpc) maintained from supply to the end of the lighting run.