Correct cable size for downlighters

[kambhari]

Hello

I am recieiving conflicting info and would be grateful for any views. Basically, our new house has been wired (on the lighting circuits) with 1.5mm T+E. However, when it comes to the two rooms that have downlighters in the ceiling, the wiring is 1.5mm upto the junction box and then 0.75mm flex to the first downlight, and then 0.75mm flex to the next downlight and so on in series.

The first room has 16 downlighters and the second room has three banks of 8 downlighters (each on its own switch). The downlighters are all mains voltage with 50w bulbs. The bulbs are such that heat is shed away from the light and not back up into the light fitting.

I have been told that the 0.75mm flex is wrong and either needs to be changed to 1.5mm cable between each downlight; or i need to change the downlighters to low voltage units. Another person has told me that the 0.75mm flex is ok to carry this load.

Any clarity would be appreciated.

 

[Taylortwocities]

A lot of questions::

What is the size of fuse protecting this lighting circuit?

and then 0.75mm flex to the next downlight and so on in series.

I seriously doubt they are in series. I hope that you mean in parallel!

or i need to change the downlighters to low voltage units

But the lights you have ARE low voltage. Perhaps the person who suggested this really means extra low voltage?
I would ignore that person - if the load is too much for 0.75mm² cable on the mains side then putting in a 12v transformer won't make things any better. Watts are watts. In fact you might need much thicker cable on the ELV side of the transformer -it depends on the layout.

Now, more important. this 0.75mm² cable that goes from light to light. Is this two core, or three core? It should be three core, if it is, are the earth conductors connected through right from start to the finish?

So, answers to your query after more info from you! Thanks!

 

[ericmark]

Table 52.3 - Minimum cross-sectional area of conductors
Gives 1.0mm² for non flexible cables
0.75mm² for flexible cables
With note on 0.75mm In multicore flexible cables containing seven or more cores. Note 1 applies which says NOTE 1: In signalling and control circuits intended for electronic equipment a minimum cross-sectional area of
0.1mm² is permitted.

413.3.4 Flexible cables and cords shall be visible throughout any part of their length liable to mechanical damage.

It could be argued that therefore with the exception of the cable from ceiling rose to lamp holder it should not be used as not visible.

But 0.75mm² is rated at 6A and since ceiling roses are also rated at 6A in most houses the maximum size overload is 6A so it will not be overloaded.

1mm is rated method 100# at 13A so where ceiling roses are not used then you could have a 10A overload and 1.5mm is full 16A

559.6.1.6 Lighting circuits incorporating B15. B22. E14. E27 or E40 lampholders shall be protected by an overcurrent protective device of maximum rating 16A.

8 x 50W < 2A so in real terms I don't see a problem but if one wanted to be pedantic I can see how someone could object.

PS even 24 x 50W < 6A but getting very close what size in the breaker or fuse? I am referring to LV lamps i.e. 230v not ELV if they are ELV then another story.

 

[TicklyT]

Could 0.75mm heat resistant flex be considered more suitable for the environment around downlighters than PVC T&E?

 

[Taylortwocities]

Not necessarily. The terminal block for the D?Ls are some distance from the lamp - if installed correctly.

 

[kambhari]

Hello All

Thank you for your replies.
The MCB protecting the ground floor lighting circuit is currently 6A, but i can change this if needed? The 0.75mm flex is 3 core and the third core (earth) is connected in the junction box (above the ceiling). The downlighters have not been installed yet so i cannot say that the earth wire is connected within each downlighter. The downlighters are from the JCC range and are known as twistlock mains voltage downlighters. Apologies if i got the series / parralel thing wrong. What meant is that each downlight has a feed flex coming into it and a feed flex going out to serve the next light. I really am trying to avoid having to change the 0.75mm cable as that would mean ripping down the ceiling due to the cross battening above. I hope this provides the info requested.

 

[ericmark]

Although it may not be exactly to regulations I don't see anything wrong in what you have.

In other words if some man from the council is saying they are wrong he is likely correct but if its just for your own piece of mind I would not worry.

Unless the flex is unsuitable because of heat there is really no problem and if it is a heat problem then bigger cable will not help it would need to be higher temperature cable.

Don't like the word heat proof as all cable even glass fibre insulated has a limit it's just a higher limit.

But size wise no real problem.

Being pedantic one could give many reasons volt drop, earth loop impedance etc. But if in my house I would not worry.