Flexible Cable from junction box to light fitting.

[nickstuart]

Hi,

I am planning on adding a couple of extra lights to an existing 1.5mm radial light circuit. in my enthusiasm I bought some 1.5mm twin and earth and some junction boxes and was quite happy.
But, I bought a 1 mm heat resistant 3 core cable to run form the junction boxes to the light fittings.

I suddenly wondered if I'm required by regs to use 1.5 mm flexible cable?

Then all sorts of doubts crept in !
1) one of my new lights is a downlighter so my plan was to feed the flexible cable throught the wall behind the light, the lighting circuit runs behind this wall anyhow in a ceiling void of an adjacent room.

2) The other lights were 2 led spots in the ceiling , again I was going to use the flexible cable to run from the junction box to the fitting.

Am I right to use flexible cable for this and is 1 mm ok ....?

Many thanks in advance !

 

[Taylortwocities]

What is the amp value of the fuse/MCB for the lighting circuit?
Its probably 6, or 10amp ?

 

[nickstuart]

Hi,

There is a 6A fuse

Thanks

 

[PrenticeBoyofDerry]

Flexible cable can be used providing it is suitable for the location and is durable enough not become damaged whilst being installed or during service. This providing the correct calculation have been made and the flex is suitable for the load it will carry.
Also and junction or joint must be accessible for maintenance, inspection and testing or the joint must be made in a method that is considered maintenance free as detailed in BS7671.
Any cables/flex buried within walls less than 50mm will require RCD protection if not mechanically protected and the cable/flex must also be installed within the permitted safe zones within walls and ceiling voids.
Also there is a legal responsibility to make reasonable provision for the prevention of injury and fire, that could be caused by the design and installation of this work. So you need to prove this is safe!

 

[nickstuart]

Many thanks for your replies.

So the cable size may be smaller than the size of the circuit ?

The basis I have used for the cable size of 1mm is that it connects a single light fitting ... even if I fitted 100w halogen bulbs it would be less than 0.4 A.

Does that sound sensisble?

Many thanks in advance.

 

[JohnW2]

So the cable size may be smaller than the size of the circuit ?

All that matters is that no cable, anywhere in the entire circuit, is too small to be adequately protected by the fuse/MCB serving the circuit.

Hence, if your circuit is protected by a 6A fuse/MCB, all that really matters is that all cable in that circuit has a current-carrying capacity of at least 6A (and 1 mm² easily satisfies that). That would remain the case even if (unnecessarily), there were 'enormous' cables elsewhere in the circuit. There is never a problem using a cable of bigger size than is necessary.

For some reason, 1.5 mm² has become quite fashionable for domestic lighting circuits. However, except in enormous houses, or very high lighting loads, 1 mm² is usually quite adequate. If cable runs are extremely long, there could be a problem with 'voltage drop' (rather than current-carrying capacity) with 1 mm² cable, but that's an unusual situation.

Kind Regards, John

 

[PrenticeBoyofDerry]

Ideally you want to keep all conductors the same CSA throughout the hard wiring of the circuit, unless discrimination was to be offered via a fuse connection.
If you circuit breaker is rated at 6A, there would be nothing unsafe by using 1.00mm2 t&e, some flex ratings may differ so that should be checked through the manufacturer's data, but generally 1.0mm flex can handle 10A, but use of this could require further calculation through de-rating factors along it's cable route as a de-rating factor of 50% would reduce the safe current carrying capacity of the flex to less than the protective device, but lets not over complicate things, but be aware of this when designing whole or parts of circuits!
As you state the load you intend to use is one that will not make the cable under full load to become unsafe through current demand, but that demand one day could change to one it cannot handle, unlikely you may say but you never know!!

 

[JohnW2]

Ideally you want to keep all conductors the same CSA throughout the hard wiring of the circuit, unless discrimination was to be offered via a fuse connection.

In the interests of 'neat design', perhaps - but, as I said, in electrical terms there is really no such requirement - the only requirement being that none of the cable(s) has a CSA such that (with the installation method used), it's CCC is less than that of the circuit's protective device.

Kind Regards, John

 

[nickstuart]

Cheers guys, I'm better informed!

I will stick with the 1mm flex and make note not to wire my cooker into the light socket

Many Thanks

 

[PrenticeBoyofDerry]

but, as I said, in electrical terms there is really no such requirement - the only requirement being that none of the cable(s) has a CSA such that (with the installation method used), it's CCC is less than that of the circuit's protective device.

I think I covered all that. What you failed to mention was the factors that concern de-ratings, only that 1.00mm flex would be safe on a 6A MCB, which is not strictly true. As we are talking about 1.00mm2 flex rather than T&E cable, as you know 1.00mm2 flex is rated less than the standard 1.00mm T&E. Therefore awareness of any de-rating is more important, as I said a de-rating of 50% (which is not that unusual on lighting circuits) will then make the safe current carrying capacity of this flex below the rating of the device protecting it. (which is electrically in correct and potentially unsafe)
So some investigation into this is advised. Regardless of any potential cooking appliance being connected to the lighting circuit, if the cable/flex is rated correctly and de-ratings applied, when required to achieve this and cable/flex rated suitably. A cable/flex with a calculated current rating greater than the protective device, this would then be safe, as the MCB would trip before the cable/flex sets on fire!

The point I am making on behalf of safety, that adding a cable/flex is not always that simple!

I see 1.5mm2 T&E installed, the first question I ask myself "is there an electrical reason why?"

 

[nickstuart]

Many thanks - I get your loast point !

 

[JohnW2]

but, as I said, in electrical terms there is really no such requirement - the only requirement being that none of the cable(s) has a CSA such that (with the installation method used), it's CCC is less than that of the circuit's protective device.

I think I covered all that. What you failed to mention was the factors that concern de-ratings, only that 1.00mm flex would be safe on a 6A MCB, which is not strictly true.

I agree with all that, but you appear to be refering back to my initial comments, when I agree that I did not mention de-rating of the cable. However, I subsequently did mention that ("with the installation method used" includes 'de-rating factors'), including in the quote above to which you are responding.

It was, as you will have seen, you assertion that all fixed wiring in a circuit should ideally be of a same size that I was commenting on - since, as I said, I can see no electrical basis for that, per se. I suppose I really had T+E in my mind when I said that 1 mm² on a 6A MCB would be OK. With flex, there clearly is somewhat of a difficulty in establishing its CCC other than 'alone in free air', since the regs do not, AFAIAA, give figures for any other situations.

I see 1.5mm2 T&E installed, the first question I ask myself "is there an electrical reason why?"

A perfectly reasonable question to ask - but, as I wrote earlier, when 1.5 mm² T&E is (as it commonly is) found in lighting circuits, it seems that there generally is no electrical reason. I think you would have to try quite hard to get the CCC of such cable below 6A and, as I said, only very long circuits would make it unsuitable for VD reasons.

Kind Regards, John