.75mm flex used from junction box to downlights

[Exhausted Pigeon]

Hi Everyone.

I'd really appreciate some advice. I have a little cafe which was all wired up by someone qualified. The issue is that someone put in a few 50w halogen bulbs and they didn't check the positioning of the wires after a ceiling skim. There had been no issues for over 4 years whilst using LED GU10's, but I've read quite a few things in the forum about the 50w bulbs causing fires regardless of the cable being 1.5mm twin and earth or .75mm flex.

There are currently 3 rings of downlights each containing around 15 bulbs on the ring. Junction boxes have been used with 1.5mm cable running between them and the consumer unit. The downlights have then been wired from the junction boxes with .75mm flex.

As the .75mm flex only has a single downlight with a max of 50w going through this, I'm of the opinion this is adequate, but I've got a 20 year old electrician telling me it has to be 1mm twin and earth.

I've already established this is wrong as the downlights are double insulated.

My question is... does the flex have to be 1mm?

As halogen bulbs are no longer being sold in the UK, I cannot see any future issues, but obviously I wouldn't want to have incorrect wiring.

Thanks in advance.

 

[Midwest]

Not quite sure what you are saying, but in the past, down lights installed adjacent to wooden joists with halogen lamps, have been known to cause thermal damage to the joists.

LED lamps/luminaires do not get as hot, but if install too close to any surface, can overheat & fail. 0.75mm flex has a current carry capacity of approx. 6 amps. A singular LED GU10 lamp would only be a few watts, so 0.75mm flex would be more that sufficient for one such luminaire.

 

[Exhausted Pigeon]

Thanks Midwest.

The ceiling has no timber. It's all steelwork.

There was localised burns on the flex which I think was down to the way the downlights were put back after the ceiling skim. I'm assuming that the heat of the 50w bulb must have had a wire too close to it, hence the local burn.

It's a relief to hear that .75mm is ok. I was sure my original electrician would have got that right. Thank you.

 

[Wilko]

Hi EP and Welcome to the Forum!
A drop down flex of 0.75mm is fine for the load of the lamp and by itself won't be a fire risk. But it's not ok to use it in the fixed wiring, which maybe what he's thinking or has seen (?). As per @Midwest its the heat from the 50W lamps that may be a problem, particularly with insulation around. I've replaced many 12v 50W lamps and transformers that have heat damage.

 

[Exhausted Pigeon]

Hi Wilko.

Thanks for the reply.

From the junction box to the consumer unit is 1.5mm which the young electrician confirmed. Is he perhaps not aware that when it's coming off a junction box it can be a lower spec wire? The run from the junction box to the light is about 75cm.

If LED bulbs are used as opposed to the offending 50w ones, am I right to assume the likelihood of this happening again is very low?

 

[Taylortwocities]

If LED bulbs are used as opposed to the offending 50w ones, am I right to assume the likelihood of this happening again is very low?

When doing electrical work, one must assume the worst case scenario. In this case, you cannot assume that LED bulbs will always be used on this wiring.
You know.........the LED bulb pops and someone puts in one of those old 50W halogens, just as a "temporary" measure. Two years later, its still there.......

 

[Exhausted Pigeon]

Thanks Taylortwocities. Yes, that's exactly what I'm trying to work out.

Even with a 50w bulb tho, from the sound of it, the .75 flex should be sufficient.

As a precaution can I perhaps put a sticker to say LED only? That way as long as people can read it will reduce the risk of anything further going wrong. From the sound of it 50w bulbs are an issue regardless of the cable size.

 

[Midwest]

When doing electrical work, one must assume the worst case scenario. In this case, you cannot assume that LED bulbs will always be used on this wiring.
You know.........the LED bulb pops and someone puts in one of those old 50W halogens, just as a "temporary" measure. Two years later, its still there.......

OP has 1.5mm fixed wiring, and 0.75mm flex from JB to each singular luminare.

 

[alasdairp]

Surely that does then require a 6A MCB (i.e. not 10A) for the 1.5mm feeder due to the capacity of all the (concealed) 0.75 mm flexes to the indivudual luminaires? That should be checked.

 

[Andy78]

524.1 which references table 52.3 allows a minimum of 0.75mm copper flexible cable for use in fixed wiring for any application.

I would not have any concerns regarding the size of cable. A single light will not be subject to overload.

 

[Deleted member 26818]

The minimum size of cable for lighting circuits is now 1mm2.
0.75mm2 flex is allowed if the product instructions recommend it and for Extra Low Voltage lighting.

 

[Midwest]

The OP (I think) has mentioned the luminaire has been connected to a joint box with 0.75mm flex.The fixed wiring between JB's, has been installed in 1.5mm.

 

[alasdairp]

524.1 which references table 52.3 allows a minimum of 0.75mm copper flexible cable for use in fixed wiring for any application.

I would not have any concerns regarding the size of cable. A single light will not be subject to overload.

So in a lightweight suspended ceiling with lots of downlighters and often with some sharp edges and corners there is no risk of flex damage? Mouse damage? Poor workmanship, even? Lots of downlighters and hence trailing flexes.

I thought that the fuse/MCB was meant to protect the wires and cables from overheating... Surely that is why the public is meant to put a 3A fuse in a lamp plug (and not leave the 13A one that came with the plug). So how come this does not matter in cables lying on a suspended ceiling?
I agree it would probably be OK, but legally?

I thought that the circuit should be fused/protected at a value primarily set by the size of the wiring. The expected design load (with diversity) sets the size(s) of the cables needed and the fuse value relates to this. 0.75mm^2 cable should be protected at 6A as I understand the rules.

 

[Shoei]

Hi Everyone.

I'd really appreciate some advice. I have a little cafe which was all wired up by someone qualified. The issue is that someone put in a few 50w halogen bulbs and they didn't check the positioning of the wires after a ceiling skim. There had been no issues for over 4 years whilst using LED GU10's, but I've read quite a few things in the forum about the 50w bulbs causing fires regardless of the cable being 1.5mm twin and earth or .75mm flex.

There are currently 3 rings of downlights each containing around 15 bulbs on the ring. Junction boxes have been used with 1.5mm cable running between them and the consumer unit. The downlights have then been wired from the junction boxes with .75mm flex.

As the .75mm flex only has a single downlight with a max of 50w going through this, I'm of the opinion this is adequate, but I've got a 20 year old electrician telling me it has to be 1mm twin and earth.

I've already established this is wrong as the downlights are double insulated.

My question is... does the flex have to be 1mm?

As halogen bulbs are no longer being sold in the UK, I cannot see any future issues, but obviously I wouldn't want to have incorrect wiring.

Thanks in advance.

The load power rating isn't the only factor for cable size, fitting LEDs doesn't mean you can use thinner wire. The cable must be heavy enough to carry fault current for long enough to trip the protection without risk of fire, so if one of the LED's fails short, the MCB must trip well before the cable overheats and sets the building alight. Check the rating of the MCB on that circuit, if it's 6A then don't worry about it.

 

[Andy78]

So in a lightweight suspended ceiling with lots of downlighters and often with some sharp edges and corners there is no risk of flex damage? Mouse damage? Poor workmanship, even? Lots of downlighters and hence trailing flexes.

I thought that the fuse/MCB was meant to protect the wires and cables from overheating... Surely that is why the public is meant to put a 3A fuse in a lamp plug (and not leave the 13A one that came with the plug). So how come this does not matter in cables lying on a suspended ceiling?
I agree it would probably be OK, but legally?

I thought that the circuit should be fused/protected at a value primarily set by the size of the wiring. The expected design load (with diversity) sets the size(s) of the cables needed and the fuse value relates to this. 0.75mm^2 cable should be protected at 6A as I understand the rules.

I'm not sure what your assumptions about the installation surroundings have to do with cable size. My comment you quoted related purely to cable size.

I'm also not sure why you have assumed the OCPD size, it's not been mentioned.

 

[alasdairp]

I'm not sure what your assumptions about the installation surroundings have to do with cable size. My comment you quoted related purely to cable size.

I'm also not sure why you have assumed the OCPD size, it's not been mentioned.

I was also referring to cable size and the fact that the OCPD size was not stated did concern me. The ring to the junction boxes is wired with 1.5 mm2 (see original post) and I would expect that to be protected by a 10A MCB - maybe even a 16A MCB, whereas Table 4F3A states that any installation circuit with 0.75 main conductors (phase and neutral) must be protected by a maximum 6A MCB (or fuse) and 0.5mm2 cable by a 3A device.

I was surprised this had not been mentioned by anyone else (unless I missed it), and several people said they use 0.75 mm2 for downlighters. The OnSite Guide only gives tables down to 1mm2. However BS7671 does allow thinner wires if protected by a suitable over-current device. The ratings are conservative and a 10A MCB would possibly be adequate to avoid over-heating due to a fault, but we are required to work to BS7671, and that states 6A max.

If you had mentioned the 6A MCB requirement, I would not have responded. I read your comment as "it is of no concern".

Well, sorry, but it is if he is to comply with the wiring regulations. The only other way would be to provide a local lower-value (5A or 6A) SFC for the downlighter circuit, wall mounted to be easily accessible.

Maybe I am mis-understanding something?

 

[Wilko]

I was also referring to cable size and the fact that the OCPD size was not stated did concern me. The ring to the junction boxes is wired with 1.5 mm2 (see original post) and I would expect that to be protected by a 10A MCB - maybe even a 16A MCB, whereas Table 4F3A states that any installation circuit with 0.75 main conductors (phase and neutral) must be protected by a maximum 6A MCB (or fuse) and 0.5mm2 cable by a 3A ...

Hi - Table 4F3A is part of Appendix 4 (Informative) Current Carrying Capacity ++ . It is the current carrying capacity of 0.75mm flexible cable that is listed as 6A, while protection from overload and fault currents are not mentioned. In this case I think it's fair for us to assume the load current for the drop down flex to an individual lamp was always going to be less than 6A.

 

[alasdairp]

Hi - Table 4F3A is part of Appendix 4 (Informative) Current Carrying Capacity ++ . It is the current carrying capacity of 0.75mm flexible cable that is listed as 6A, while protection from overload and fault currents are not mentioned. In this case I think it's fair for us to assume the load current for the drop down flex to an individual lamp was always going to be less than 6A.

Yes, of course it will be much less than 6A - BUT an electronic switch mode power supply could badly fail and cause a short (I have seen this) or the cable could be damaged on the suspended ceiling metalwork.

I would welcome some high-level feedback. PLEASE, someone!

My understanding of BS7671 requirements is that all 'installed' circuit wiring overload protection device (MCB or Fuse) MUST be appropriate for the cable current carrying capacity. i.e. if you have a section of 0.75 mm2 which can only officially carry 6A, then the circuit needs to be overload protected at 6A. Either at the CU or by means of fused wall switch for that part of the circuit.

Is that not correct?

 

[Midwest]

Yes, of course it will be much less than 6A - BUT an electronic switch mode power supply could badly fail and cause a short (I have seen this) or the cable could be damaged on the suspended ceiling metalwork.

I would welcome some high-level feedback. PLEASE, someone!

My understanding of BS7671 requirements is that all 'installed' circuit wiring overload protection device (MCB or Fuse) MUST be appropriate for the cable current carrying capacity. i.e. if you have a section of 0.75 mm2 which can only officially carry 6A, then the circuit needs to be overload protected at 6A. Either at the CU or by means of fused wall switch for that part of the circuit.

Is that not correct?

I'm lost a bit now. Where did 10A mcb/fuse come from?

 

[Ian1981]

Yes, of course it will be much less than 6A - BUT an electronic switch mode power supply could badly fail and cause a short (I have seen this) or the cable could be damaged on the suspended ceiling metalwork.

I would welcome some high-level feedback. PLEASE, someone!

My understanding of BS7671 requirements is that all 'installed' circuit wiring overload protection device (MCB or Fuse) MUST be appropriate for the cable current carrying capacity. i.e. if you have a section of 0.75 mm2 which can only officially carry 6A, then the circuit needs to be overload protected at 6A. Either at the CU or by means of fused wall switch for that part of the circuit.

Is that not correct?

No.
The regulations allow omission of overload protection as detailed in section 433 for certain applications.
After all for example, a pendant lamp holder does not require overload protection and is allowed to be protected by an ocpd of up to 16 amp( see section 559)