.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)

 

[alasdairp]

Thanks for responding.
However:
599 just allows "flexible cable" and has nothing about the cable rating. The drop flex is clearly visible - effectively an appliance lead wired to the rose instead of plugged - so is not really a part of the "main wiring installation". It may be replaced by a non-electrician.

433.1.1 states that the ocpd must be rated no more than 1.45 times the rating of the lowest cable (here 6A for 0.75mm2 cable) so MCB needs to be less than 8.9A - hence it has to be a 6A one - to meet the Regulations.

So, I still believe that my interpretation is correct.

 

[Ian1981]

Thanks for responding.
However:
599 just allows "flexible cable" and has nothing about the cable rating. The drop flex is clearly visible - effectively an appliance lead wired to the rose instead of plugged - so is not really a part of the "main wiring installation". It may be replaced by a non-electrician.

433.1.1 states that the ocpd must be rated no more than 1.45 times the rating of the lowest cable (here 6A for 0.75mm2 cable) so MCB needs to be less than 8.9A - hence it has to be a 6A one - to meet the Regulations.

So, I still believe that my interpretation is correct.

433.3.1 indent (ii)

Where the ocpd is intended to afford fault protection only, In can be greater than Iz and I2 can be greater than 1.45 Iz ,as long as the protective device is selected for compliance with 434.5.2

What overload requirements does a cable supplying a single downlight require other than that the cable can take the proposed load?

 

[Ian1981]

433.3.1 indent (ii)

559.5.1.204

Perhaps try reading past the first regulations in the section
That’s in regards to lamp holders but my point is with regards to the ocpd and the fact that overload protection is clearly not a requirement

 

[Midwest]

Ian, have you been reading the regs book all day.

 

[Ian1981]

Ian, have you been reading the regs book all day.

It was from the 17th edition ( a much better read)
Don’t like that 18th edition, there never as good as the one before it

The authors are loosing their touch​

 

[Deleted member 26818]

I think the red one was the best.

 

[Ian1981]

I keep the red one around just for the fact it has the curves for bs 1361 fuses
Extremely sad I know

 

[Shoei]

Interesting points raised here and in view of the safety implications, worth working out a clear interpretation. I can't agree that there are any parts of a normal installation where it's allowed to compromise fault protection, 434.3 suggests a few odd situations where fitting a current trip may be unsafe, but they are odd situations.

559.5.1.204 specifies the maximum overcurrent protection for circuits containing one of 5 different lampholders but that's not permission to stick a 16A MCB on any circuit with one connected, regardless of cable CSA. The basic principle for overcurrent protection is set out in 131.4, and 434 sets out in detail how it must be implemented.

Sometimes the manufacturer of a luminaire specifies the required current limit, but often they just tell the installer to comply with regulations, as these MIs from MK illustrate. If a 0.75mm wire is fitted the protection should be designed accordingly, the design calculation is in 434.5.2, a 0.75mm conductor on a 16A MCB doesn't comply with 434.5.2.

 

[Ian1981]

Interesting points raised here and in view of the safety implications, worth working out a clear interpretation. I can't agree that there are any parts of a normal installation where it's allowed to compromise fault protection, 434.3 suggests a few odd situations where fitting a current trip may be unsafe, but they are odd situations.

559.5.1.204 specifies the maximum overcurrent protection for circuits containing one of 5 different lampholders but that's not permission to stick a 16A MCB on any circuit with one connected, regardless of cable CSA. The basic principle for overcurrent protection is set out in 131.4, and 434 sets out in detail how it must be implemented.

Sometimes the manufacturer of a luminaire specifies the required current limit, but often they just tell the installer to comply with regulations, as these MIs from MK illustrate. If a 0.75mm wire is fitted the protection should be designed accordingly, the design calculation is in 434.5.2, a 0.75mm conductor on a 16A MCB doesn't comply with 434.5.2.

No said anything about compromising fault protection.
Unless the 0.75 flex is too small regarding the adiabatic equation for the csa of the cpc and is too small for the thermal constraints of the live conductors during fault conditions, then there’s not an issue there.( protection by a class 3 current limiting mcb say 6 amp type B will likely satisfy this )
No one said lump it on a 16 amp mcb that point was brought up with the fact that a lamp holder can have omission of overload protection.

Regarding the use of the cable, my only involvement of this thread was in response to another member about whether all aspects of the 0.75 flex required overload protection or not.

 

[Deleted member 26818]

The problem with 559.5.1.204, is that the only 16A overcurrent protective devices that would satisfy disconnection times would be MCBs or RCBOs.
As such, I don’t quite understand how it is determined that a 0.75mm2 conductor on a 16A MCB would not comply with 434.5.2?

 

[Wilko]

Hi - agree Reg 434.5.2 is important here.
So for fun I apply some values to the equation given, being k=115 (table 43.1), S = 0.75mm (drop flex conductor being discussed) and picking current as 160A, I get t = 0.29 seconds.
From Figure 3A5 we know that a 16A C type MCB will trip in less than 0.1 seconds when passing 160A.
So I think the 0.75 mm conductor is potentially up to the task of handling the fault clearance current without heat damage.

 

[Lucien Nunes]

Agreed. The crux of this is that the load on each 0.75mm² flex is well below its rating, regardless of whether the transformer is wirewound or electronic, and of whether the lamp is LED or filament. If any of the above were to pass a current sufficient to overload the cable then it will self-destruct rapidly or instantly. Therefore, it can be seen as a fixed load in all cases, and only fault protection is needed. According to Wilko's numbers, that is OK on a B6, how about 10A OCPD?

TBH, I would be more concerned to find out whether the flexes have been correctly installed at the junction boxes. I have no issues with flex having been used per se, but the boxes must be suitable for it. If they are not, and the connections may be poor or subject to stress. In any event the cable, flex or twin+earth, should be supported clear of the luminaires.

 

[Shoei]

No said anything about compromising fault protection.
Unless the 0.75 flex is too small regarding the adiabatic equation for the csa of the cpc and is too small for the thermal constraints of the live conductors during fault conditions, then there’s not an issue there.( protection by a class 3 current limiting mcb say 6 amp type B will likely satisfy this )
No one said lump it on a 16 amp mcb that point was brought up with the fact that a lamp holder can have omission of overload protection.

Regarding the use of the cable, my only involvement of this thread was in response to another member about whether all aspects of the 0.75 flex required overload protection or not.

Thanks for the clarification Ian, agree with your points about thermal constraints and that a B6 MCB would be fine when 0.75mm flex is in circuit.

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)

I took this to imply that a pendant lampholder can be installed without overload protection. Maybe I read this the wrong way,

The problem with 559.5.1.204, is that the only 16A overcurrent protective devices that would satisfy disconnection times would be MCBs or RCBOs.
As such, I don’t quite understand how it is determined that a 0.75mm2 conductor on a 16A MCB would not comply with 434.5.2?

559.5.1.204 doesn't recommend a 16A MCB, it sets the maximum limit when any of the 5 lampholders listed are used. To comply, the conductors should be sized to withstand any prospective fault current/disconnection time.

If a fault caused (say) 70A to flow in the 0.75, using a start temperature of 70 degrees and table 43.1, the limit temperature is reached in 1.5s. A compliant B16 can allow 70A to flow for over 10s (Fig 3A4) so the design fails 434.5.2, it needs more copper or a smaller/faster MCB/RCBO.

Referring back to the OP's question I think he needs to check for a 6A protective device on his 0.75mm connected circuit, otherwise the design needs to be reviewed.

 

[Wilko]

Thing is, I would say you can't get a fault of 70A constant, that would be a load. A fault should cause current to flow limited by the impedance at that point. Otherwise you could make this argument for all conductors being affected by "excessive currents that are almost faults". Just my thought .

 

[Ian1981]

A lamp holder supplying one lamp is not going to overload period.
If overload protection was required for every single pendant set and manufacturers live and neutral supplies to led drivers and the like then nothing would comply

 

[Shoei]

Thing is, I would say you can't get a fault of 70A constant, that would be a load. A fault should cause current to flow limited by the impedance at that point. Otherwise you could make this argument for all conductors being affected by "excessive currents that are almost faults". Just my thought .

Fair point Wilko, all I can say is that faults don't always follow the script and I've seen this happen. For example, a mains LED lamp contains a switch mode current source. If a transient turns the electronics into a piece of slag it's anyone's guess what current will flow. I would indeed argue that all wiring should be sized so that it can't start a fire at any current level/trip time in the range of its OCPD.

A lamp holder supplying one lamp is not going to overload period.
If overload protection was required for every single pendant set and manufacturers live and neutral supplies to led drivers and the like then nothing would comply

We'll have to agree to disagree Ian, my understanding is that the type of load is irrelevant. The regs are designed to prevent fires by holding temperature rise in wiring to safe limits in any single fault condition. The core principle is that the protective device must disconnect before any downstream conductor overheats, even on lighting circuits.

 

[Lucien Nunes]

If a transient turns the electronics into a piece of slag it's anyone's guess what current will flow

SMPSUs and transformers (unless they are non-compliant junk) include internal protection such as a fuse, thermal fuse or fusible resistor to prevent this scenario, which qualifies them as fixed loads. Even if that protective measure fails, one has to consider what will happen if this arbitrary current does flow.

To cause 0.75mm² to get dangerously hot in less than a minute or two would need maybe 15A or more. At that current, the faulty 'load' is dissipating 15 x 230 = 3.5kW, so the 'load' will be on fire long before the cable. There is no realistic situation where a small electronic device will dissipate enough power for long enough to damage the cable, without being destroyed itself and either going open-circuit and interrupting the current, or short-circuit and tripping the OCPD.

 

[Shoei]

SMPSUs and transformers (unless they are non-compliant junk) include internal protection such as a fuse, thermal fuse or fusible resistor to prevent this scenario, which qualifies them as fixed loads. Even if that protective measure fails, one has to consider what will happen if this arbitrary current does flow.

To cause 0.75mm² to get dangerously hot in less than a minute or two would need maybe 15A or more. At that current, the faulty 'load' is dissipating 15 x 230 = 3.5kW, so the 'load' will be on fire long before the cable. There is no realistic situation where a small electronic device will dissipate enough power for long enough to damage the cable, without being destroyed itself and either going open-circuit and interrupting the current, or short-circuit and tripping the OCPD.

I understand the logic Lucien, but the regs boil it down to straightforward calculation, they reasonably put the onus for safety on the fixed installation because that's what the designer controls. There is an awful lot of non-compliant junk about and common faults which don't involve the load at all.

Your scenario assumes the fault resistance is quite large, the concern is low-impedance faults where most or all of the power is dumped in the fixed wiring. For compliance the wiring must be sized to remain below the specified limit temperature under any fault current/power that the OCPD can deliver before tripping. 434.5.2 is unambiguous about routes to compliance, it doesn't make an exception for lighting circuits and it's easy to work out. I'm a bit surprised that anyone would consider deviating from it without a serious risk analysis that couldn't be worth the trouble to validate in any normal installation.

 

[Lucien Nunes]

In the case of

low-impedance faults where most or all of the power is dumped in the fixed wiring.

the impedance of the defective load must be much lower than that of the circuit, in which case the current will approximate to the PSCC and will trigger the fault protection. If the impedance of the defective load is high enough to significantly reduce the current below this value, it will dissipate enough power to self-destruct.

But I agree; my statement 'only fault protection is needed' should have been qualified with 'for safety' as I'm not claiming it to be explicitly compliant, just practically safe.

Historically, though, there was an exemption for the wiring of light fittings specifically from overload protection, subject to being of a minimum size. There was a standard conductor known as a 'fittings wire' that complied. I can't recall when that was revoked, I should read up.

 

[Shoei]

Thanks for the clarification Lucien, I agree it would be bad luck to start a fire with a B16 MCB on a 0.75 mm wire and mostly it would be fine, but if enough people do it often enough then someone will have a bad experience.