.75mm flex used from junction box to downlights

[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?