Why 1.5mm cable for lighting circuits?

[JohnW2]

Your question is very valid as I know my boss never calculated volt drop it was me as the site electrician who would suggest maybe he should use thicker cable and until the loop impedance meter was used really we had only a guess at the result.

If you are referring to my question, it did not relate to VD (which, quite honestly, I find it difficult to get excited about) but, rather, EFLI - in particular confirming that Zs is low enough to provide the required disconnection times.

If an electrician measures Zs on an unloaded circuit (with conductors at ambient temperature) and finds that it is 'just low enough' to achieve the required disconnection times, then if that circuit becomes heavily loaded (hence much higher conductor temperatures), then the Zs may well rise to a level which is no longer low enough to satisfy the ADS requirements. What are your views about this?

Kind Regards, John

 

[EFLImpudence]

Which leads me to ask, when an electrician records R1+R2, or any other variable relating to earth look, is the record the actual measured value or is it corrected for temperature rise?

I shall have to say, it depends.

The certificates and programme I use state and automatically insert the "maximum Zs permitted by BS7671" - the values stated in 411.

Therefore I insert the actual measured values of Zs.
This figure is usually so far short of the maximum (as is your 0.91+Ze compared with the maximum of 7.66) that it doesn't matter.

The same with a ring circuit maximum of 1.44 and an actual reading of 0.4, for example.

Should the values be near the limit then a more detailed calculation can be done as per Appendix 14.

 

[JohnW2]

The certificates and programme I use state and automatically insert the "maximum Zs permitted by BS7671" - the values stated in 411. ... Therefore I insert the actual measured values of Zs.
This figure is usually so far short of the maximum (as is your 0.91+Ze compared with the maximum of 7.66) that it doesn't matter. ... The same with a ring circuit maximum of 1.44 and an actual reading of 0.4, for example. ... Should the values be near the limit then a more detailed calculation can be done as per Appendix 14.

Yes, I'd forgotten about Appendix 14. In fact, that Appendix seems fairly specific about 'near to the limit', in indicating that the corrections for temperature 'may' (it doesn't say 'should' or 'must') be undertaken if the measured Zs is greater than 0.8U0/Ia.

Kind Regards, John

 

[westie101]

So nearly 40 years ago as a trainee it was suggested by our instructors that 1.0mm was more fragile and likely to break in terminations than 1.5 so it might be a better idea to use that instead.

 

[JohnW2]

So nearly 40 years ago as a trainee it was suggested by our instructors that 1.0mm was more fragile and likely to break in terminations than 1.5 so it might be a better idea to use that instead.

I suppose no-one could deny that the smaller conductor was probably 'more fragile'! Another interesting issue is that, for whatever reason, whilst it allows 1mm² for 'lighting'circuits, BS7671 requires a minimum of 1.5mm² for 'power circuits' (which it does not define), even if the CCC of 1mm² would be adequate for the circuit!

Kind Regards, John

 

[jason61c]

So nearly 40 years ago as a trainee it was suggested by our instructors that 1.0mm was more fragile and likely to break in terminations than 1.5 so it might be a better idea to use that instead.

I suppose no-one could deny that the smaller conductor was probably 'more fragile'! Another interesting issue is that, for whatever reason, whilst it allows 1mm² for 'lighting'circuits, BS7671 requires a minimum of 1.5mm² for 'power circuits' (which it does not define), even if the CCC of 1mm² would be adequate for the circuit!

Kind Regards, John

It'd be hard to argue that a transformer is something a power circuit needs? So if you've got transformers you need 1.5?

 

[flameport]

Another interesting issue is that, for whatever reason, whilst it allows 1mm² for 'lighting'circuits, BS7671 requires a minimum of 1.5mm² for 'power circuits' (which it does not define), even if the CCC of 1mm² would be adequate for the circuit!

The OSG lists various circuits, including one in 1.0mm cable with a 16A fuse or MCB.
Clearly not for lighting as it is also listed for 5% volt drop with a terminal load.

The OSG is of course only a guide, but plenty of people will use and rely on it to contain correct information.

 

[OwainDIYer]

It'd be hard to argue that a transformer is something a power circuit needs? So if you've got transformers you need 1.5?

Is that a shaver transformer or a lighting transformer?

 

[JohnW2]

Another interesting issue is that, for whatever reason, whilst it allows 1mm² for 'lighting'circuits, BS7671 requires a minimum of 1.5mm² for 'power circuits' (which it does not define), even if the CCC of 1mm² would be adequate for the circuit!

The OSG lists various circuits, including one in 1.0mm cable with a 16A fuse or MCB. ... Clearly not for lighting as it is also listed for 5% volt drop with a terminal load. ... The OSG is of course only a guide, but plenty of people will use and rely on it to contain correct information.

I hadn't noticed that (and confess that, after a quick look, I can't find it in my, albeit 'red', OSG). However, it certainly would not surprise me, and it never ceases to amaze me that someone has to carefully checked to ensure that these sort of inconsistencies/contradictions between OSG and BS7671 don't exist. As you say, the OSG is only a 'guide', but it still should not be providing 'guidance' which appears to be non-compliant with the regs!

Kind Regards, John

 

[333rocky333]

I always use 1.5 i have had a few calls to sites where 1.00 mm has broke off at acccessories.
Pub accomadation round here always seems to be 1.5.
I think the spec for the jobs sometimes demand 1.5

 

[RF Lighting]

A lot of commercial contracts still spec 1.5 as standard for lighting, but 1.0 does the job fine, it's easier to run and terminate and I've never had an issue using it.

 

[ban-all-sheds]

Another interesting issue is that, for whatever reason, whilst it allows 1mm² for 'lighting'circuits, BS7671 requires a minimum of 1.5mm² for 'power circuits' (which it does not define), even if the CCC of 1mm² would be adequate for the circuit!

It's quite simple.

Either Table 54.1 says that you can use 1mm² for lighting circuits which do not use power, or it says that you have to use 1.5mm² for circuits with an undefined attribute, or it is total nonsense and therefore must be ignored otherwise it becomes impossible to have electric lighting and comply with BS 7671.

I vote for the latter.

 

[ban-all-sheds]

1mm for me too. 1.5mm is pointless, and fills a pendent and switch back box.

Some seem to think 1mm is "against regs" in their words.

Have you ever asked them which regulation(s) it contravenes?

 

[ericmark]

I am assuming amendment 1 as my old book says power and lighting circuits 1mm² copper in Table 52.3 - Minimum cross-sectional area of conductors.

I know when I took my 2382 the on-site-guide had not been printed so there were no questions on the guide and as a result I have never bought one for BS7671:2008.

How anyone can claim lighting does not require power I don't know I realise historically we had two meters one for power and one for lights and lighting electric was cheaper. There were some court cases of people plugging items like irons into the lighting sockets but this was well before my time.

My grandmother always used the light socket for the smoothing iron there were only two power sockets in the house both 15A.

As to hot or cold cables often hard to know which you are measuring. In theroy maybe the earth loop impedance is measured before energising a circuit but in practice often companies have a limited number of test sets and a roving electrician does all the testing for many sites. Heaters used to dry out plaster are hardly the standard domestic load and in real terms loop impedance testers are not that accurate. Test a circuit three times and likely three different results.

Trying to do home brew beer I have realised how long things take to warm up and cool down even the direction on the wind can alter readings. In industry yes we can look at a machine with cables on cable tray and have a good idea of running temperature but domestic we look at a consumer unit 12 way and adding up all the MCB's maybe 200 amp may be drawn but in real terms likely no more than 60 amp. Even central heating running or not will alter readings so all this hot or cold really does not work with domestic.

When we look at a B6 MCB it should trip with a loop impedance of between 6.99Ω and 8.43Ω depending on the voltage present assuming it needs 30A to trip the magnetic part. But the loop impedance to ensure volt drop is within the 6.9 volts is 1.5Ω so in real terms we with a lighting circuit are no where near the limit for the MCB even if type C was used in fact even type D with 209.714 volt will only require a loop impedance of 1.75Ω which is still above the 1.5Ω needing to ensure no more than 6.9 (3%) volt drop.

So assuming volt drop is complied with the MCB will trip on the magnetic part of the trip. At least for a 6A lighting supply.

29 meters seems a lot of cable and using the ceiling rose as a JB unlikely to be exceeded with separate up and down stairs lighting. But where the switch is used as a junction box far more cable is used. Unlike the ring working out how much cable is used does not work as all drops to switches other than the final light are not really adding to the worst reading. So one could well use a 100 meter role of cable and still be within the 29 meters maximum to any one point.

Although looking at 44 meters with 1.5mm² this also raises the loop impedance to 1.8Ω mainly as I am calculating on the incomer being 0.35Ω this is the problem of course as said in other posts if the volt drop at origin is less then the loop impedance readings will change still 29 or 44 meters but clearly if incomer is 0.25Ω then 1.5Ω drops to 1.4Ω. And in real terms you could exceed the 6.9 volt drop because the incoming supply is more stable.

Again in real terms fluorescent using wire wound ballast may have a problem at 209 volt but most other forms of lighting will not really have a problem. Most LED lights have some built in switch mode regulation and most extra low voltage lighting will also have some switch mode regulation also HF fluorescent has switch mode regulation and in real terms if you have a problem with volt drop then simply change the lamp.

Until Part P finding electricians with full test set was unusual and I am sure many will admit like me the test set was in the office but rarely used be it read the regulation book or sign out the test set you were met with the comment "What do you want that for". Even when I started using the meters on a daily basis it was earth - line loop impedance not neutral - line loop impedance and only when we got RCD protection on all circuits did I question volt drop and even consider how to measure it.

We throughout my working live would attend tool box talks and lectures to keep us updated by 2008 I was not working but still attended IET lectures and it was only then that I considered how to measure volt drop.

I must have installed many circuits which had over the allowed volt drop since 1980 worked mainly abroad or on very large contracts so most work was on temporary installation or following detailed plans. But when I took my degree late in life around 2008 we as students were not taught about how to comply with regulations. Two of us were already electricians and we would comment many times about what seemed to us a blatant disregard to regulations. So not sure if the people drawing up those detailed plans really worried about regulations?

And to conclude I must ask does volt drop really matter? Ensuring a MCB will trip with a short circuit yes, but volt drop if everything works and is safe what's the problem if it does drop to 180 volt?

 

[aesmith]

Thanks for the comments. I think I've answered one of my questions ..

Which leads me to ask, when an electrician records R1+R2, or any other variable relating to earth look, is the record the actual measured value or is it corrected for temperature rise?

On our certificate where it states "Maximum Zs permitted" it looks like they use a de-rated value, as they list 6.13 ohm for the lighting circuits, which equates to 230/30 x 0.8. So I think what they're doing is noting the max permissible Zs at ambient temperature so it can be directly compared with actual measured R1+R2 and Zs.

Regarding voltage drop, is it not the convention that lighting circuits are assessed as if the total distributed load was a single point load at 50% of the length? I can't remember where I heard that, so may not be up to date.