Voltage drop

[EFLImpudence]

Well, you will be surprised to learn that I could not actually find anywhere it was actually mentioned in relation to voltage drop.

However, all calculations are done with nominal voltage - I suppose that's what it is.

As for measuring the supply voltage - as above, I think the regulations have allowed for the varying voltage so it wouldn't really matter.

 

[JohnW2]

Well, you will be surprised to learn that I could not actually find anywhere it was actually mentioned in relation to voltage drop.

Ah! Maybe that explains why I hadn't seen it

WellHowever, all calculations are done with nominal voltage - I suppose that's what it is.

Maybe - but, with apologies for continuing to be dim, where does it say that?

As for measuring the supply voltage - as above, I think the regulations have allowed for the varying voltage so it wouldn't really matter.

One assumes that they have (i.e. they are happy with the permissible voltage drop even if the supply voltage is that the bottom of it's declared range). However, in terms of design, it is obviously necessary to know what denominator applies to the "3%". If, as you said, it's 3% of nominal, then fair enough - but, as above, I've yet to see that actually specified anywhere.

I have to say that I would have thought the most obvious interpretation of 525.3 is the 'natural' one - i.e. with the denominator being the prevailing supply voltage, whatever that may be (in which case one would presumably have to design on the basis of the lowest possible supply voltage). If we were told that the maximum permissible shrinkage of curtains at their first wash was 3%, I think most people would take that to mean 3% of whatever length they were pre-washing, not 3% of some other figure!

Kind Regards, John

 

[westie101]

Got to admit it seems a badly thought out part of the regs compared to the specific reg in ESCQR where the reference voltage is specified

 

[EFLImpudence]

When I wrote that I could not find the nominal voltage mentioned in relation to voltage drop I meant just that as that was the subject of your query.

The same can be said for all the other pages which quote calculations therefore I would think that the nominal voltage of 230V be used. This is stated in Appendix 2.

I would think common sense would rule out using the measured voltage at the time because this would lead to ridicule.

Mr.Customer, if he knew, would be asking us to come back after lunch so that he then would be able to have an extra metre on the circuit (I haven't worked it out).

 

[westie101]

Which is why I take the view that as long as it is above 216.2-3% it should be permissible!

Though it could be interpreted as 216.2 - 3% of 230V also!

 

[JohnW2]

When I wrote that I could not find the nominal voltage mentioned in relation to voltage drop I meant just that as that was the subject of your query.

Indeed. I realised that - and, as you say, what currently interests me.

The same can be said for all the other pages which quote calculations therefore I would think that the nominal voltage of 230V be used. This is stated in Appendix 2.

I don't really understand "The same can be said for all the other pages ...". Are you saying that you can't find nominal voltage mentioned on any of those pages, either? Appendix 2 tells us what the nominal voltage is, but says nothing relevant about it, or about when it should be used in calculations.

I agree that one generally uses nominal voltage for most calculations which require a voltage (even though safety would often be maximised by calculating on the basis of the 'worst case' voltage). However, at risk of seeming a pain, I think it's very different when one is talking about a 'percentage voltage drop'. On reflection, I imagine that you are probably right, in that their intention is that 3% means "3% of nominal" - but one reaches that conclusion by application of engineering common sense, rather than by reading what they have written. Anyone who knows anything about mathematics is likely to say that the "percentage voltage drop" when a voltage falls from X to Y is 100(X-Y)/X, and not 100(X-Y)/Z (where Z is some 'nominal' value of A) - don't you agree? It seems we have another badly worded bit of the regs!

...and they have dug this hole for themselves. It's only necessary to specify the maximum permissible voltage drop as a percentage if the denominator varies, so specifying it as a percentage makes one think that the denominator does vary. If they do mean "3%" of nominal, and given that Appendix 2 tells us that nominal is (and always is) 230V, then why on earth don't they just say that the maximum voltage drop on a lighting circuit is 6.9V?

I would think common sense would rule out using the measured voltage at the time because this would lead to ridicule.
Mr.Customer, if he knew, would be asking us to come back after lunch so that he then would be able to have an extra metre on the circuit (I haven't worked it out).

I suspect that the authors of BS7671 simply didn't think this through properly. I now tend to agree that their intent was probably that the maximum permissible voltage drop on a lighting circuit is 6.9V (since, as you say, it's otherwise a silly 'moving target'), so why didn't they just say so? Assuming that was their intent, by specifying a percentage, they merely set the scene for anyone with a mathematical mind to question what they really meant!

Kind Regards, John.

 

[JohnW2]

Which is why I take the view that as long as it is above 216.2-3% it should be permissible!
Though it could be interpreted as 216.2 - 3% of 230V also!

As you will have seen from my other posts, I'm now almost persuaded that it's probably the latter of those - but (assuming their intent is to have a constant denomination, whether 216.2V or 230V) they really have confused the issue totally by choosing to unnecessarily specify the maximum permissible drop as a percentage!

Kind Regards, John

 

[Stoday]

they really have confused the issue totally by choosing to unnecessarily specify the maximum permissible drop as a percentage!

If you were to work in an industrial environment, where the criterion for choosing cable sizes is volt drop, you would see the reason for choosing %ages. In these situations, the current is well below the cable's maximum, but the volt drop is critical. The crunch comes when you consider a distance where the cable changes sizes along its length.

 

[JohnW2]

they really have confused the issue totally by choosing to unnecessarily specify the maximum permissible drop as a percentage!

If you were to work in an industrial environment, where the criterion for choosing cable sizes is volt drop, you would see the reason for choosing %ages. In these situations, the current is well below the cable's maximum, but the volt drop is critical. The crunch comes when you consider a distance where the cable changes sizes along its length.

I guess that there must be something about the industrial environment I don't know which makes it useful to express voltage drops as percentages (of something).

If those voltage drops are expressed as a percentage of something which varies, then fine - that's a very valid way of expressing such things. However, if (as I'm being told), the maximum permissible voltage drops specified in BS7671 are 'percentages of 230V', that's just an unnecessarily contrived way of stating the actual maximum permissible drops in volts. Which would you find more useful - for me to tell you that my neighbour is 44 years old, or to tell you that his age is 55% of 80 years?

In the industrial environment you refer to, what is the denominator used to calculate the percentages (i.e. % of what?) - is it something which varies?

Kind Regards, John.

 

[EFLImpudence]

I don't really understand "The same can be said for all the other pages ...

What I meant was that on pages where equations and calculations are given nominal voltage (in words or Uo) is frequently mentioned but the figure '230' only seems to appear in Appendix 2. unless I have missed any.
I suppose this is the format of documents of this type. Allowing values to be changed without having to wade through every page to alter a number.

This could be the answer to your question of "why a percentage?" Appendix 12 quotes - for a lighting circuit, for example - 3% of nominal voltage rather than 6.9V. The 3% or 230V can be amended in ONE alteration and would apply throughout the book.

However, noone has definitively stated why it is 3% and 5%.
4% of 240 is close enough to 5% of 230, although a slight reduction, without the need for a figure to decimal places but 4% of 240 is reduced to 70% (approx.) of its value with 3% of 230. Do we know why?

Another teaser for you with regard to varying voltage: It states in Appendix 12 that the voltage drop should (should again, not shall ???) not be greater than, for lighting, 3% of nominal voltage i.e. 6,9V.
Therefore, I read this to mean that the figure of 6.9V would be applied whether the voltage was at its maximum or minimum. Another quirk in the wording?
This would only apply when you were testing a circuit that had been wrongly designed in the first place (heaven forbid there should be such a thing) if you were to measure the voltages rather than work it out from R1 etc. but, obviously, does not affect the design of new circuits which would be the primary reason for the regulation.

 

[JohnW2]

This could be the answer to your question of "why a percentage?" Appendix 12 quotes - for a lighting circuit, for example - 3% of nominal voltage rather than 6.9V. The 3% or 230V can be amended in ONE alteration and would apply throughout the book.

Good point - not that nominal voltage changes that often; I suspect it had been the same for decades prior to the 'harmonising' 1995 change. However, in practical terms, I think the 3% figure only appears on one page (Appendix 12, p358), so it would not take a lot of searching to make any changes!

However, noone has definitively stated why it is 3% and 5%.
4% of 240 is close enough to 5% of 230, although a slight reduction, without the need for a figure to decimal places but 4% of 240 is reduced to 70% (approx.) of its value with 3% of 230. Do we know why?

I obviously don't - that's why I started this thread to ask 'why?'! The suggestion that this may be due to the increasingly widespread use of non-incandescent lighting, some of which is more voltage-sensitive, seems pretty credible.

However, I don't quite get the figures you're quoting. 4% of 240 is 9.6 but 5% of 230 is 11.5 - those are pretty different figures, not as close as you suggest. Furthermore, as you go on to imply, even if the percentage figures had been revised such that the absolute permissible voltage drops (in volts) were the same as there were in 14th Ed., that would still mean that the 17th Ed was allowing (in worst case supply voltage situation) about 10V less to reach the current-using equipment - since the worst case voltage became 216.2V, rather than 225.6, following the 'harmonisation'.

This Another teaser for you with regard to varying voltage: It states in Appendix 12 that the voltage drop should (should again, not shall ???) not be greater than, for lighting, 3% of nominal voltage i.e. 6,9V.

Eureka!! You seem to have modestly missed the point that you have just found what we have all be unsuccessfully hunting for!! The first sentence of Appendix 12 says that the (percentage) figures in Table 12A are "expressed with respect to the value of the nominal voltage of the installation". So, the figures definitely are (currently) 'percentages of 230V' - How did we all miss this?!

This Therefore, I read this to mean that the figure of 6.9V would be applied whether the voltage was at its maximum or minimum. Another quirk in the wording?

Quite - but that's one of the major issues we've been discussing in this thread.

This would only apply when you were testing a circuit that had been wrongly designed in the first place (heaven forbid there should be such a thing) if you were to measure the voltages rather than work it out from R1 etc. but, obviously, does not affect the design of new circuits which would be the primary reason for the regulation.

As you say, the issue only arises if one measures, rather than calculates, the voltage drop - but what you're saying is really one of the points I've been making. However, now that we know (first sentence of Appendix 12) that the maximum permissible drop is 3% of 230V (i.e. 6.9V), regardless of the actual supply voltage, I don't any more see any problem or ambiguity.

One has to assume that the authors of the regs were satisfied that (per the 'worst case' scenario) providing a lamp with only 209.3V (216.2 - 6.9) would be acceptable - or, similarly, that providing any other load with 204.7V (216.2 - 11.5) would also be acceptable.

Kind Regards, John

 

[westie101]

One has to assume that the authors of the regs were satisfied that (per the 'worst case' scenario) providing a lamp with only 209.3V (216.2 - 6.9) would be acceptable - or, similarly, that providing any other load with 204.7V (216.2 - 11.5) would also be acceptable.

Which equates to 230 - 9% for lighting and 230 - 10% for power, now where have we heard the 230V - 10% before!

 

[JohnW2]

One has to assume that the authors of the regs were satisfied that (per the 'worst case' scenario) providing a lamp with only 209.3V (216.2 - 6.9) would be acceptable - or, similarly, that providing any other load with 204.7V (216.2 - 11.5) would also be acceptable.

Which equates to 230 - 9% for lighting and 230 - 10% for power, now where have we heard the 230V - 10% before!

Well, actually, 230 - 9% for lighting and 230 - 11% for power, but it's a very interesting point. It could be a pure co-incidence, but it might be at least part of the thinking which resulted in the figures in the regs.

If the much-banded-about 230V±10% supply ever happened, and if the surmising above is correct, then that would produce a very 'interesting' situation. If 'they' have decided that the minimum acceptable voltage to be delivered to a load is about 230V - 10%, then if the supply itself became 230V±10%, that would presumably mean that the permissible voltage drop in installation cables would have to be reduced to around zero

Kind Regards, John

 

[westie101]

or that one hell of a lot of appliances would stop working if the existing installation volt dropwas retained, i.e. 230v - 15% (195.5V)

I know it is common in a lot of countries to fit voltasge regulators to individual supplies to maintain a "sensible" voltage 9we fit them from time to time). But if this was instigated it would cause mayhem!

 

[JohnW2]

I know it is common in a lot of countries to fit voltasge regulators to individual supplies to maintain a "sensible" voltage 9we fit them from time to time). But if this was instigated it would cause mayhem!

What sort of voltage regulators? 'Constant voltage transformers', transformers with auto tap-changing, electronic regulators or what?

Kind Regards, John