10.5kw shower on 40a mcb

[JohnW2]

Probably not a formal requirement but it makes sense to use the worse case voltage to ensure that the instalation is safe at that voltage.

Yes, of course. Not only does it 'make sense', but I personally strongly believe that is should be 'the requirement'.

The situation we have is, in my opinion, totally ridiculous and completely unlike what I am used to in the safety-critical world in which I usually operate...

... it is (again 'IMO') ridiculous that if, given a resitive load that draws a current of, say, 29A at 230V, an OP proposing to use a cable with a CCC of 27A woul;d be told that that was 'unacceptable', possibly that it would be 'unsafe' and certainly that it would be 'non-compliant with regulations, yet if the calculated current (at 230V) were 26.5A, we would say that was (just) 'OK', even though the current drawn through the cable would more than 29A (29.15A) if his supply voltage happened to be 253V ! Why is putting 29A (or more) through the cable acceptable, safe and 'compliant' in one of those situations but unacceptable, possibly unsafe and 'non-compliant' in the other ??

Put simply (and 'obviously') safety critical calculations should be undertaken in relation to 'worst-case scenarios', not a scenario which probably exists in less than half of cases!

I would think that well over half the installations in the UK have (at least sometimes) a supply voltage above 230V. That means that the present situation is analogous to saying something along the lines of "this medicine is 'safe' in at least 40% of the patients who take it" - something which quite obviously would not be regarded as acceptable for most medicines!

Kind Regards, John

 

[scousespark]

As I just wrote (and as I'm sure you know), there is no "253V requirement" in relation to calculations (nor a "216.2V requirement" in relation to other calcs - give or take the fact that the recently introduced Cmin almost achieves that for some calcs) - but, as I also wrote, I strongly believed that there should be!

Kind Regards, John

I was not being facetious and quoted where this requirement came from. As it transpires it is just something Bernard has decided should be done. As people check this site for electrical guidance, I would say this should follow the regs or be clearly stated as personal opinion.

 

[scousespark]

So your statement that we must take the max supply voltage into account is misleading as that is just your personal opinion. The accepted practice to use Uo (230V) to calculate the load and this is used when checking trip times.

Added text

I meant this for Bernard and had copied this from his post. Using an IPad and did not see the paste had not worked properly.

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The maximum supply voltage of 253 volts ( 230 +10% ) must be taken into account when calculating the trip time of an MCB .
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[JohnW2]

I was not being facetious and quoted where this requirement came from. As it transpires it is just something Bernard has decided should be done. ...

I didn't think you were being facetious but rather, since I'm sure you knew that there was no such 'requirement', assumed that your question was essentially rhetorical.

... As people check this site for electrical guidance, I would say this should follow the regs or be clearly stated as personal opinion.

That's quite a difficult one. I agree that those coming to this site (or anywhere else) should be told what are the minimum requirements to comply with the regs. However when, as in this case for me, one believes that those minimum requirements theoretically leave a substantial proportion of installations 'at risk', it is questionable as to how ethical it is to not also mention that.

[I would, however, add that it was bernard, not me, who 'started this' in this particular thread - although it is an issue that I have often 'spoken up' about in the past]

Kind Regards, John

 

[JohnW2]

So your statement that we must take the max supply voltage into account is misleading as that is just your personal opinion.

I said maximum or minimum supply voltage, as appropriate for the calculation in question.

The accepted practice to use Uo (230V) to calculate the load and this is used when checking trip times.

I'm not totally sure what calculations you're talking about.

If you are talking about 'maximum Zs' to achieve required disconnection times (ADS) then, for decades prior to Amd 3 of BS7671:2008 (in 2015), the maximum Zs figures were, indeed, based on the assumption that the supply voltage was Uo (initially 240V, later 230V) - a crazy (in my opinion) approach, since it left all installations with a supply voltage less than Uo potentially 'at risk' (not necessarily achieving required disconnection times when Zs was at, or close to, the calculated 'maximum').

For such calculations the minimum possible supply voltage (currently 216.2V) should have been used. That Amd 3 (at long last!) went most of the way to correcting that anomaly, with the introduction of the concept of Cmin. However, since they gave it a value of 0.95 (rather than 0.94), it only made things 'safe' down to a supply voltage of 218.5V - so any installations with (theoretically possible, but probably extremely rare) supply voltages between 216.2V and 218.5V remain theoretically at potential risk.

Kind Regards, John

 

[bernardgreen]

As people check this site for electrical guidance, I would say this should follow the regs or be clearly stated as personal opinion.

Yes it is a personal opinion about the validity of the method of calculation of the current that could flow in a circuit.

As it transpires it is just something Bernard has decided should be done.

I would like to see it included as part of the method used to ensure an installation was safe.
How would you ensure that the current carrying capacity of a cable was adequate for the sustained current when the supply voltage was at the maximum of 253 volts.

Do not forget that some of the posters here believe ( or did believe ) that increasing the voltage to a 3 kW heater will result in a lower current.

 

[JohnW2]

I meant this for Bernard and had copied this from his post. Using an IPad and did not see the paste had not worked properly.

Fair enough. I obviously didn't realise that, but thought it was a response to my saying that safety-related calculations should use the maximum or minimum supply voltage (as appropriate), and not the nominal voltage.

Having said that, I strongly suspect that Bernard was trying to say essentially the same as me, but did not word his stament very clearly.

Kind Regards, John

 

[JohnW2]

I would like to see it included as part of the method used to ensure an installation was safe.

As you know, so would I.

It does seem rather extraordinary that this 'anomaly' has persisted for decades, despite the considerable amount of relevant knowledge and expertise which exists in the committees that have been responsible for the Wiring Regulations and, more recently, BS7671.

I have personally raised this issue during the consultation processes for recent editions/Amendments of/to BS7671. I very much doubt that it is anything to do with my comments, but they have now at least introduced "Cmin" which goes most of the way to 'correcting' the issue in one context - but, as yet have left other manifestations of the issue unaddressed. So long as that remains the case, I will probably continue to provide my input in relation to future DPCs!

Kind Regards, John

 

[scousespark]

I was taught to 16th edition and the nominal voltage used in cable calcs (Uo) was 240v and the only change I can recall is to drop this to 230v in later editions. Is the 253v requirement new?

This is what I posted. It was a genuine question, as I wondered if I had missed a change introduced in the 18th edition. It was not a rhetorical question.

This was in response to Bernard posting the following: The maximum supply voltage of 253 volts ( 230 +10% ) must be taken into account when calculating the trip time of an MCB .

As it transpires, this is personal preference and not something we must do. I would say that the vast majority of single-phase installations I have worked on have read a supply around 240v. If the OP had 253v supply, as Bernard posted he would experience trips (and the breaker would be doing it's job).

 

[JohnW2]

This is what I posted. It was a genuine question, as I wondered if I had missed a change introduced in the 18th edition. It was not a rhetorical question.

Fair enough. Please try not to interpret this statement in any 'negative' way (as criticism, 'sarcasm' or whatever), since such is certainly not intended, but I genuinely assumed that you would know that there was no such change in the 18th edition, in which case your statement would presumably have been rhetorical. However, I was wrong.

As it transpires, this is personal preference and not something we must do.

It is certainly not something "we must do" (because of rules/regulations). 'Personal preference' is, I suppose, one way of putting it, but the 'preference' expressed by Bernard (and myself) is something which would be required in safety-realted issues in virtually any other field.

I would say that the vast majority of single-phase installations I have worked on have read a supply around 240v.

Indeed - and that is the point. Although it would still not be consistent with standard safety practices, one might attempt to justify undertaking calculations using the assumption of some sort of 'average' supply voltage (say 240V), rather than the maximum possible supply voltage, but I would suggest that it is impossible to find a logical or sensible justification for undertaking calculations on the arbitrary assumption of a supply voltage which is, say, 10V less than the average supply voltage.

If the OP had 253v supply, as Bernard posted he would experience trips (and the breaker would be doing it's job).

Well, for a start, there should not be trips unless the breaker is faulty. If a situation were "just compliant" with an assumption of 230V, with a simple resistive load, an MCB should not trip unless the supply voltage was 259.9V (230V x 1.13) or higher (and, even at that supply voltage, would probably only trip if the current were sustained for literally 'hours').

However, that's not really the point. We are generally very 'pedantic' ('strict') in the advice we give ... if the situation is even 'slightly non-compliant' with an assumed supply voltage of 230V, the person is likely to be told that such is 'not acceptable' - but we would say that a situation which was complaint with an assumption with an assumption of 230V was 'OK', despite the fact that 'non-compliance' of an even greater magnitude (than that present in the 'unacceptable' situation) would arise if the supply voltage happened to be appreciably greater than 230V.

Kind Regards, John

 

[scousespark]

As you know the 40A breaker should not trip up to 1.13 times the rating (45.2A). Taking 5.5 as the resistance and the supply at 253V the load is 46A. I was only saying that a trip at this load would not necessarily indicate a faulty breaker. It is obviously unlikely for a shower to run long enough, but another appliance may do so. I would like to stress that, if the correct breaker had been fitted there would be absolutely no issue. Even with the 40A breaker, calculating the load at 230V still has a safety margin which would limit the danger should the supply rise to 253V. A 45A breaker should never trip up to and including 50.85A so this thread is now moot. I’m happy to leave this now, so feel free to have the last word.

 

[JohnW2]

... Even with the 40A breaker, calculating the load at 230V still has a safety margin which would limit the danger should the supply rise to 253V.

In terms of the general situation (not just the specific issue in this thread) I don't think that anything that anyone can say will alter my personal opinion that it's just plain wrong to undertake a safety-related calculation on the basis of an assumption which is far from the 'worst case' scenario - since that is contrary to everything I have ever learned or practised.

More specifically, I would be hesitant to use the phrase 'safety margin' when it related to a situation that could involve subjecting an MCB to a current greater than its In (even if not enough current to result in a 'trip') - 'non-trip margin', perhaps, but I'm not so sure about that other phrase.