Shower switch wiring

DrJimFranklin · 31 Aug 2015

winston1 said:
WOW. We've already established you don't know your physics, don't embarrass yourself more by not understanding Ohm's Law. For a resistive load, such as a shower, if you increase the voltage, the current increases, and the load increases by a square law.

You didn't follow the conversation did you...we did not use a fixed resistance for the calculations or the discussion, but it was fixed at 9.5kW for the power...

If we fix the resistance then the current drawn will increase with a change in the voltage, that is correct, but for the purposes of the discussion it was the POWER rating that was fixed, thus the current decreases as the voltage rises.

This is yet another reason why we have harmonised voltages...you have no demonstrated that you have a little knowledge but now so good at understanding.

securespark · 31 Aug 2015

DrJimFranklin said:
The AVERAGE voltage across Europe is actually 229.2V (hence the 230V) but there are massive variations. I have personally measured voltages as low as 218V and as high as 256V.

This is what I have told Winston myself. But he does not accept the evidence.

I have taken thousands of voltage readings during the last 30 years. Figures of 207V and 259V ring a bell.

winston1 · 31 Aug 2015

DrJimFranklin said:
winston1 said:

WOW. We've already established you don't know your physics, don't embarrass yourself more by not understanding Ohm's Law. For a resistive load, such as a shower, if you increase the voltage, the current increases, and the load increases by a square law.

Click to expand...

You didn't follow the conversation did you...we did not use a fixed resistance for the calculations or the discussion, but it was fixed at 9.5kW for the power...

If we fix the resistance then the current drawn will increase with a change in the voltage, that is correct, but for the purposes of the discussion it was the POWER rating that was fixed, thus the current decreases as the voltage rises.

This is yet another reason why we have harmonised voltages...you have no demonstrated that you have a little knowledge but now so good at understanding.

Some modern equipment with switch mode power supplies does have constant power over a range of voltage.

But we were discussing a shower rated at 9.5kW at 240v.

flameport · 31 Aug 2015

DrJimFranklin said:
You didn't follow the conversation did you...we did not use a fixed resistance for the calculations or the discussion, but it was fixed at 9.5kW for the power...

Previous quotes indicate otherwise, and your original reply to winston1 was totally different, containing only a few words.

winston1 · 31 Aug 2015

securespark said:
DrJimFranklin said:

The AVERAGE voltage across Europe is actually 229.2V (hence the 230V) but there are massive variations. I have personally measured voltages as low as 218V and as high as 256V.

Click to expand...

This is what I have told Winston myself. But he does not accept the evidence.

I have taken thousands of voltage readings during the last 30 years. Figures of 207V and 259V ring a bell.

I have also taken voltage readings over the years.
The 60's Europe around 125 v or 216v depending on location. UK 240 to 250v.
The 70's, 80's & 90's Europe around 217v UK 240 to 250v.
Now Europe 230 to 240v UK 238 to 248v.

bernardgreen · 31 Aug 2015

DrJimFranklin said:
You didn't follow the conversation did you...we did not use a fixed resistance for the calculations or the discussion, but it was fixed at 9.5kW for the power...

So this shower has internal controls that adjust the current taken to ensure it is always drawing 9.5 kW no matter what the supply voltage is.

You had better tell the manufacturers they have got it wrong because they say it takes less power at 230 volts than it does at 240 volts.

DrJimFranklin · 31 Aug 2015

Bernard...please don't be facecious...in the real world of course things are not as simple. The resistance is marginally variable and as has been established voltage varies.

I fixed the load for example purposes to 9.5kW because it saved getting over technical....and to be honest I was being a bit of a git because I was hoping someone would come along and make a comment that meant I could sit back and see Winston simply accept the point being made about the 230V discussion.

Perhaps in future I'll be more to the point and less sneaky...

EFLImpudence · 31 Aug 2015

The origin of the discussion.

EFLImpudence said:
45A or 50A switch will be satisfactory.
9.5kW is just under 40A.

No more needed to be said, but...

DrJimFranklin said:
Hate to **** on your parade but your incorrect, I assume you erroneously calculated using 240V, but you should, as per BS7671 (In fact as per the 15th and 16th Editions of the IEE Regs - Before it became BS7671) the correct voltage for design calculations is 230V and for a 9.5kW load that is 41.3A, as such all cable calcs etc should be based on this.

ban-all-sheds · 1 Sep 2015

DrJimFranklin said:
Picky as usual, however he is still wrong and your later comments in this posts and the following one about Winston demonstrate that.

You think so?

Shame you didn't use any of your time away from here to attend some basic reading classes.

I KNOW he is wrong because the Regulation state he is WRONG. It is not my opinion, it is a Fact as per BS 7671 Appendix 2 section 14. The UK supply 230 V +10% - 6% and this is what ALL design calculation are based on for single phase services.

Please pay attention. How do you KNOW that the value of 9.5kW for the load is at 230V?

Totally irrelevant. The unit is rated at 9.5kW and the supply voltage has a NOMINAL rating of 230V, thus to comply all calculation must be based on these figures and not one you pick out of your head to try to win an argument with yourself...

But someone would have to be spectacularly stupid, surely, to use the wrong figure for the size of the load when doing calculations? For example, only a complete idiot with no understanding would use a value of 9500 in a calculation when they should be using 8700, as doing that would produce the wrong answers.

See the section in Bold...are you the only person in the UK who does not understand that is enquiring about cable length?

Doubt it.

Are you the only such person who does not understand that I was pointing out that the length of the cable is irrelevant to it overheating at the switch terminals?

or is it that you simply have to have the last work all the time to make yourself look big Kevin? really no need for this nonsense is there?

I don't think there is, but as long as you keep writing it I'll keep challenging it.

Who's Kevin, BTW?

ban-all-sheds · 1 Sep 2015

DrJimFranklin said:
I fixed the load for example purposes to 9.5kW because it saved getting over technical

We do, it is true, sometimes get people here who don't understand that with a simple resistive load the power dissipated reduces when the voltage does, but please don't think that we are all like that.

I was hoping someone would come along and make a comment that meant I could sit back and see Winston simply accept the point being made about the 230V discussion.

Sorry - that wasn't meant to be derogatory. You have no idea how many times we have tried to get Winston to understand that point. Feel free to have a go yourself, but you might want to check out a few of these first:

//www.diynot.com/diy/search/505042/?q=230v|240v|nominal&o=date&c[node]=8&c[user][0]=134775

JohnW2 · 1 Sep 2015

DrJimFranklin said:
I fixed the load for example purposes to 9.5kW because it saved getting over technical....

Fiar enough - but the problem is that in the name of "avoiding getting over technical" you wrote things that would almost certainly totally mislead people who did not already know about these things - and, as I have already reminded you, this is a DIY forum.

Kind Regards, John

bernardgreen · 1 Sep 2015

DrJimFranklin said:
Bernard...please don't be facecious...in the real world of course things are not as simple. The resistance is marginally variable and as has been established voltage varies.

I wasn't being facecious. I was asking for confirmation that in this case the shower was able to control the current it took so as to ensure that the load was a constant 9.5 kW irrespective of supply voltage. Perhaps my wording was not the most polite.

Alfie25 · 1 Sep 2015

Going back to the original question... I've read that a common cause of the burning out is due to the cables not being sufficiently tightened in the terminals and that it's good practice to screw them in very tightly. The question is how tight is too tight? Copper is quite soft so isn't it possible to crush or deform some of the strands if you screw it in super tight?

ban-all-sheds · 1 Sep 2015

And brass isn't the strongest material either.

It's hard to describe - it's something you need to feel for. Do the screws up tight, but not with huge force applied, give the conductors a bit of a wiggle and a tug, tighten a bit more.

Then a few days later check the tightness again.

endecotp · 1 Sep 2015

Crushing or deforming the strands is what you should be aiming for. Too tight is when you're in danger of stripping the screw thread.

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Shower switch wiring

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