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Topic Title: Shower cable and protection..again!
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Created On: 29 March 2013 04:57 PM
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 29 March 2013 04:57 PM
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misterben

Posts: 408
Joined: 11 June 2007

Hi all,

I,ve been on a job where the shower is existing but It has a 6mm wired to it, the data on the shower says 8.7-9.8kw. I have worked it out as follows:
9.8 @230 = 42A
9.8@240= 40.83
I know the shower doesnt need overload protection and as such there is a 40A MCB installed. The cable is in PVC trunking and only 5Metres long from the consumer unit.
My question is should It be on a 40A or a 45A MCB?
I know it will withold 42A for a long shower but I dont believe this to be good practice as a permanent fixture.
Your thoughts appreciated

Misterben
 29 March 2013 05:19 PM
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Zs

Posts: 2629
Joined: 20 July 2006

Misterben, do you know what the actual voltage in the property is? I expect it to be closer to 240V and probably higher so a calculation from the true voltage might be just the ticket for you.

What make is the distribution board? You may struggle to obtain a new 45A MCB. So that might also sort out your dilemma.

Assume RCD protection is dealt with upstream?

I am a fan of fusing as low as possible but I do get pulled up on that from time to time because of what is described as a 'standard' circuit.

Zs
 29 March 2013 05:45 PM
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AdrianWint

Posts: 247
Joined: 25 May 2006

Misterben,

I'm afraid your maths is flawed. The shower is a resistance heater. Its resistance will stay almost constant (well, it will actually rise a little as the element heats up, but we'll ignore that). So it can't produce the same power at 230V as it does at 240V ..... infact it must produce more power at 240V and also pull MORE current not less. Hence why the manufacturer has given a range of powers.

To be able to work out the actual current draw we need to know the voltages at which the manufacturer has declared the powers stated.

Adrian

Edited: 29 March 2013 at 06:57 PM by AdrianWint
 29 March 2013 05:48 PM
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misterben

Posts: 408
Joined: 11 June 2007

Hi Zs

Im not sure what the voltage is off the top of my head, however arent we supposed to design on 230V worst case scenario? Or sometimes as low as 220V?
It is a Hager VC755H1 im pretty sure there isnt a 45 just 40 and 50Amp.
If that is the case I guess it will have to stay!

regards
Misterben
 29 March 2013 05:51 PM
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Jaymack

Posts: 4480
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Originally posted by: AdrianWint
The shower is a resistance heater. Its resistance will stay almost constant (well, it will actually fall a little as the element heats up, but we'll ignore that).

Shower with a negative temperature co-efficient? Sorry, just choking.

Regards
 29 March 2013 05:52 PM
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dbullard

Posts: 1166
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I have and will never put any 8.5kw + shower on a 40a mcb, I always install showers on 6mm / 10mm on a 32a mcb (if RCD protected) or RCBO if required, Never had any tripping issues with load.

Simple solution to a simple problem.


Hope it helps

Regards

Daren

-------------------------
..... Dont pee in my pocket and tell me its raining ......


www.quest-electrical-sw.co.uk
 29 March 2013 06:01 PM
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misterben

Posts: 408
Joined: 11 June 2007

Hi Adrian,

I dont have that information , if I did you are correct I could work out the resistance.
I guess I could measure the resistance if I wanted to for the purposes of learning! If the power was one fixed value say 10KW and rated at 230V then resistance is 5.29 ohms?

regards
Misterben
 29 March 2013 06:13 PM
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AdrianWint

Posts: 247
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Originally posted by: Jaymack

Originally posted by: AdrianWint

The shower is a resistance heater. Its resistance will stay almost constant (well, it will actually fall a little as the element heats up, but we'll ignore that).


Shower with a negative temperature co-efficient? Sorry, just choking.



Regards




Oooops..... yes, you are correct....it has a positive temperature coefficient & its resistance will, of course, RISE as the element gets hotter..... and hence the current will fall ... abit.

I guess it would be fun with a negative coefficient.... thermal runaway anyone?

Adrian
 29 March 2013 06:43 PM
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GB

Posts: 346
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40A Mcb will do the trick, no need to go for the 45A (if available) you would have to shower for several years to have any issues and even my kids dont take that long!!
 29 March 2013 06:49 PM
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Zs

Posts: 2629
Joined: 20 July 2006

Phew.

I was on my fourteenth triangle when you posted that last one Adrian. I shall stop trying to make them work now, and sell this page to Tate modern instead.

'variation on an Ohm, with oddness' Mixed media including bic, on stolen corporate note-pad. By Zs ( I quite like the one on bottom left though, might keep that).
 29 March 2013 07:21 PM
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weirdbeard

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Originally posted by: Zs


I was on my fourteenth triangle when you posted that last one Adrian. I shall stop trying to make them work now, and sell this page to Tate modern instead.




With you on that one, I think the oddity is the OPs given Kw's,
shower makers usually give the highest wattage @240V first to look impressive, then 230V with the lower power as per this link:


http://www.mirashowers.co.uk/o...ra_sport_multifit.pdf



9.8 Kw at 240V would= 5.87 ohm, that works out to 9.0 at 230V, and 8.7 Kw would need to be 226V

based on that it seems the OPs shower has outputs given for 226-240V which seems a bit unusual?
 29 March 2013 07:27 PM
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tillie

Posts: 754
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Hi misterben , could you not carry out a load test and if the current drawn is above 40 amps then you could install a 50a mcb and check that short circuit s catered for .

Regards
 29 March 2013 07:45 PM
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redtoblackblewtopieces

Posts: 111
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Given the installation Reference Method is not the max current-carrying capacity of the cable only 41 amps , or have I missed something ?
Kevin

-------------------------
Safety through a Standard
Compliance by Approved Documents
 29 March 2013 08:11 PM
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mikejumper

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Why not clamp the cable while the shower is running, this should give you a good idea of the amount of current it is drawing.
 29 March 2013 08:38 PM
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Thripster

Posts: 558
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As there is no possibility of overload, as Tillie says, put in a higher MCB and make sure the cable is thermally protected for fault current. I think BS7671 frowns upon a circuit designed with the protective device constantly operated with an overload, albeit small.

Regards
 29 March 2013 08:49 PM
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dbullard

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Originally posted by: mikejumper

Why not clamp the cable while the shower is running, this should give you a good idea of the amount of current it is drawing.



Did this very thing a few years ago, Mira Advance 9.8 in my own home as a experiment, set to boil and the conclusion was about 26amps (from memory) max current drawn so no requirement IMO to have a shower on a 40 amp MCB.

Removed the shower a few years later and went pumped direct from the boiler as the daughter thinks a quick shower is 30mins.

Regards

Daren

-------------------------
..... Dont pee in my pocket and tell me its raining ......


www.quest-electrical-sw.co.uk
 29 March 2013 09:15 PM
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weirdbeard

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Originally posted by: dbullard

Did this very thing a few years ago, Mira Advance 9.8 in my own home as a experiment, set to boil and the conclusion was about 26amps (from memory) max current drawn so no requirement IMO to have a shower on a 40 amp MCB.



So that would be 9.8@240V = 5.87 ohms?

Either your clamp meters way out or your voltage was dropping to 153V
 29 March 2013 09:27 PM
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stateit

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Originally posted by: dbullard


Did this very thing a few years ago, Mira Advance 9.8 in my own home as a experiment, set to boil and the conclusion was about 26amps (from memory) max current drawn so no requirement IMO to have a shower on a 40 amp MCB.


I always get the advertised/expected current from my clamp meter when I do this.

Either something is wrong with the element in your shower, or your clamp meter is way out .

-------------------------
S George
http://www.sg-electrical.com
 29 March 2013 10:29 PM
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dbullard

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Originally posted by: stateit

Originally posted by: dbullard





Did this very thing a few years ago, Mira Advance 9.8 in my own home as a experiment, set to boil and the conclusion was about 26amps (from memory) max current drawn so no requirement IMO to have a shower on a 40 amp MCB.




I always get the advertised/expected current from my clamp meter when I do this.



Either something is wrong with the element in your shower, or your clamp meter is way out .


Shower function was perfect, as was the meter (calibrated), still have the shower in the shed incase one day I run water out there and then I would install it as a means of hand washing instead of having the wife moan about oily scum in the kitchen sink .

I will on the next shower install do the tests again and verify.

Regards

D

-------------------------
..... Dont pee in my pocket and tell me its raining ......


www.quest-electrical-sw.co.uk
 29 March 2013 11:13 PM
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Zs

Posts: 2629
Joined: 20 July 2006

Funny the things that catch us. It wasn't the OP that threw me into modern art weirdbeard, it was Adrian's post about resistance on a shower. I'd never challenge in public but I had a bit of an 'is it me?' moment over it.

But now the flowers in the ohms law garden are blooming again.

Misteben, if you are genuinely concerned over this one let me have all the details and I will run it through amtech for you in my lunch hour and send you the calculation and cable analysis. Week after next though, I'm on other things next week. I'd need a Ze to do it properly for you.

Zs
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