11kw shower

[reedluke]

Hi All,

I'm having trouble finding the answer to this...
My father in law wants to have a new 10.8kw shower installed, I have offered to buy and run the cable, breakers etc. but am not sure what to go for....

The cable will be run through a loft (not insulated now but will be) and the entire cable run will be about 15metres including drops.

Should this be a 10mm cable with a 45A breaker or 16mm cable with a 50A breaker?
The board is protected with an RCD.

Please let me know what you think

Thanks L

 

[ricicle]

As 10.8kW is almost 47A @ 230v this is right on the limit for a clipped direct 6mm².
It would be worth your while keeping the cable away from any likely insulation. Then you can use 10mm² and a 50A MCB.(provided it is clipped direct)
Is the board capable of taking a 50A device ? Is the RCD 30mA ?

Do you know the tests you will be required to do to comply with building regulations ?

 

[xr4x4]

http://tinyurl.com/42sftgn

 

[reedluke]

Thanks guys,
Thats a handy link to have, but this is the issue I'm already stuck with - a recommendation for 10mm & the link suggesting 16mm - although it seems a 50A breaker is correct!
Different sites are suggesting different cable!
The board will be new and everything fitted and tested by part p qualified etc. I'm just doing the donkey work!
Cheers

 

[ricicle]

Then the correct procedure will be to let whoever signs the certification decide on the correct cable as they will have to sign a design declaration.

 

[xr4x4]

http://tinyurl.com/42sftgn

It's the insulation that's the killer, it you raise the insulation above 100mm, it says there's no cable available.

Also clipped direct says 6mm!!!

 

[riveralt]

Thanks guys,
Thats a handy link to have, but this is the issue I'm already stuck with - a recommendation for 10mm & the link suggesting 16mm - although it seems a 50A breaker is correct!
Different sites are suggesting different cable!
The board will be new and everything fitted and tested by part p qualified etc. I'm just doing the donkey work!
Cheers

If it a 10.8kw shower, check what voltage they have calculated it with.
For example the Mira Sport Max is rated at 10.8kw at 240v. But since we use 230v to calculate power that equates to 9.9kw.
Furthermore, the manufacturers instructions for this shower state that a 45Amp MCB is required.
So firstly check with the manufacturers instructions.
Secondly check with the electrician who will fit the shower, because s(he) will need to determine the size of the cable to be used and the run it should take, not you.

 

[EFLImpudence]

the link suggesting 16mm

It gave me 6mm but as said that is right on the limit.

Try again

 

[ban-all-sheds]

Hi All,

I'm having trouble finding the answer to this...

Everything you need to know to work it out is in the Wiring Regulations. Why don't you have a copy if you're doing circuit design work?

Should this be a 10mm cable with a 45A breaker or 16mm cable with a 50A breaker?

• 
  
  
  
• //www.diynot.com/wiki/electrics:mcbtocable
  
  
• http://www.tlc-direct.co.uk/Book/4.3.1.htm
  
  
• http://www.tlc-direct.co.uk/Book/4.3.11.htm
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/cableselection_web.pdf
  
  
• http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
  
  
• http://www.tlc-direct.co.uk/Book/5.3.1.htm
  
  
• //www.diynot.com/wiki/electrics:mcb2
  
  
• http://www.tlc-direct.co.uk/Book/8.1.1.htm
  
  
• //www.diynot.com/wiki/electrics:part-p[/list]
  
  
  

  Please let me know what you think

  I think you should let an electrician do the design.
  
  
  

  Thanks guys,
  Thats a handy link to have, but this is the issue I'm already stuck with - a recommendation for 10mm

  Please read it again.
  
  See where it says "It would be worth your while keeping the cable away from any likely insulation. Then you can use 10mm²..."
  
  Why don't you appreciate the effect that thermal insulation has on the current carrying capacity of cables?
  
  
  

  & the link suggesting 16mm

  Because of insulation.
  
  
  

  Different sites are suggesting different cable!

  Did you give these sites different input data?
  
  Did they make different assumptions on supply voltage, installation method, grouping factors, ambient temperature....?
  
  Calculators like the TLC one are to be used in the same way as a regular calculator - to do the mechanics. They are absolutely NOT a substitute for having a genuine knowledge of how to do the calculations yourself.
  
  
  

  The board will be new and everything fitted and tested by part p qualified etc. I'm just doing the donkey work!

  But you're not. You're choosing cable size and breaker ratings - that's design. You're choosing a cable route - that's design. You're planning on installing the cable - that's construction.
  
  The electrician will not sign off your design and construction as his work.
  
  
  

  I have offered to buy and run the cable, breakers etc

  You do realise that the electrician will pay less for the materials than you will?
  
  
  Just get an electrician to do the whole job.

 

[Alarm]

Just get an electrician to do the whole job.

I`m really getting cheesed off with you being correct.

 

[JohnW2]

If it a 10.8kw shower, check what voltage they have calculated it with.
For example the Mira Sport Max is rated at 10.8kw at 240v. But since we use 230v to calculate power that equates to 9.9kw.

That seems gto be common practice, but is it actually good, or even correct, design practice when it comes to matters such as cable sizing?

As far as I can make out, despite the nominal 230v, many/most of us have supplies of at least 240v. If the designer knows that the supply voltage is likely to be at least 240v, is it really good/correct practice to design on the assumption that it is only 230v?

Kind Regards, John.

 

[ban-all-sheds]

10.8kW @ 240V = 45A

9.9kW @ 230V = 43A

Our supply used to be between 226V and 254V. It's now between 216V and 253V.

Did people traditionally design on the basis of "well it might be over 250V"?

 

[JohnW2]

Our supply used to be between 226V and 254V. It's now between 216V and 253V.
Did people traditionally design on the basis of "well it might be over 250V"?

That's not what I'm asking - and (although one might debate the wisdom of this) there doesn't seem to be any requirement to design on the basis of the highest possible supply voltage.

I wasn't talking about the range of legally possible supply voltages, but of designer's knowledge of what the supply voltage was likely to be. In most disciplines, unless there was a requirement to design for 'worst case' scenarios, a designer would normally be expected to design on the basis of their best judgement as to the most probable actual operating conditions.

Kind Regards, John.

 

[riveralt]

If it a 10.8kw shower, check what voltage they have calculated it with.
For example the Mira Sport Max is rated at 10.8kw at 240v. But since we use 230v to calculate power that equates to 9.9kw.

That seems gto be common practice, but is it actually good, or even correct, design practice when it comes to matters such as cable sizing?

As far as I can make out, despite the nominal 230v, many/most of us have supplies of at least 240v. If the designer knows that the supply voltage is likely to be at least 240v, is it really good/correct practice to design on the assumption that it is only 230v?

Kind Regards, John.

The BRB, now going slightly green, says we use 230v - all the cable calculations etc are based on that, so I will design all my circuits on that basis. Particularly as I haven't got the time nor the inclination to worry about variations in voltage.
A quick voltage test at the beginning of process tells you whether it is within the range allowed or not. If it is, get on with it. If it isn't then inform the DNO accordingly.
In my view there is already over design built into the figures used in the BRB, so I am more than happy to follow the guidance given there.
Its also good practice to follow the manufacturers instructions regarding MCB sizes as well.

 

[JohnW2]

The BRB, now going slightly green, says we use 230v - all the cable calculations etc are based on that, so I will design all my circuits on that basis.

Fair enough - I have no doubt that is the 'official' answer. I was just thinking beyond the BRB/BGB to 'good engineering design practice' itself.

Particularly as I haven't got the time nor the inclination to worry about variations in voltage.

I certainly wasn't imagining that anyone would consider (worry about) this on a case-by-case basis. Rather, I was considering the fact that (I think) 'we all know' that 240v is closer to the truth for most UK supplies than is 230v. FWIW, mine is rarely below 245v.

In my view there is already over design built into the figures used in the BRB, so I am more than happy to follow the guidance given there.

That, of course, is the 'saving grace' - but it's also the potential start of a slippery slope if designers take that into account too often.

Its also good practice to follow the manufacturers instructions regarding MCB sizes as well.

Again, I'm sure that's the official line. However, I'm not so sure what 'absolute' good engineering design practice would say about installing a 45A MCB when one knows that (certainly 'now', and probably always), the appliance is likely to be drawing 47A. Again, one is playing with 'margins'.

Kind Regards, John.