Replacing 9kw shower with 10.8kw - been told 6mm cable is fine

[wwwebber]

Hi, after lots of complaints that our shower is not powerful enough (just moved house and its a 9kw Mira Sport) I've been looking at putting in a new unit - ideally a 10.8kw Mira Sport. Just had an electrician friend round who has checked out the wiring and it's apparently 6mm and he suggests that it'll cope with 10.8kw no problem. The wiring is a mixture of being buried in plaster and running under floor boards with no insulation. So a couple of questions :-

I've seen various specs & articles suggesting that 10.8kw needs a 10mm cable - so who is correct here ?

Was the 6mm cable even providing enough power to the 9kw to run it at full strength ?

Thank you very much in advance.

 

[JohnW2]

Just had an electrician friend round who has checked out the wiring and it's apparently 6mm and he suggests that it'll cope with 10.8kw no problem. The wiring is a mixture of being buried in plaster and running under floor boards with no insulation. So a couple of questions :- I've seen various specs & articles suggesting that 10.8kw needs a 10mm cable - so who is correct here ?

The short answer is that (on the basis of the information you have supplied) your electrician is right. Assuming that the 10.8kW is quoted at 240V (almost certainly the case), that would be 45A (less if the supply were the 'nominal' 230V). 6mm² cable installed in the manner you have described (e.g. no insulation, no conduit etc.) has a current-carrying capacity of 47A - so OK for a 45A load. Were the cable installed differently, 6mm² may well not be adequate - which is perhaps why some people say it needs 10mm² ('to be safe under all installation situations')

Was the 6mm cable even providing enough power to the 9kw to run it at full strength

It's not a case of "providing enough power" - the shower will use as much power as it wants. It's a matter of whether the cable is safe (from overheating) - and, since, as above, 6mm² installed as you describe would be OK for a 10kW shower, it obviously was OK for a 9kW one.

Kind Regards, John

 

[jg321]

Hi, after lots of complaints that our shower is not powerful enough (just moved house and its a 9kw Mira Sport) I've been looking at putting in a new unit - ideally a 10.8kw Mira Sport.

You'll probably find that the complaints don't stop after you've fitted the 10.8kw shower, as it will still be pathetic, especially so in winter.

 

[wwwebber]

The short answer is that (on the basis of the information you have supplied) your electrician is right. Assuming that the 10.8kW is quoted at 240V (almost certainly the case), that would be 45A (less if the supply were the 'nominal' 230V). 6mm² cable installed in the manner you have described (e.g. no insulation, no conduit etc.) has a current-carrying capacity of 47A - so OK for a 45A load. Were the cable installed differently, 6mm² may well not be adequate - which is perhaps why some people say it needs 10mm² ('to be safe under all installation situations')
It's not a case of "providing enough power" - the shower will use as much power as it wants. It's a matter of whether the cable is safe (from overheating) - and, since, as above, 6mm² installed as you describe would be OK for a 10kW shower, it obviously was OK for a 9kW one.

Kind Regards, John

Thank you - so all is well. Much appreciated.

 

[wwwebber]

You'll probably find that the complaints don't stop after you've fitted the 10.8kw shower, as it will still be pathetic, especially so in winter.

Well that'll be the bathroom shower. Will be installing a shower into the en suite next year which will use a mixer, so we'll be using that one and guests will use the leccy one

 

[jg321]

Ahh makes sense. Despite having a combi, the previous owners of our house stuck with an electric shower. Useful when the boiler packs up maybe, but dreadful on a cold January morning.

4 rooms skimmed, decorated and carpeted. After a little break it's time for a new bathroom, with a shower off the combi!

Good luck

 

[JohnW2]

Thank you - so all is well. Much appreciated.

You're welcome.

Kind Regards, John

 

[JohnW2]

After a little break it's time for a new bathroom, with a shower off the combi!

Just remember to educate people not to turn a tap on whilst you're having a shower!

Kind Regards, John

 

[securespark]

Which shower?

Our electric Mira is terribly susceptible to taps being turned on.

but the boiler-fed Aqualisa is not at all affected. You can turn a cold tap on full and the temperature does not even twitch.

 

[JohnW2]

Which shower?

The mixer shower which jg321 proposed to fit, fed from his combi-boiler.

... Our electric Mira is terribly susceptible to taps being turned on.

That obviously depends upon your water pressure and the size of the pipework etc. Most electric showers are incapable of heating water at anything like the rate au which the mains cold supply could potentially supply.

... but the boiler-fed Aqualisa is not at all affected. You can turn a cold tap on full and the temperature does not even twitch.

Are you talking about a combi-boiler? If so, then the same comments as above relating to water pressure/pipe size etc. again apply in relation to turning on a cold tap.. However, I was actually talking/thinking about someone turning on a hot tap (supplied by the same combi) - unless you have a fairly massive combi, that's usually the killer!

Kind Regards, John

 

[securespark]

My Aqualisa mixer has bi-metallic thermostatic control and seems pretty unshakeable.

The only thing that upsets it is turning off one or other of the feeds to the unit....

 

[JohnW2]

My Aqualisa mixer has bi-metallic thermostatic control and seems pretty unshakeable.

Yes, all my mixer showers are Aqualisa, and they are very stable - but they are running off stored HW, not a combi-boiler.

My comments were related to running a mixer shower from a combi-boiler (which is what jg321 proposes to do) - is that what you are doing?

Kind Regards, John

 

[ban-all-sheds]

Which shower?

Our electric Mira is terribly susceptible to taps being turned on.

Our Redring Selectronic is not even susceptible to the WC next door on the same run of pipe being flushed.

The flow rate drops, yes, but the temperature does not rise.

 

[securespark]

Yes, all my mixer showers are Aqualisa, and they are very stable - but they are running off stored HW, not a combi-boiler.

My comments were related to running a mixer shower from a combi-boiler (which is what jg321 proposes to do) - is that what you are doing?

Kind Regards, John

Nah, it's coming gravity-fed from an Ideal Mexico.

 

[JohnW2]

Nah, it's coming gravity-fed from an Ideal Mexico.

As I said, that's no problem then. I was commenting on jg321's intention to feed a mixer shower from a combi-boiler. As I said, if he does that, unless it's an uncommonly large combi, the shower will suffer if anyone turns on a hot tap whilst he's showering. What happens depends on how good is the thermostatic control of the mixer. If it's good (like Aqualisa's), the the temperature of the shower will remain unchanged, but the shower flow rate will decrease. If the thermostatic control is 'less good', then the shower temperature might fall.

Kind Regards, John