Shower 8.5kw v 10.5kw - The winner is ?

[Smiffy008]

Hello
I am considering ( actually the wife is demanding ) fitting a new electric shower into a downstairs shower room. We currently have a 7.5kw fitted with 6mm² cable on a 32amp MCB. The cable run from the CU is 10m. I just have a couple of questions I would appreciate opinions on.
1 - can I fit a 8.5kw shower to this cable size?
2 - is a 10.5kw shower noticably better ( flow/temp ) than an 8.5kw ? I know it must be better but can you feel the difference ( the 10.5kw would be fitted with 10mm² cable - and that will be another story )
Thanks for any advice

 

[ricicle]

Some points to consider:-

8.5 kW equates to around 35A at 240v (if an older shower it will more than likely be rated at 240V)

This is too big for a 32A MCB. 6mm² can take up to 47A if the installation conditions are favourable (ie not ran in trunking, or covered in insulation etc)
If the conditons were ok the mcb could be swapped for a 40A (CU allowing)
Has the existing shower got 30mA RCD protection - it needs it !

A 10.5kW would be noticably different to a 8.5 kW.(more flow at higher temp.)

Any of this work would be notifiable apart from a straight swap of shower units. (This could also involve checking/upgrading earthing and bonding)

 

[Adam_151]

How is the cable to the shower run? (whats it supported on/clipped to?/near?) and is it grouped with any others or surrounded by insulation for any length?

8.5kw can go on 6mm depending on how the cable is run (but not on a 32A MCB, it would have to be uprated to 40A assuming circuit is suitable, and of course the circuit should be fully tested afterwards, also for what its worth, its also notifiable to your local building control department)

2- Simply put a 10.5kw shower is 23 and a half percent better than a 8.5kw one , its that simple, it takes a given amount of energy to raise the temperature of one litre of water by 1 degree celesis (4.184 Kjoules, or one Kcal if you prefer), an 8.5kw shower (assuming its not thermostatic controlled one), will simply put out 8.5Kjoules into the water passing through it every second, the temperature control on the front is simply a flow regulator, so if you want it hotter, what you actually do is reduce the flow, which is why you have to have it set higher in winter as it controls the amount the water is raised in temperature, rather than absolute temperture. QED

As to noticing the difference, there is a bit of difference between the two, obviously, but both will feel poor compared to say a pumped shower off the DHW system, that can deliver upto say 12 litres a minute* at upto whatever temperature the tank is at, until you run out of hot water

If its complete refit of shower room, I'd be tempted to put in a bigger cable and a 10.5kw shower, its a simple shower swap, then I'd put in the biggest shower that the existing cable could support within the bounds of BS7671

At the end of the day, its a downstairs shower room, I expect it only sees use if you want a quick shower while the missus is trying for the record of how long a person can stay in a bathtub, or if the gas boiler breaks down

*or even more if you have a more powerful pump, but than you start to get in wasteful territory....

 

[Goldberg]

2 - is a 10.5kw shower noticably better ( flow/temp ) than an 8.5kw ? I know it must be better...

Not necessarily - it depends on the maximum achievable flow rate of your cold mains supply.

Have you measured it?

 

[Smiffy008]

Thanks for the quick response , Ricicle , Adam , Goldberg !
The water flow rate is 18 litres per minute , dunno the pressure. The cable is a single 6mm² cable running in the kitchen ceiling - so no insulation there.
I have just read the Creda shower instructions, they produce a 9.5kw version of the shower we like. Quoting their manual.........
"9.5kw , 230/240v , 6mm² cable , 45A BS1361 fuse , 12 metre max cable run "
Would this be OK to fit on my existing cable ? The connection and testing would be completed by a certified electrician. Running a 10mm² cable would be a nightmare - meaning lifting 2 bedroom laminate floors.
TIA

 

[Goldberg]

The water flow rate is 18 litres per minute , dunno the pressure.

That flow is good enough, and the pressure is irrelevant in this context.

The cable is a single 6mm² cable running in the kitchen ceiling - so no insulation there.

If you know that it's clear of insulation throughout its run, then in theory it will safely support an 8.5kW load, but you don't have much headroom without it all being clipped direct, so personally I wouldn't fit a 9.5kW unit without going up to 10mm².