10.8kw showers and cables!

[Dunce]

Hi all

I have a few questions that hopefully someone will be able to clear up….

First off:

Is my shower 10.8kw? On the box it says 10.8kw for 240v, but then in slightly small writing it say 9.9kw at 230v.

Now I know UK supply rms is 230v so maybe that answers my own question but…. why? Mira are confusing me slightly by advertising it as 10.8kw when quite clearly, it can never be!

My next question is probably a bit trickier - it's to do with cable size and length. I spoken to a good handful of electricians and an even healthier handful of 'DIY know-it-alls' at work and have had slightly different answers of each. I've also done a lot of interenet searching and found nothing definitive. Sooooooooo……..

Is a 10mm cable sufficient for my 9.9kw/10.8kw (?) shower?

I know there are lots of factors to consider here:
- Cable length
- routing
- etc

So any advice would be great. At the moment my set-up is this:

(And this is going to create some controversy I know!)

I'm running a 10mm cable from the consumer unit which has a 45amp circuit breaker. The consumer unit is in the attached garage and the cable runs along the garage, through the house wall, under the bath (I know!!), up into the loft, into the pull chord switch, and then back down again into the shower.

Overall I'm using 17m of cable with one join (two if you count the pull chord switch).

Before I connect it all up though, anyone who can be bothered reading all this is more than welcome to point out any glaring shodiness! I welcome the feedback because I'm no expert.

I know the underlying theme is "Get an electrician in", my problem with that is I've so much conflicting advice from qualified electricians, it seems to me to be whatever that particular electrician feels like.

Where are the hard and fast rules?

Cheers for the advice.

 

[holmslaw]

..

 

[OilLecky]

The voltage of the electrical supply in Europe is 230 Volts + 10% - 6%. ( i.e. 216.2 Volts to 253.0) so you see that there can be a variation that the manufacturer of the shower has to allow for. (BS 7671:2008 Appendix 20)
So when you say that the shower cannot ever be 10.8kW you are not quite correct. The voltage could very well be 240Volts where you live and thus the shower could consume 10.8kW of power. 10.

If your shower is 10.8kW at 240Volts and 9.9kW at 230 Volts then, the worst (permitted) case is at 253.0 Volts when it will be 12kW. (and 47.43 amps)

However all the calculations for installation are carried out using the nominal voltage i.e. 230volts.

1. So we have 230volts, and thus 9.9kW and therefore 43 amps current draw.
2. We have 17m of (multi-core PVC? i.e. twin and earth?)10.00mm cable which will safely carry 43 amps if the cable is in conduit in a thermally insulated wall (worst case) and up to 70 amps for a cable in free air or on a perforated tray. Your installation will be somewhere in between these two figures. (BS 7671:2008 Appendix 4)
3. The maximum permitted voltage drop along a cable length permitted is 5% (BS 7671:2008 Appendix 12).
4. The voltage drop on your installation is 4.4mV/A/m = 4.4 X 43 X 17 = 3.216 Volts. The maximum voltage drop permitted is 5% of 230 volts or 11.5 Volts (BS 7671:2008 Appendix 4)

So. The shower will draw 43 amps, the cable will carry 43 amps safely, the voltage drop in the circuit is within limits and the circuit breaker will protect the circuit if your cable installation is “normal”. If the cable installation is actually in conduit and insulation through its length, which I doubt, then I would down-size the circuit breaker to 40amps. The shower will probably never be on for long enough to trip it.

 

[Dunce]

Thank you very much for the response. Much appreciated.

Out of curiosity, what's the reason for avoiding loft insulation?

 

[londonboy]

It derates the current carrying capacity.
I.e the cable can get hot due to the heat not being able to escape.

 

[Dunce]

Brilliant.

Thank you very much.

OilLecky thank you very much for the detail - much appreciated. It helps with the understanding.

I was getting worried that I might have had to start dragging out the 10mm twin and earth and replacing it with 16mm.

I seem to remember from electronics theory at college that resistance goes up as temperature goes up. Pulling this sort of power you want as little resistance as possible.

It's all starting to make a bit of sense!

 

[londonboy]

You say the cable will have a join in it, where and why? Joins are not necessary on a new installation.

 

[Dunce]

Ah! Yes. That old chestnut!

I know - I'd rather not have it.

I stupidly bought 10meters of the cable and then another 10meters. £35 per roll from B&Q!

If it's an absolute no-no then I'll have to go and buy just one lot of 20meters. I'd rather another £70 layout than a burned down house.

 

[Dunce]

Also - another question!

If at worst case as OilLecky mentions the shower is drawing 47amps, why is it that a 40amp circuit breaker will suffice?

Won't it trip at 40amps?

 

[sparkwright]

Where will the join be?

How will you do the joint?

Is there an alternative route you can take the cable?

Can you take the cable back to the shop?

 

[Dunce]

Ah sorry! Yeah the join will be in the attic space of the garage, just before the cable goes into the main body of the house. I'm going to use a junction box at the moment. although finding one big enough is a struggle! Any suggestions?

I've deliberated for weeks over the route to take. This at the moment is looking to be the only choice before I start having to remove walls and such.

I don't think B&Q will have the cable back now! I've dragged it through several walls!

 

[sparkwright]

If you decide to have the joint, a 60 amp junction box is available. Click make them. Some wholesalers have them, some wholesalers don't - so phone around first. It is unlikely you will find these in a DIY shop.

 

[sparkwright]

A tip would be to leave a little slack in the cable in the loft above each acessory, to allow for any re-terminating. You must make sure your connections are tight - give a final tight check before replacing covers. Shower cable connections have a tendancy of burning out if you don't make tight, secure connections.

Also use a good quality shower switch, like MK or Crabtree.

 

[Dunce]

Thank you very much for all the responses. I was fully expecting the internet version of sucking through teeth but this has all been really useful.

Excellent tip sparkwright, thanks for that.

I don't for one second underestimate the seriousness of getting this stuff wrong. By nature I err on the side of caution and consider myself a bit of a perfectionist. So I'm feeling positive about my workmanship.

It's just my knowledge on the subject.

I noticed the 60A junction boxes on TLC direct. That'll do nicely!

 

[sparkwright]

Will the circuit be having 30 mA RCD protection?

It really, really should do.