Hi, Our shower is not working properly and we wanted to fit this - http://www.screwfix.com/p/mira-sport-multi-fit-manual-electric-shower-white-9kw/82661. The manual says this shower needs a 40a mcb. A few months ago we had our consumer unit upgraded, the electrician fitted a 32a mcb to the shower circuit, the test certificate shows the cabling is 6mm. The only shower I can see that mentions a 32a mcb is 7.5kw, for example http://www.screwfix.com/p/mira-sport-electric-shower-white-chrome-7-5kw/9105r. We really wanted a more powerful shower than that. Can anyone advise what shower I can fit? Can I change the mcb for 40a and fit the 9kw Mira? Or with 6mm cable am I limited to 7.5kw? Thanks for your help.
The cabling, as installed, is only adequate to support the shower that you have. At nominal mains voltage, that shower requires a full 32amps. A 9kW shower requires nearly a full 40amps. However, depending on the installation method, a sufficient EFLI and a couple of other considerations, the 6mm cable MAY be able to support the 40amp load required. You need to get your electrician back to check that the 32A MCB can be uprated to 40amps, then you could get the 9kW shower.
You could fit a 8.5kw on the 32a, depends a lot on length of cable and how it is run. i.e. in/under insulation etc
That is a very bad suggestion. You could, providing you do not want a long shower. 8500w @ 230v = 37amps. So the 32A MCB may trip when the OP is all nice and soapy. I it is very bad practice to provision a circuit with a CPD lower than the design current. Circuit design, the first thing you consider is that In > Ib ! Or do they not teach you that on the 5WW course?
There is a topic posted only today that demonstrates exactly what happens if you get it wrong... http://community.screwfix.com/threads/shower-tripping-out.184623/
Hi Bazza, Thanks for your reply and your help. I'll contact the electrician who did the work to check, although it seems like the safest thing to do is to fit the 7.5kw shower for now. Changing the cable will be a big job that's best done with other work sometime in the future.
Hmmm didn't see this when I was replying. I think that's decided it, I'll fit a 7.5kw shower for now. Thanks for your help and advice.
A shower above 7.5 kW will need a 10 mm cable to be safe and you do not want to take a chance with cable calculations especially with a shower. So get a 7.5 kW or upgrade your cable to 10 mm and the man to 40 amp
Have you got shares in a copper mine? Cable calculations are calculations. 6mm is ample to support a 40A current, providing the installation method is correct.
That Mira shower is rated at 9KW at 240V If your working out calculations based on the nominal current of 230V 240Vx240V=57600 57600/9000W=6.4 Ohms 230V/6.4 Ohms=35.93A The shower is a fixed load so if the installation method gives a CCC of 36A or over then the shower can be installed on a 40A circuit breaker.
I did say it wasn't worth trying to be clever and pushing cable calculations to near the limit especially in a shower circuit,have you got shares in a funeral directors?
You're a typical "new member" here Andy, a slight change of attitude will get you a much better acceptance!
My attitude is nothing to do with you so chill out and stop giving silly advice and then getting mardy when someone has the good grace to put you right. Arrogance is quite common among electricians but I tend not to suffer from it,either personally or from others.
Andy, @harlice has come here asking for advice. I have told him exactly all that he needs to do. It will provide what he needs for his new 9kW shower. All he needs is someone who is competent (not you) to review the existing cabling and judge if the CPD can be increased to 40amps. If it is then he needs to do no more. 40amps is within the CCC of 6mm cable providing the installation method and ambient temperature, etc meet the requirements. You have put forward a solution that involves him ripping out all of his existing wiring for the shower and putting in new, and for no reason. Good grief, are you the person responsible for the upgrade work being done on the Great western Rail electrification project? Is that why it is 200% over budget and 5 years late?