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Upgrading shower from 7.5kw to 9kw

Postby mattio111 » Wed Oct 22, 2008 4:00 pm

Hello all, sorry i'm a bit of a novice so all help welcome and sorry if this is a stupid question.

My 7.5kw mira sports shower has given up on me (stopped heating water) so am looking to do a like for like replacement. I can get hold of a 9kw mira sports on the cheap so was wondering if there would be much issue in swapping for this one?

I guess it might trip if I ever put it on the max setting, but if i only used medium or low would i be alright?

Many thanks for anyone who takes the time to reply. :)
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Simply Build It

Postby ericmark » Wed Oct 22, 2008 8:35 pm

Although one may stretch the like for like statement in the Part P regulations to include the upgrade from original within the same power rating once you increase the power it is no longer like for like.
The power settings on a shower are normally two stage half and full power and the other setting are only to do with the temperature which is controlled by adjusting the water flow rather than the power being used by the shower.
4.5Kw is unlikely to be enough to shower with and only with the larger 11Kw showers is the half power setting normally available.
Assuming then you want to increase from 7.5Kw (32 Amp) to 9Kw (40 Amp) then the cable must be able to handle the increase. In the main this lifts one up a size 6mm² installed to reference method 100 or 102 is rated 34/35 amp so it would require upgrading to 10mm² but even if 10mm² is already used reference method 101/103 will only give 36/32 amp so unless 16mm² cable is used one could not guarantee the cable is man enough for the job. Only if the cable is enclosed in conduit on a wall or in trunking etc. can you know it will be OK if under 16mm².
As soon as you modify anything then the latest regulations need to be followed which means the RCD will in most cases now be at origin of supply and changing a MCB will require careful note of the earth loop impedance to ensure it still complies. And this would all need documenting in the minor works certificate you will need to raise to keep insurance valid.
As one looks at all this one can understand why bathrooms are special locations and why either the LABC or a registered electrician has to raise a completion certificate under Part P.
The MCB will allow over load for a limited time and some people do not change the MCB and as long as they keep the shower time short they get away with the overload. Well at least for some time, but as time goes on the MCB is gradually being damaged by these overloads and in the end fails normally by allowing shorter and shorter shower times.
I don’t know what your cable size or route is so I can’t advise as to best way forward except to call a registered electrician and rely on his expertise and with the issue of a completion certificate retain insurance cover should anything go wrong.

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