New fuse for shower upgrade?

26 Nov 2005
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United Kingdom
Upgraded my old 7.2kW shower to 8.5kW. Old fuse was 32A but new suggests 40A in instructions (and Mira helpline). Seems to work (so far) but how do I go about replacing a 32A trip switch for a 40A trip switch? (There is an RCD pull-cord device installed as well.) Nothin obvious, but wil change out if required by whatever means necessary

All help appreciated....
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If electric is coming in at 230v now instead of 240v then I think that the amperage is watts divided by volts which I got as 36.96amps.
I think that from a safety point of view there is nothing wrong with a fuse that is slightly under sized.
I am not a quallified sparky but feel the only thing that could happen is that the MCB could trip out so leave you all lathered with either no water or cold water!
On a serious note, have you had a competant person test the shower circuit with test gear that is within its calibration date?
As your life could depend on the safety devices working it may be a good idea to have them tested.
One more thing, how often do you press the test button on your RCCD, andare you sure the pull swich you refer to in your post is an RCD?
The Baker.
PS there are a lot of useful books in the Libraries in this country.
You should only upgrade your MCB to a 40A if you're sure the cable that it's going to be connected to is suitable for being connected to a 40A MCB.

You wouldn't wire you power sockets in speaker cable would you?

......would you?

If you've got 6mm T&E then the max MCB should really be be a 32A. Although in certain circumstances you could get away with 40A through a 6mm the majority of cases would mean that you should really upgrade to a 10mm T&E if you were to change to a 40A MCB.

Of course this work applies to Part P.
Hello 1974stephen,
you have made me wonder about existing cables,
has the mains voltage change from 240v at the consumer unit, to 230v, has made some parts, of some installations, which may have cables that are at the top end of their current carrying capacity,
sub standard or even potenially dangerous?
Some how I cannot imagine that any Government Department would make such an error of judgemant as to endanger the lives of the public.
However I would appriciate it if any of you electricians out there could enlighten me on this point.
The Baker.
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thebaker, you are excessively worried about this. you emasure the voltage at your consumerunit. I bet its nearer 240 than 230. some parts of the country, the voltage hasn't changed at all.
Reduceing the voltage would not increase the current, that flows, it would reduce it, its ohms law. And besides the voltage hasn't *really* changed...
I suspect that if it is like the shower I fitted yesterday it is 8.5KW at 230V and 9.5KW at 240V.
As stated the voltage in most places still remains 240V and if it is reduced to 230V then the current (and power consumed in this case as the shower has a fixed resistance) goes down. The MCB should be 40A or 45A and the cable 10mm². An RCD is advisable and the supplementary bonding should be checked and improved if not up to standard.
Not such an easy swap!!
Running mcb's over their rating in normal operation is not advised.

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