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How do i check the fuse rating for an electric shower and if a new electric shower is a suitable replacement?

Our old shower is a Mira 8.78 advance ATL (I'd assume 8.78kw but the label doesn't actually say). We're looking for a new replacement electric shower and hope to replace it with a 9kw as we don't want the hassle (and extra cost) of fitting a 10mm cable. (This one we think

As this is the first time I've had to buy a shower I have no idea what is or how to check a fuse rating... how do I do this and how do I know if the new shower can just be put in without any other problems? Is there anything else I need to consider?

Thanks for your help!

I should add to recent answers that no I am not intending to do the work myself, however I need enough information to first buy the replacement shower. I do not want to purchase it only then to realise that I can't afford to install it!

Also if the old shower is an 8.78kw then surely I should already have a 10mm cable and RCD fitted? We have a separate switch in the old boiler room (next to the bsthroom) that turns the power to the shower off completely. The wire to which is over 1cm wide. Is that then a 10mm cable?

15 Answers from MyBuilder Electricians

Best Answer

Hi Kylie. Firstly go to your consumer unit (fusebox), under the stairs, in a cupbaord somewhere etc and have a look to see if each of the fuses or circuit breakers is lablled. - Hopefully they are, although all too often they are not!

Assuming that you find one labelled shower, you should be able to get some details from it on what amperage the fuse/circuit breaker is. Normally for a shower it woul be a 40 or 45amp fuse/circuit breaker.

If so then this should be fine for a 9kW shower as its
9000watts divided by 230volts = just over 39 amps.

Its also worth checking that you have an RCD (redisual current device) fitted, as all showers must now be RCD protected. The RCD provides additional protection against a fatal shock in the event of a fault.

If you have a circuit breaker type consumer unit, you will know if there is an RCD fitted as it will be a bit wider than compared to one of the circuit breakers and it will also have a little button with either a "T" or "test" marked on it, that when you press, trips out the RCD.

As for checking whether you have a 6mm or 10mm cable on your existing shower circuit, then thats a bit more difficult as you need to start takeing the cover of the old shower unit or at the switch, so not really recommended for your average DIY'er? Just beacuse the cable is over 1cm across when you measure it, does not mean its 10mm. The size of electrical cables is the cross sectional area of the conductors and it has no real relation to teh width of the cable.

Also I would not assume that your old shower has an RCD without checking. Its was not mandatory to fit an RCD for a shower until July2008, so if the old shower was installed before that it may well not have one. Also many 8kW plus showers are fitted to 6mm cables and work ok, but do not comply with the regulations.

Hope this helps, please clikck on "like" if it does.


Answered 24th Jan 2012

There are too many unanswered questions, how big is the existing cable? is a bearing on what size shower go in, also, is it on an RCD?
The way of converting KW to Amps is KW divided by Volts.
In your case 9000/240=37.5 amps


Answered 22nd Jan 2012

Unfortunately the previous answers contain errors and poor advice. Cable sizing depends on many factors not just the wattage of the shower. Length of cable run, the route it takes ie through insulation, and another Important factors. Note that when calculating the ampage the voltage used for this calculation is 230 and not 240. Having spent many years teaching electrical science and principle I find it frustrating that "qualified" electricians cannot calculate it.


Answered 11th Jul 2016

The only way I would even consider using a 6.0mm cable on a 9kw shower was if it was clipped directly to the surface of a wall and even then it could only really be a very short run. In short, just invest in 10mm or a smaller shower for your safety if nothing else.

The amp rating of the shower is 39.13 amps.


Answered 22nd Jan 2012

Bottom line! Your questions all revolve around you doing the job! dont do diy electrics if you aint got a clue! You should call a qualified part p domestic electrician. Dont take any advice DIY! My advice, you need an RCD Protected consumer unit or Rcd fuse for the shower cable on your board! any shower over 8kw needs a 10mm cable for safety! only a sparks can tell you the diameter. if you haven't got a 10mm cable you will need one for a 9kw shower!!

Don't cheap skate on electrics!



Answered 22nd Jan 2012

for every 1000 watts it works out at roughly 4 amps so a 9 kw shower should be protected by 40 amp breaker


Answered 22nd Jan 2012

I'm sorry if this is going to disappoint, but any shower over about 7.2KW is going to need to be wired with 10mm cable. In theory, you might be able to "get away" with 6mm cable, BUT Part P of the building regulations states a minimum cable size of 10mm for a shower over 7.2KW.

You asked if there was anything else to consider:

Whether there is RCD protection already (pretty much essential now to meet regulations within a bathroom).

With respect the fuse or circuit breaker rating...

You're probably going to need a 40A BS EN 60898 type B circuit breaker, or 45A BS3036 rewirable fuse, or 45A BS1361 cartridge fuse - dependent upon what kind of fusebox/consumer unit you have.

Your earth loop impedance needs to be tested by an electrician to ensure it's below the maximum required for the fuse/breaker type and rating.

Hope that helps.


Answered 22nd Jan 2012

Hi kylie,

Before you check the fuse rating you need to ensure that the cable is sufficent. You seem to suggest that your cable is not 10mm so I presume that it must be 6mm. The fuse size should be 32 amp. If it is rewireable then it should be 30 amp, the way of being 100% accurate with a rewireable is to rewire the fuse with 30 amp wire. Even a 8.5 kw could trip a 32 amp depending on factors such as the flow / incoming water temp ect.
I do appreciate that you will not want the extra hassle / cost of upgrading your shower / cable ect but to comply with the regulations and thus it would be getting installed by a registered competent installer ( + certification ) your options are 2 fold ;

1) Replace with a 7.5kw ( plenty hot enough for most domestic showers )
2) Upgrade the supply

Hope this helps

NJM Electrical


Answered 22nd Jan 2012

Most shower outputs are rated at 240v, so 9000w/240v = 37.5A, so your fuse (MCB) will have to be minimum 40A or 45A max.

It will hopefully be a circuit breaker, not a fuse. You can look in the consumer unit to see if you have a circuit labeled 'shower' and then read the number. If it is a 32A rewirable fuse with two red dots, then call an electrician to upgrade.

I advise you to seek professional to check all is safe, because lots of potential issues, this is probably the most potentially dangerous electrical appliance in the house, so not worth risking your life with.


Answered 22nd Jan 2012

Hi all electrical work in a bathroom is subject to part P building regulation and should be done by a competent registered person with NICEIC NAPIT ELECSA OR OTHERS or you can apply to your local authority building control to inspect and test your work they charge around 200 quid for this depends on your local authority (council)

you can be prosecuted for doing electrical work without following the above if found guilty can be a fine of upto £5000.00

Get a qualified electrician for a free quotation this way you will find out what is required and if the circuit is suitable including isolator cable size protective device rating and additional RCD protection is all in order.


Answered 6th Feb 2012

Hi Kylie, as a guide 6mm cable should only be used for showers upto 7.5kw (though some will argue 8kw), and anything higher would be 10mm cable. So if you want a higher rating of shower then you're going to have to shell out I'm afraid.
Different showers have different inlet positions so the electric feed and water feed may be in a different position to the existing ones. Triton do a range that allows the water feed to be supplied from the bottom left/right, or top (T80Z is one such model).
Has the current shower been tiled upto, or has the tiling been done first and the shower surface mounted? If the latter then the tiles should also be grouted so you shouldn't have any issues on that front...
AC Plumbing.


Answered 22nd Jan 2012

As with most of the advice given it is always best to get electrician to carry out any electrical work, however I don’t see the issue with the 6mm cable as it’s current carry capacity would be 47 amps if clipped direct (reference method c)
In terms of rcd protectionist the circuithowever I find controversial! in views of electrical safety i would always advise that all electrical circuits have rcd protection as this is now the most up today’s requirement in bs7671 18th edition working regs however this is. It retrospective! Meaning if the the shower circuit or wiring is being altered there is not a need to comply to the current wiring regs! d
But as said by many of in doubt contact an electrician!

Kind regards


Answered 23rd Mar 2019

There is already so many answers on here so I will help with cable sizes. An easy way to tell the difference between 6mm twin and earth and 10mm twin and earth is that the earth on 6mm is solid and on 10mm it is stranded


Answered 22nd Mar 2021

Most shower outputs are rated at 240v, so 9000w/240v = 37.5A, so your fuse (MCB) will have to be minimum 40A or 45A max.


Answered 21st Aug 2018

Strong advice is for a 9kw shower you must have 10mm cable fitted and a 40 amp breaker protected by an rcd ..
You must not do the work yourself and the fuseboard needs to be made of metal to comply with the current regulations
I hope this helps


Answered 30th Apr 2019

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