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Question for electricians about shower size and cable capacity

Stuart Wright

AVForums Founder
Staff member
Hello folks,
we have a shower which has an Opus 4 electric unit on the wall.
It went bang the other day (literally) and tripped the dedicated 30 amp circuit breaker for the shower.

There is no information on the Opus 4 on the 'net so I emailed the manufacturers who mailed back today (impressively promptly) with the specs. 7kw, 1 bar minimum. The shower hasn't been made for several years.
The cold water supply is from a pipe which is about 10mm in diameter. There is no sign of an electrical cable as it's been tiled in, but there is a separate pull chord in the ceiling with an amber light which, after the circuit breaker was reset, does come back on. The shower also still pumps water out, but doesn't heat it any more.
The closest source for the electricity supply is a next-door-but one boiler room which has a dark grey rectangular cable, about 9mm x 5mm which disappears into the wall up high in the direction of the shower.

The water pressure is good.

So my question is whether a 7.5kw replacement shower (don't seem to be able to get lower than 7.5) would be ok, or whether we could even step up to a higher powered one.
I would want a decent shower as it's used daily by the family. I do want a thermostatic shower.

Thanks for any advice you can give.
 

noiseboy72

Distinguished Member
Watts is calcualted as volts x amps, so if you know the power in watts, you can work out the current required. 7500W / 230V = 32A.

The cable to your shower should be a minimum of 6mm (The diameter of the live and neutral conductor.

From the sounds of it, your current installation will be fine for a 7.5KW shower, but insufficent for a 9.4-10Kw type.

As this is a bathroom installation, it will need to be carried out or at least signed off by an electrician in any case.
 

Stuart Wright

AVForums Founder
Staff member
Watts is calcualted as volts x amps, so if you know the power in watts, you can work out the current required. 7500W / 230V = 32A.

The cable to your shower should be a minimum of 6mm (The diameter of the live and neutral conductor.

From the sounds of it, your current installation will be fine for a 7.5KW shower, but insufficent for a 9.4-10Kw type.

As this is a bathroom installation, it will need to be carried out or at least signed off by an electrician in any case.
So I'd need to upgrade the trip to higher than 30A to get a bigger than 7kw shower?
It's a dedicated shower, not a bathroom. Does that make any difference?

[Edit : The rooms in question are in a separate part of the house. The photo of the fuse box serving those rooms is attached. I can't read the writing above the left 20a fuse. The 15a fuse says plugs (there is just a double socket in the boiler room), the 20a fuse second from right says sauna (it has a 4.5kw heater), and the 30a fuse says shower. That one had tripped. An electrician will recognise the cables and know their rating, I'm sure.]
 

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noiseboy72

Distinguished Member
It looks like you will need to upgrade the trip and cable, but an electrician would need to advise as the picture is not totally clear. You might also want to upgrade to a more modern fuse box with built in RCDs.

But in answer to your question, a direct swap out will limit you to a 7KW shower.

Any "Wet Room" electrical install needs to be carried out or signed off by an electrician. Failure to do so will invalidate your household insurance and could make your home unsaleable.
 

Berties

Banned
Apparently they can't do any electrical work until the fuse box is changed to meet latest standard.

Any "Wet Room" electrical install needs to be carried out or signed off by an electrician. Failure to do so will invalidate your household insurance and could make your home unsaleable.
That includes the shower socket? And fan extractor? We did those ourselves (dad is an electrician)
 

noiseboy72

Distinguished Member
Part P states the following as notifiable and not notifiable. Put simply a qualified electrician must carry out the work.

Areas within a kitchen, bathroom, garden or special location
NOTIFIABLE:
A complete new installation or rewiring
Installing a new circuit (eg, for socket outlets, lighting or shower)
Fitting and connecting a shower to an existing point
Replacing a fuse board
Adding a socket outlet to an existing circuit
Adding a lighting point to an existing circuit
Fitting a storage heater to an existing adjacent point
Installing a supply to a garden shed, garage or other outbuilding
Installing a lighting point or socket outlet in a garden shed, garage or other outbuilding
Installing a garden pond pump including the supply
Installing electric under floor or ceiling heating
Installing an electricity generator

NOT NOTIFIABLE:
Connecting a cooker to an existing connection unit
Replacing a damaged cable for a single circuit (cable to be the same size and type and installed as original)
Replacing a damaged accessory such as a light switch or socket outlet
Replacing a light fitting
Connecting an item of equipment to an existing adjacent connection point
Fitting a storage heater to an existing adjacent point
Replacing an immersion heater

Areas not in a kitchen, bathroom, garden or special location
NOTIFIABLE:
A complete new installation or rewiring
Installing a new circuit e.g. for socket outlets, lighting or shower
Replacing a fuse board
Installing a lighting point or socket outlet outdoors
Installing and fitting a storage heater including the circuit
Installing electric under floor or ceiling heating
Installing an electricity generator

NOT NOTIFIABLE:
Adding a socket outlet to an existing circuit
Adding a lighting point to an existing circuit
Connecting a cooker to an existing connection unit
Replacing a damaged cable for a single circuit (cable to be the same size and type and installed as original)
Replacing a damaged accessory such as a light switch or socket outlet
Replacing a light fitting
Connecting an item of equipment to an existing adjacent connection point
Fitting a storage heater to an existing adjacent point
Replacing an immersion heater

I incorrectly stated above that you can carry out the work and ask an electrician to sign it off. By the book, the electrician must carry out the work. In reality, most electricians will sign off a simple installation if all parts of the work are visible.
 

Stuart Wright

AVForums Founder
Staff member
Changing the fuse box is not a problem. We have an electrician. However, changing the cable is not an option as it travels too far through non-accessible roof spaces and walls. Is the existing cable not good enough for a 9.5kw shower?
 

nheather

Distinguished Member
Are you sure that you have a dedicated circuit for the shower at the moment.

I can see

2x 20A - lighting circuits, I guess
1x 30A - ring main sockets
1x 45A - what is this for

Are you sure the 45A is for shower and not for an electric oven outlet?

Also looking at the picture, I can't see any 45A size cable leaving that consumer unit.

I guess the question I'm asking is "has the shower been installed on a dedicated high amp circuit or has it been taken off the normal ring mains".

You can get away with the latter electrically, but it wouldn't pass inspection.

Cheers,

Nigel

Actually, the more I read it the more the fuses look like 15, 20, 20, 30.

On my consumer unit the two ring main socket circuits are on 32A, the lights are on 6A, the imersion heater on 20A and the cooker on 40A. I don't have an electric heated shower because it was going to be a real pain putting a 50A fuse in and routing the massive cable upto the bathroom.
 
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lynx

Well-known Member
Stuart, your board is a little dated and could benefit from an update...Dual RCD 17th Edition Consumer Units

Ask your electrician his thoughts on this.

Is there another board not shown in the image?
 

noiseboy72

Distinguished Member
For 9.4KW, you need 10mm diameter cable and a 45A fuse. Your electictrician will be able to tell in an instant if its up to the job.

6mm cable is simply not up to the job and could / will get hot in use. This is dangerous and will also limit the amount of power getting to the shower. If you cannot pull in a new cable, go for a 7KW shower.
 
Glad to see voltage drop has been mentioned.

And 6mm is NOT the diameter of the cable, it is the cross sectional area of the conductors.

Moving on... The new shower WILL need 30ma RCD protection if it states it in the instruction manual.

You DO NOT need to touch your existing consumer unit, as a stand alone unit can be installed alongside, however, main earthing and bonding will need to be upgraded if needed.

You DO NOT need to upgrade anything else in bathroom, if all you are renewing is the shower.

You CAN just swap out the shower for a new one of similar rating and you DO NOT need to notify, however, as stated above, if it requires an RCD then it will.

If you install a new shower and supply cable, it WILL need to installed and certified by a registered competent electrician.

If you have an electrician, why are you asking on here?
 
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Stuart Wright

AVForums Founder
Staff member
Are you sure that you have a dedicated circuit for the shower at the moment.[snip]

Actually, the more I read it the more the fuses look like 15, 20, 20, 30.

On my consumer unit the two ring main socket circuits are on 32A, the lights are on 6A, the imersion heater on 20A and the cooker on 40A. I don't have an electric heated shower because it was going to be a real pain putting a 50A fuse in and routing the massive cable upto the bathroom.
As I mentioned above, it's 20,15,20,30 supplying the electrics in the far section of the house.

Stuart, your board is a little dated and could benefit from an update...Dual RCD 17th Edition Consumer Units

Ask your electrician his thoughts on this.

Is there another board not shown in the image?
Yes there is the main fusebox which has 16 or so trips in it.

Glad to see voltage drop has been mentioned.

And 6mm is NOT the diameter of the cable, it is the cross sectional area of the conductors.

Moving on... The new shower WILL need 30ma RCD protection if it states it in the instruction manual.

You DO NOT need to touch your existing consumer unit, as a stand alone unit can be installed alongside, however, main earthing and bonding will need to be upgraded if needed.

You DO NOT need to upgrade anything else in bathroom, if all you are renewing is the shower.

You CAN just swap out the shower for a new one of similar rating and you DO NOT need to notify, however, as stated above, if it requires an RCD then it will.

If you install a new shower and supply cable, it WILL need to installed and certified by a registered competent electrician.

If you have an electrician, why are you asking on here?
I wanted to get other opinions and suggestions.
Most showers for sale are 7.5kw which I guess is in keeping with 32A fuses.
Thanks for your input.
 

noiseboy72

Distinguished Member
Glad to see voltage drop has been mentioned.

And 6mm is NOT the diameter of the cable, it is the cross sectional area of the conductors.

Moving on... The new shower WILL need 30ma RCD protection if it states it in the instruction manual.

You DO NOT need to touch your existing consumer unit, as a stand alone unit can be installed alongside, however, main earthing and bonding will need to be upgraded if needed.

You DO NOT need to upgrade anything else in bathroom, if all you are renewing is the shower.

You CAN just swap out the shower for a new one of similar rating and you DO NOT need to notify, however, as stated above, if it requires an RCD then it will.

If you install a new shower and supply cable, it WILL need to installed and certified by a registered competent electrician.

If you have an electrician, why are you asking on here?
I would only disagree on the fitting of the shower. My understanding of the regs is that swapping out of a shower qualifies as fitting a new one to an existing cable.

And yes, my use of the phrase diameter for the cable was in error. It should of course be cross sectional area.
 
Berties said:
Apparently they can't do any electrical work until the fuse box is changed to meet latest standard.

That includes the shower socket? And fan extractor? We did those ourselves (dad is an electrician)
Rubbish, the regs do not cover what has been installed previously. The sparky involved would simply have to ensure any new installation was up to regs and that can be achieved by installing a seperate RCD or RCBO circuit and ensuring all bonding requirements are met.

Edit:- beaten by niceguy
 

IronGiant

Moderator
Would I be correct that swapping like for like is not notifiable, but fitting a shower to an existing point, where there previously was no shower would? Even if the point was installed with a future shower in mind?

And in similar vein, upgrading is notifiable? And how flexible is like for like? My shower is getting a bit clogged internally, if I swap it for an identical wattage is that good enough?
 
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Jesus Dave, you dont half make things complicated.:p

Allow me..

Quote..

"m. The fitting and replacement of cookers and electric showers is not notifiable unless a new circuit is needed."

Edit:

And after your edit and in answer to your final question, yes, providing the circuit has RCD protection if the manufacturers specify it.

Assuming you were happy with the previous shower rating.
 
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Stuart Wright

AVForums Founder
Staff member
Anyone know whether there is a 7.5kw thermostatic shower? Mira and Triton don't do them. 8.5kw is the minimum power with those brands.
 

IronGiant

Moderator
You get used to that in GC :laugh:

Which I think is a good thing, just goes to show how many helpful people there are on here :thumbsup:
 

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