Dodgy advice on shower rating?

[JackDwight123]

Hi everybody,

I have recently had an electrician in to give me a quote and Im not sure I trust what he's saying, Id like to see what you all think...

Im looking to put an electric shower in my bathroom. I think there used to be one as there because there is already wiring hooked up. I got a guy in to tell me if I could re-use this existing wiring, and if so what shower (max kw) could I put in. He told me the cable is only 10mm2 and would need to be upgraded and put on the outside of the wall in order for me to fit any 'normal' shower (7.5kw and up), also the MCB would need upgrading. He quoted £260 to run the new cable and upgrade the MCB.

Now Ive looked at everything and these are the details: The consumer unit is wylex with a 40A MCB for the dedicated shower circuit, I believe the cable is 10mm2 (based on what I can see) and is around 15m long, The shower cut off switch is rated 50A (I took it off the wall and checked), and the cable runs under the floorboards and up inside a plasterboard wall (not insulated). I think the voltage in my house is 240V (that's what it says on the meter).

I know from school that 240V * 40A = 9600kw. So even given voltage drops and changes in voltage at the supply I believe I should be ok to fit up to 9kw shower?!

Im really not sure about what he tells me, also I really dont want the cable on the outside of the wall?!

Sorry about the long post guys, but what do you think? Any help appreciated.

 

[riveralt]

As long as the cable runs in the safe zones and its current size is 10mm2 T&E then there is no need to move it.
A cable rating of this size can allow you to have an electric shower up to 63Amps which equates to over 14kw.
So for example a 10.5kw shower by Mira would easily be catered for by this cable.

However.

The MCB size of 40Amps could be an issue because the shower manufacturers have a different interpretation of the purpose of the MCB than most electricians. I believe the purpose of the MCB is to protect the cable and therefore must always be lower than the cables current carrying capacity. Where as the shower supplies seem to focus on the power of the shower - because in their view they cannot control the size of the cable.
So from their perspective the 40Amp MCB you have 40A*230v = 9.2kw is the maximum rated shower you can have and still comply with the manufacturers instructions.
The other issue is that when fitting a new shower the manufacturers state that you MUST have the circuit additionally protected by an RCD.

So if I was designing your circuit and you wanted a 9.6kw shower then I would utilise the current 10mm2 T&E cable. If your consumer unit already had RCD protection for that circuit then I would replace the existing MCB for a 45Amp MCB (or whatever size the manufactures required).
If your current consumer unit didn't have an RCD and I couldn't fit an RCBO then I would install a separate appropriate RCD(MCB) /RCBO rated shower unit next to the consumer unit.

If you only wanted an 8.5kw electric shower then the 40Amp MCB would be okay but you would still need RCD protection.

Best advice would be to get several quotes. Unless there are other issues that you have not outlined £200 does seem a lot to hook up a shower that you have purchased.
If everything I have described is already in place then the price should be nearer £100.

 

[Taylortwocities]

You say the cable is 10mm². How have you measured this?

 

[riveralt]

You say the cable is 10mm². How have you measured this?

I thought the electrician he obtained the quote told him that....

I got a guy in to tell me if I could re-use this existing wiring, and if so what shower (max kw) could I put in. He told me the cable is only 10mm2 and would need to be upgraded and put on the outside of the wall in order for me to fit any 'normal' shower (7.5kw and up),

 

[ban-all-sheds]

So from their perspective the 40Amp MCB you have 40A*230v = 9.2kw is the maximum rated shower you can have and still comply with the manufacturers instructions.

It's also the most you can have and comply with the Wiring Regulations -
Ib ≤ In ≤ Iz and all that....

 

[PrenticeBoyofDerry]

Get another electrician in,
If I have read your post correctly, the only reason other than being ripped off, for a new cable to be installed would be if your current cable has insulation resistance failure or continuity issues.

 

[ban-all-sheds]

The guy you had in was not a real electrician. He may have claimed he was. He may even believe he is.

But he's an incompetetent charlatan - kick him into touch.

 

[Taylortwocities]

Ahh yes, you are right

He told me the cable is only 10mm2 and would need to be upgraded

So that's tosh 10mm² would be plenty big enough.

 

[EFLImpudence]

He told me the cable is only 10mm2 and would need to be upgraded

How would he connect the next size up to the shower?

 

[sparkwright]

The important thing here is to check the size of the cable.

Are you aware how cable is measured?

X.X mm2 is the cross sectional area of each conductor, so there is no easy way for you to measure this.

Usually, cable sizes for showers are 6.0 mm2 or 10.0 mm2. Even as small as 4.0 mm2 some thirty years ago.

So you need to find out for certain the cable size, then find a shower to suit.

Send photos if possible.

 

[ban-all-sheds]

The important thing here is to check the size of the cable.

Are you aware how cable is measured?

I have recently had an electrician in ... He told me the cable is only 10mm2

X.X mm2 is the cross sectional area of each conductor, so there is no easy way for you to measure this.

I have recently had an electrician in ... He told me the cable is only 10mm2

So you need to find out for certain the cable size, then find a shower to suit.

But it is quite possible that this fake electrician doesn't know what 10mm² means.

 

[RF Lighting]

If you can see any of the cable, such as at the back of the shower switch, then post a clear photo on here, and someone on here should be able to size it by eye for you

 

[JackDwight123]

Thanks for all the advice everyone...

I can confirm I have got RCD protection over the Shower circuit (I have one RCD covering all circuits on the left of my CB - all the sockets, Oven, and shower).

Im not able to actually measure the cable myself, Im just going by what the 'electrician' told me and the fact that there was already a 40A MCB connected (I know this is a big assumption, but Im thinking youd always have an MCB thats a lower rating than the cable). So like you guys are all saying I dont know for sure about the size of the cable (I will get a picture up later).

If I can confirm the cable is infact 10mm2 and I know the MCB is 40A and I know how the cable is laid (under floor boards and in plasterboard wall, no insulation) and is no longer than 15m would it be possible and more importantly safe to hook up a 9.5kw shower?

Thanks again everyone.

 

[sparkwright]

If it 10 mm2 then this should be fine for most showers.

I wouldn't be volunteering to fit the next size up which is 16 mm2, this would be a complete sod to terminate in a switch.

With all power off, take a picture. Try to include some of the bare earth wire in the picture, as this may assist in determining the size.