what size cable for 8.5kw shower

[Gary80gas]

Ive got an electric shower to replace, currenty 8.5 kw on a 6mm cable.
Is it dumb to install 8.5kw on 6mm, im sure i used to do this years ago without thinking too hard about it.
A quick bit of research recommends 7.5kw max rating. what would you do?
Rewire for 10mm too expensive. Cheers.

 

[Burger]

Hi Gary,

There are some factors that are taken into account:

1. Manufacturers instructions - these will tell you whether it’s suitable to use 6mm or 10mm.
2. The length of run
3. Insulation thicknesses

 

[Welder]

Ive got an electric shower to replace, currenty 8.5 kw on a 6mm cable.
Is it dumb to install 8.5kw on 6mm, im sure i used to do this years ago without thinking too hard about it.
A quick bit of research recommends 7.5kw max rating. what would you do?
Rewire for 10mm too expensive. Cheers.

Personally, I'd simply replace like for like. However, since I have no idea what I'm doing (only an HND in Electronics - not Part P) I'd ignore everything I say to cover your backside ... 🙄🙄🙄

 

[Riley]

MIs and don’t be cheap if it calls for it 😀

 

[Gary80gas]

Hi all, Manufacturers always cover themselves by saying get an electrical engineers advice. Fair enough.
I understand a cable can overheat but i cant remember last time i fitted 7.5 kw for shower. Is there a simple calculation that would cover me for installing 8.5kw on 6mm. Length of run, route and power etc?

Im not looking to take risks just give the customer the best deal. I gotta make a decision soon and to cover myself the generic cable sizing means to be safe its 7.5 kw, just sounds weak for a shower.

 

[Welder]

You can use 6mm cable for up 9.2 kW. Only condition is if you have a 9.2 kWshower on a 6 mm cable it should not be more then 6 meter away from the consumer unit. But 7.5 kW is ok. If you have a shower more then 9.2 kW, 10mm cable must be used.
Shower cable 6mm or 10mm - MyBuilder

https://www.mybuilder.com/questions/v/17924/shower-cable-6mm-or-10mm

 

[Welder]

From the current Bristan Joy 8.5KW shower installation instructions...

 

[Gary80gas]

My understanding is if the 6mm cable is fitted where heat cant build up then 8.5kw is ok. Any insulation then it has to be 10mm. Part of the run goes through the roof so its bound to be covered in insulation.

 

[Harvest Fields]

I would like for like if I’m honest. I only get a sparks in if they want a bigger output shower.

 

[mattfrance]

2 days ago someone asks how deep he should dig a trench to put a gas pipe in, everyone jumps on him because he's not in the gas safe cartel, the guys just trying to save a bit of money by digging a trench.

but its cool to advise someone about the size of cable that they should use for an electric shower? something if you get wrong your house burns down, you get electrocuted, i really don't understand this forum

 

[Riley]

2 days ago someone asks how deep he should dig a trench to put a gas pipe in, everyone jumps on him because he's not in the gas safe cartel, the guys just trying to save a bit of money by digging a trench.

but its cool to advise someone about the size of cable that they should use for an electric shower? something if you get wrong your house burns down, you get electrocuted, i really don't understand this forum

Chalk and cheese really mate. I would suggest anybody running a new wire back to the consumer would get a sparks in to connect and sign off. I think this guy has been told a few times to not upsize the shower but unfortunately people do what people do

 

[Harvest Fields]

Agree with @Riley

 

[Deleted member 96286]

10mm twin and earth is the recommended minimum for all electric shower installations. Even the manual in budget Triton Enrich showers tells you that. Don't skimp or try to save pennies on safety regulations, especially those made by the manufacturer. You'll need a RCD at the consumer box and either a pull switch or a shower switch at the bathroom.
In Scotland any electrical works to be carried out in a bathroom, must be done by a qualified electrician, I don't know how this applies in England and Wales.

 

[Gary80gas]

The dilema for this installation was replacing an 8.5kw shower which may have been overated for the 6mm cable. I would'nt do it because someone 10yrs earlier thought it ok, if it caused a fire that would be a poor defence. I ran the details through TLCs cable calculator and it recommended 10mm, interestingly if it was surface clipped to a stone wall i could have used 4mm cable for 8.5kw. New installation would always be 10mm in my book.
I cant be sure how the cable has been run and how much insulation it comes into contact with, so im opting for a 7,5kw shower. Thanks all.

 

[markadams]

As a sparky and heating engineer I can say that according to BS7671 : 18th ed, a 6mm T&E cable clipped direct or buried in a wall with no insulation will take 41 amps.

8.5kw - 8500/230 = 36.95 amps and this doesnt take into account the diversity factor that can be taken into account.

6mm is more than enough for an 8.5 kw shower, just to be tecnically correct there is NO WHERE in the regulations that states you must have a double pole isolation switch for an electric shower, it is just good practice. All the regs state is that you must have a means of isolation for all circuits which you have at the consumer unit if you really wanted to avoid the switch (which I never do but just saying).

You do need to have a 30ma RCD protecting the circuit with the correct rated over current protective device.

You should also have a qualified Part P electrician doing the work as you have no idea if the circuit you are connecting to is safe and the safety devices work.

 

[markadams]

The only issue you may have with 6mm T&E is if you have a very long run then volt drop may not be within tolerance.

 

[markadams]

if you are going for a 7.5 kw shower then use a 32 amp BS60898 MCB or a 30 amp BS3036 fuse

 

[Murdoch]

if you are going for a 7.5 kw shower then use a 32 amp BS60898 MCB or a 30 amp BS3036 fuse

Or a BS61009 rcbo

And a DP isolation switch is recommended

 

[Murdoch]

Or a BS61009 rcbo

And a DP isolation switch is recommended

And I forgot to mention, nobody has commented on the fuse or breaker capacity ..... That's where the consideration is key

 

[Burger]

And I forgot to mention, nobody has commented on the fuse or breaker capacity ... That's where the consideration is key

True, but some of us mentioned MI's, which would usually cover this, but a good point all the same 😀

 

[Murdoch]

True, but some of us mentioned MI's, which would usually cover this, but a good point all the same 😀

MI's can't allow for the installation method ....