Can I add a 10mm cable spur to an existing 10mm cable supply?

[Moggy B]

I have an electric shower on a 10mm cable direct from the board with its own MCB and it has worked perfectly since installation. I would now like to fit a 9.6kW instant water heater to supply one tap in the bathroom and one in the kitchen. It would be difficult to run another 10mm cable back to the board, so, can I fit a 45amp junction box from the existing 10mm feed to the shower, to power the instant water heater? Distance from the JB to the water heater would not be more than 3 metres.

 

[omega015]

What power rating is your shower? What MCB do you have for that circuit? Where are you going to be placing the instant water heater? Where do you intend on fitting the junction box? Likely the answer will no to what you are proposing but we need a bit more information to explain why.

 

[Moggy B]

Many thanks for your interest. Your questions answered:
Existing shower is 9.6kW; existing MCB is 40A; the heater will be in an airing cupboard taking the place of a copper storage tank; the JB would be in the attic, close to the existing isolator pull switch for the shower. Thanks again.

 

[flameport]

You can, but ONLY if an additional device is installed to prevent both the shower and heater being used simultaneously.

The existing circuit is not suitable for an 80A load, and it's fairly likely the supply to the property isn't suitable for that either.

 

[Moggy B]

Forgive me for asking but can you suggest a suitable switching device which would accomplish what you advise?

 

[flameport]

One example: http://www.electricalwholesaler365.co.uk/Products/Priority-Shower-Unit-Eaton__269141.aspx

However what is the purpose of installing this water heater instead of the stored water system, and how many outlets is it intended to supply?

 

[Moggy B]

The planned changes are to improve the effeciency and reduce the carbon footprint of an old, back boiler, open fired heating system in my cottage by installing a wood burning stove and the water heater which will supply only the tap in the bathroom and the tap at the kitchen sink thereby removing the need to light a fire or switch on an immersion heater.

 

[Moggy B]

What if I dropped the load for the instant water heater to 5.7kW? Would that allow me to use a 6mm cable and alter things significantly enough for me to avoid having to fit the switching device?

 

[EFLImpudence]

What if I dropped the load for the instant water heater to 5.7kW?

That's still an additional 24A so - not really.

Would that allow me to use a 6mm cable and alter things significantly enough for me to avoid having to fit the switching device?

What do you mean? 6mm² cable can carry a maximum of 47A (assuming suitable installation method) so the cable is not the problem.

I think you will find the immersion (with insulated cylinder) is not as wasteful as you imagine.



Certainly going 'green' is not a good enough reason for electrical bodges.

 

[JohnW2]

One example: http://www.electricalwholesaler365.co.uk/Products/Priority-Shower-Unit-Eaton__269141.aspx

Might a cheaper option for the OP to consider be to have a manual changeover switch (outside of the bathroom) - normally switched to the water heater but manually switched to the shower when that was needed? Admittedly, one would have to remember to do the manual switching.

Kind Regards, John

 

[flameport]

improve the effeciency and reduce the carbon footprint

Using electricity will not achieve that.
By the time electricity reaches your home, 2/3rds of the energy has already been wasted.

For the water heater, most of the losses are due to the pipework, where hot water is left in the pipes and cools rapidly every time it is used.

 

[Moggy B]

Many thanks for your contributions thus far and for any more to come. I fully appreciate the importance of not doing a bodge job, even in the interests of 'green' wishes. Safety first, always!