Spur off lighting circut to power underfloor heating

17 Mar 2012
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United Kingdom

We are currently redoing a bathroom.
We would like to install under floor heating, but have a few issues trying to power it.

We are looking at installing Warmup DWS300
300 watt, 1.3 amps.

There are no powerpoints near the bathroom to take a spur to power the underfloor heating.

The easiest thing I can see is to take a spur off the lighting circut (from the light). Run the cable through the ceiling were there are allready some cable runs. Then run the cable down the inside of frame/stud work to the thermostat/switch (on the outside of bathroom) then down to the floor to the underfloor heating.

The lighting ring has a 5 amp fuse at the fuse box. The total aperage of all the lights and the warmup should remain under the 5 amp limit of the fuse.

Is it within regulation to take a spur off the light circut to power the underfloor heating?

Can I do this myself? As technically I am not adding any points inside the bathroom does it count as electrical work in the bathroom?

What cable size would I need? Considering the lighting circut/ring that I can see is either 1.5 or 1mm. I would asume 1.5 would be sufficient as it will carry much more than the warmup will draw. And theres no point making it larger than the lighting ring.

Thanks for your help
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A few things:-

If you have a 5amp fuse, it sounds very old and probably means its time to consider a new consumer unit.

Based again on the above, its unlikely there is any RCD protection on any circuits. If you go with your plan, you would need to install an RCD with fuse unit outside the bathroom, maybe next to the controller for the underfloor heating.

I cannot see a major issue powering it off the lighting circuit. Its not ideal, but with the correct protection would be safe.

1mm T&E would be more than enough to do the job, 1.5mm will just be more fiddly to use.
It states Toughest and thinnest wire you can buy: 2mm, multi-strand core, double-insulated using advanced flouropolymers so I would not think this is suitable for use in a bathroom unless you fit an earthed mesh.

701.753 Electric floor heating systems
For electric floor heating systems, only heating cables according to relevant product standards or thin sheet flexible heating elements according to the relevant equipment standard shall be erected provided that they have either a metal sheath or a metal enclosure or a fine mesh metallic grid. The fine mesh metallic grid, metal sheath or metal enclosure shall be connected to the protective conductor of the supply circuit Compliance with the latter requirement is not required if the protective measure SELV is provided for the floor heating system.
For electric floor heating systems the protective measure "protection by electrical separation is not permitted.

So I am guessing you can't use that type anyway in a bathroom.

If one considers normal lighting in a house say 200W living room 100W kitchen etc. Then likely the loading is already 3A or more so that would only give you a 2A max not a 5A max.

As already said you will need RCD protection.

I fitted underfloor heating in my parents house in wet room and had to get a reply from manufacturer to say suitable for bathroom before I could fit. And it was useless after all that.
It was only to dry floor but once cooled with shower water the re-heat time was around 1/2 hour by which time my mother had left the room. The thermostat failed three times and now remains non functional.
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Thanks for your reply.

The basement was converted to a flat in the late 70's.
They installed Wylex Consumer Unit (what id call a fuse box, from Australia) BS1595.

It has
2 wylex nb05 type 2 MCBs
1 wylex nb30 type 2 MCBs
And an isolator switch (wylex BS5419) which I assume isn't an RCD.

Are there any rules that state an RCD must be installed when using underfloor heating, or just changing the electrics? Is it a must do or just good practice.

Its not that I am avoiding safety. I want to stay as safe as possible. I just want to understand what the regulations are so I know what can and cant be done.

My general plan was

- Spur off the lighting ring from the light in the ceiling (there is allready a cable spured at the light powering an exhaust fan)
- Run cable through ceiling down stud to fused/mcb/rcd (what is available?) switch.
- then wire to the thermostat
- then run cable to the underfloor heating in stud work.

Are there also any rules about where the cabling that goes into the tiling must be located. I.e. proximity to shower, bath or basin?

Thank you.
Thanks ericmark.

In the install manual for the warm up it makes note:

"Installation of an additional earth metallic grid is NOT required, as an earth metallic sheath is allready incorporated into the wire design, as per 701.753 of the BS7671:2008 wiring regulations."

So I assume this covers your concern with using the wire in the bathroom.

The lighting ring currently has:
- 4 light sockets. max 1.7amps - with 100watt globes
- 1 small chandelier (3 light sockets). max 1.3amps - with 100watt globes
- set of 3 12 v down lights in kitchen. max 0.6 amps.
- 1 extractor fan in bathroom. max 0.1 amps
Total max load currently = 3.7amps.

The warmup uses 1.3 amps.
which would take loading on the ring to 5amps.
Since we arent using 100watt globes in all the light sockets, actual loading will remain under 5amps on the lighting ring.

Would you just upgrade the consumer unit for a new one with an RCD in it or install an RCD just for the installation of the under floor heating? Which is likely to cost more?

Thank you
Underfloor heating MUST be RCD protected.

The registered electrician who would be doing this notifiable work could provide an RCD/fused connection unit to provide the RCD protection.

If you are doing other electrical works in teh bathroom then all of this must be RCD protected, so an RCD would need to be provided at the "fusebox" end to protect all circuits in the bathroom.

Why a registered electrician? You could do it yourself but you must first sign up with the local authority and pay their fee. For smallish jobs its cheaper to use a registered sparky.

More info at //www.diynot.com/wiki/electrics:part_p:diy_electrical_work_and_the_law
as an aside

300w will not heat the room you will need other heating sources to overcome the losses
I could not find link to manual when I looked however reading your link it clearly says it has an earth braid so is OK for bathroom use. However a number of your questions are also answered in the same sheet.
Install the RCD
The electrician must install a dedicated RCD or use an existing RCD. The electrician may wish to use a combination ‘double pole switched’ fused-spur/RCD. No more than 4.8kW of heating may be connected to each 30 milliamp RCD. For larger loads, use multiple RCD’s.

Note: It is possible to run the heater(s) from an existing circuit. The electrician will be able to determine whether or not the circuit can handle the load and if it is RCD protected.

It warns that there is likely some leakage when it says max of 4.8kW per RCD and also warns about the neutral - earth leakage problem which may exist when it suggests a double pole RCD.

In order to use a local RCD rather then built into the consumer unit the feed cables may need to be Ali-tube to avoid requiring a RCD for the feed cables.

Having fitted one and seeing the results I would not fit one again. However learn from my mistakes. Do make sure the pocket used for the temperature sensing probe has a genital bend when it goes up the wall. Our probe failed and although we used a pocket the bend was too sharp and we were unable to remove the probe to renew it.
Do make sure the pocket used for the temperature sensing probe has a genital bend when it goes up the wall.
Sorry, Eric, I know I'm the last person who can/should talk about typos ....

...but does that perhaps qualify as "typo of the month"?? :)

Kind Regards, John
It's great, isn't it?

The assistive technology which Eric uses does have flaws, but rarely do they result in such sublime results.

It's almost as good as the spell checker on our work email system which does not recognise the word "helpdesk", and suggests "helpless", a parapraxis more exquisite than Freud ever encountered.
Can I do this myself? As technically I am not adding any points inside the bathroom does it count as electrical work in the bathroom?
I don't know whether you can.

You may DIY, but UFH is a Special Installation, so you are required to apply for Building Regulations approval in advance.
I would not take the feed for the UFH from the lighting circuit as quite literally you could be in the dark when trying to fault find when the UFH trips the RCD feeding the lights. If you must take it from the lighting circuit do fit a two pole isolator so the failed / .faulty UFH can be fully disconnected to enable the RCD to be reset.

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