Installing doorbell transformer in CU

4 May 2008
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United Kingdom
Hi All,

I would like to install a doorbell transformer in my consumer unit to power a door chime. I've never taken the front of a CU before, and would like some advice as to whether this is a viable plan for a DIYer.

As I understand it, what I would need to do is:
Disable the power to the CU by turning off the main switch on the unit.
Remove front of CU.
Install transformer and connect wires to door chime unit.
Replace cover on unit.

My main questions are:
Is this a safe and straightforward job for a DIYer?
Would any Wylex doorbell transformer be likely to fit my Wylex CU, or would there be different types?
What type of wire would I need to use between the transformer and the door chime unit? - the transformer outputs 8V.
I presume that a doorbell transformer will output a DC current - in which case am I right in thinking that both + and - terminals will be located on the transformer itself rather than using a common negative supply in the CU?
Is there anything else I should be aware of?

Many thanks for any advice :)
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If you've never worked in a CU before, I would generally suggest this is not a job for you. If you do decide to go ahead with it though, then some points:

* Turning off the main switch still leaves the tails that feed it live, so be careful as those terminals (and any exposed copper if they haven't been fitted properly) will be live.
* Make sure you supply your transformer from a suitable circuit breaker (e.g. a lighting circuit) - don't just supply it from the bus bar, as at that point there is no overcurrent protection for it. You may have to cut the bus bar, if a full one has been installed all the way across. It's possible the transformer will have it's own overcurrent protection or similar, I've never used one, so check the manufacturer's instructions.
* Take the feed to your bell from the output of the transformer only - it is most likely a DC supply (but even if it's AC), it will have it's own 2 terminals, don't try and use the main neutral bar.
* If you decide to install a new MCB for this transformer, then note that that would be notifiable under Part P of the building regulations, as it's a new final circuit. I believe adding the transformer to an existing circuit would not be (unless your CU is in a kitchen or other special location).
Hi Guys,

Many thanks for the advice, - given that I don't fully understand the replys (I was unaware that it would be possible to supply this sort of transformer from another circuit breaker in the CU - I thought that all units within the CU would connect directly to the busbar) I think this is probably an indication that I should leave well alone and install a battery operated doorchime instead!

Thanks for the help!
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1) Any cables within you consumer unit must be rated for 230 volt even when they only carry 8 volt.
2) I would think you will need to pay council their £100 plus fee under Part P to work inside a consumer unit.
3) Transformers are normally A.C. output.
4) It will be considered as SELV and should not connect in any way to the 230 low voltage supply.

As to failure affecting other items within the unit I would think this is unlikely but unlike the external units the in-built protective devices are often not accessible so any fault will likely require whole unit change. Safety feature to stop wrong size fuses being fitted.

I can see some advantages as initial install but not as retro fit and I use radio linked chimes.
Transformer mounted inside the CU is tidy but is definately not a DIY retro-fit task.

Fit a transformer outside the CU and feed it 230 volts from one of the lighting circuit MCBs. That way if the MCB trips for some reason and de-powers the door bell you will be aware as the lights will not be working.

And do not forget that the door bell push connects between transformer and the bell. NOT between transformer and 230 volt supply I have seen two such installations by people who thought the power to the transformer had to be switched by the bell push.

Radio linked door bells work but are dependent on batteries and other wireless related "difficulties".
could you give me a link or information on where to get a wylex bell transformer.

I didn't think they made them,

Only ever used DIN rail mount transformers for bells once - in a small block of flats, landlord explicitly wanted them supplied from his supply, was a good use of a completely stripped down hager CU!

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