Installing a new junction box - which wire to run it off?

[djmwr]

Hi all

I want to install a mains powered doorbell in my home and I plan to run a junction box off of a current mains wire to do this.

I have access to the wires above where I want to put the doorchime (I have some floorboards up on my first floor) but there are plenty of wires leading to places where I can’t really see. Is there an easy way to work out which wire goes where?

Also, when running a new junction box spur is it recommended to go from a wire that serves a socket or a light fitting? Or does it not really matter?

All help appreciated!

Thanks

Matt

 

[Taylortwocities]

It all depends. Usually the doorbell runs from its own breaker. Sometimes it will share the feed from a lighting circuiit.

So you'll need to identify a lighting circuit permanent feed.
Not all the wires up there will be a permanent feed.

In any case, this is academic as you can't put a junction box under floorboards. It has to be accessible.

 

[djmwr]

Thanks for the reply.

What's the best way to identify a lighting circuit permenant feed?

 

[Space cat]

Is there an easy way to work out which wire goes where?

Not really.

Also, when running a new junction box spur is it recommended to go from a wire that serves a socket or a light fitting? Or does it not really matter?

If you want to tap a socket cable you'll have to put a fused connection unit (FCU) in there AND you'll have to make sure you're tapping into a ring main cable and not a spur. It's an elegant solution - you can even label the FCU "Doorbell" - but it's difficult and un-necssary. Tap a lighting cable.

PS: Make sure it's not a switched cable - or the cable from the switch itself! I've been in a house where the bell only worked if the hall light was on.

Edit:

What's the best way to identify a lighting circuit permenant feed?

Can you find at least one end of it?

 

[djmwr]

What's the best way to identify a lighting circuit permenant feed?

Can you find at least one end of it?

The only lighting feeds that I can see run between the light fitting and a switch, so I probably can't tap into that as it would only work when the light is on (like that other house you've been in)??

There is a permenant feed to a smoke alarm though, maybe I should tap into that?

 

[Taylortwocities]

The only lighting feeds that I can see run between the light fitting and a switch, so I probably can't tap into that as it would only work when the light is on (like that other house you've been in)??

It wouldn't even work then - because there is only a live and a switched live in that cable. And you need a permanent live and a neutral (and an earth).

There is a permenant feed to a smoke alarm though, maybe I should tap into that?

That may be a possibility. The smoke alarm may be fed from a lighting circuit. follow the wire from the smoke and see where that leads you.

Is this your house, or a rented one?

PS Why didn't you get a wireless, plug in doorbell?

 

[ban-all-sheds]

I have access to the wires above where I want to put the doorchime (I have some floorboards up on my first floor)

Will a JB there be accessible for testing and inspection in the future?

but there are plenty of wires leading to places where I can’t really see. Is there an easy way to work out which wire goes where?

Not really.

Also, when running a new junction box spur is it recommended to go from a wire that serves a socket or a light fitting? Or does it not really matter?

What matters are:

that the cable you use be appropriately sized for the rating of the fuse/MCB on the circuit. Note that if spurring from a 2.5mm² 30/32A ring final there are restrictions on how long a 2.5mm² cable can be.

that you do not add a spur to an unfused spur from a ring final.

that you are happy for the new <whatever> to be on the same circuit as the existing things served by that circuit.

that any new concealed cabling complies with the rules for concealed cables.

that the circuit fuse/MCB provides adequate protection as per any manufacturer instructions.

 

[djmwr]

Thanks for the advice all.

I will make sure the JB is in a place where it can be expected, and that the wiring meets all of those standards. It is in my home so I want to make sure it's done correctly.

 

[Space cat]

There is a permenant feed to a smoke alarm though, maybe I should tap into that?

If the smoke alarm is already on a lighting circuit, that would be reasonable. If it's on its own dedicated fuse/breaker, I wouldn't want anything else on there because that would diminish its integrity as a smoke alarm. (Ie faulty doorbell transformer trips breaker so alarm doesn't work. )

If you aren't sure, switch off all the lighting circuit breakers (or pull the fuses) and see if the alarm still works.

 

[sparkwright]

The usual method is to get a feed from the consumer unit, usually a lighting circuit, and site the bell transformer next to the consumer unit. You can even fit a switch in between to isolate the bell circuit if a fault develops. You would use a switched fused spur if wiring to a socket circuit.

It is done this way to keep the transformer out of sight.

You may be able to get a feed from the lighting circuit from a ceiling rose of there is a permanent live there. If there is, there will be more than two cables here, with wires to live, neutral, earth, and permanent live - which is usually the centre terminal with two or more lives connected.

 

[Taylortwocities]

If you aren't sure, switch off all the lighting circuit breakers (or pull the fuses) and see if the alarm still works.

It will probably still work. There is probably a battery back up. The alarm will have a mains power indicator that will go out when you turn off the appropriate breaker.

 

[Space cat]

It will probably still work. There is probably a battery back up. The alarm will have a mains power indicator that will go out when you turn off the appropriate breaker.

Good point!

 

[RF Lighting]

Sparkwright is dead on the money.

Site the transformer next to your fuse box and run a wire from there to your doorbell.

Don't mess about trying to botch it onto some random cables you've found under the floor which you have no idea of if they're suitable or not.

 

[ban-all-sheds]

How do we know it has a transformer?

 

[RF Lighting]

It's a doorbell.