Door bell transformer installation

[Londonmuscle]

I am installing a transformer to power a door bell and wanted advice.

I am going to run 1.5 cable from the CU to a switched FCU which will be earthed.

As the transformer has no earth terminal, can I run another 1.5 cable from the FCU without an earth? I take it as the transformer is double insulated, it does not affect what mains terminals I connect it to?

 

[Taylortwocities]

You should understand that the fuse is there to protect the cable.

I am going to run 1.5 cable from the CU to a switched FCU which will be earthed.

The size of the cable will depend on the size of the fuse/cb in the CU. 1.5mm is OK if the fuse is 6amp. If the fuse is 32amp then your cable will need to be at least 4mm²

As the transformer has no earth terminal, can I run another 1.5 cable from the FCU without an earth? I take it as the transformer is double insulated, it does not affect what mains terminals I connect it to?

NO. The cable from the FCU to the transformer must be earthed. The central core is called the cpc (circuit protective conductor) it protects because it will be connected to earth (at the FCU end).
The transformer is double insulated so the earth does not connect at that end.

 

[Londonmuscle]

You should understand that the fuse is there to protect the cable.

I am going to run 1.5 cable from the CU to a switched FCU which will be earthed.

The size of the cable will depend on the size of the fuse/cb in the CU. 1.5mm is OK if the fuse is 6amp. If the fuse is 32amp then your cable will need to be at least 4mm²

As the transformer has no earth terminal, can I run another 1.5 cable from the FCU without an earth? I take it as the transformer is double insulated, it does not affect what mains terminals I connect it to?

NO. The cable from the FCU to the transformer must be earthed. The central core is called the cpc (circuit protective conductor) it protects because it will be connected to earth (at the FCU end).
The transformer is double insulated so the earth does not connect at that end.

I take it then, that I run the 1.5mm cable from the CU to the FCU, palce a 3amp fuse in the FCU then from the FCU to the transformer, I connect the earth at the FCU end but not at the transformer?

 

[Taylortwocities]

Yes. Assuming the contact breaker in teh consumer unit is 6amp.

Are you going to be connecting into an existing circuit at the CU (eg lighting?)

 

[ericmark]

That would be an expensive way by time you pay the LABC Part P charges but yes earth cable all way to the transformer would be norm.

At £100 plus for Part P fees it would be far cheaper to get a registered electrician to do the work.

There is a max of 3 meters of cable where the fuse is at end of cable not at origin so if tapping into ring main at CU then no more than 3 meters to FCU.

After FCU then only restriction is for loop impedance and volt drop which with 1.5mm and 3A would be very long.

Remember is buried less than 50mm then RCD protection or Ali-tube cable is required.

 

[Londonmuscle]

It would be from an existing 6 amp lighting fuse in the CU. I want to run 1.5mm cable to a FCU which would be earthed, fit that with a 3amp fuse, then run the cable from the FCU to the transformer.

I did not know whether the section between the FCU and the transformer had to be earthed, as there is no earth connection on the transformer, that is what I was not sure of as I thought it would not make sense to connect the earth from the load of the FCU when it won't connect anywhere.

It is going to be installed next to the CU as the door is right next to it.

 

[Londonmuscle]

Or is 3amp fuse in the swtiched FCU to low, should I fit a 5amp fuse ?

 

[Londonmuscle]

Or is 3amp fuse in the swtiched FCU to low, should I fit a 5amp fuse ?

 

[Paul_C]

A 3-amp fuse will be more than adequate for any normal domestic doorbell transformer. You could fit a 1-amp fuse if you wanted to.

 

[Londonmuscle]

Could it also be possible to run a cable from the transformer and connect it to a 3 pin plug or is it designed for hard wiring?

 

[bernardgreen]

Yes and it gives the easy option to turn the door bell off when you do not want to be disturbed.

 

[Londonmuscle]

What cable would I use from the transformer to the plug?

 

[JameZZ]

http://www.tlc-direct.co.uk/Products/CA0dot5F3.html - Use a 3A fuse in the plug.