electric bell wired into 16amp mcb

Postby danny001 » Fri Jun 13, 2008 8:17 pm

i need some advice on a electric bell,
i have ssen a bell wired straight in a 16amp mcb which also controls a radial curcuit
i can see how this is correct should the radial be at 20amp amd can some tell me what the bell should be in

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Joined: Fri Feb 22, 2008 11:35 pm


Simply Build It

Postby ericmark » Sat Jun 14, 2008 4:54 am

The manufactures instructions should be followed. Bell transformers are available to connect into the consumer unit in the same way as the MCB’s and these are designed to be protected by the main incoming 100 Amp fuse. But most bell transformers only require milliamps and would normally be supplied through a 1 amp fuse in a fused connection unit. 230 volt bells are rare but are around and would not use a normal bell push which is normally rated to a max of 16 volt.
A radial could be supplied by any size fuse for example a radial to the cooker could be 45 amp and a radial to immersion heater 16 amp. There are two things with would decide to size of a MCB.
1) The equipment it feeds, for example a built in oven may require a 16 amp MCB even though the cable is 6mm².
2) The cable and it’s route for example a 2.5mm² could if in a stud wall with thermal insulation have a maximum of 13.5 Amp but the same cable clipped direct could carry 27 Amp only flexible cords have a current carry capacity which is fixed 2.5mm² is ratted 25 Amp as it is not permitted to run flexes in or under anything they must always be exposed to the air.
So if you wanted to increase the size of any MCB feeding an existing cable you would need to look at the installation certificate and see what the installation method is. Unfortunately the method is not always included which could mean knocking a few holes in the wall and lifting a few floor boards before it could be done.
One good reason to hang on to all installation, completion, and periodic inspection reports.

Yours Eric

Postby danny001 » Sun Jun 15, 2008 7:51 pm

thanks for the info i have had periodic inspection reports done and its says that

protective device over rated for the cable - part of circuit wired in 1mm flex

the mcb is a 16amp and this feeds a radial circuit and a bell is also wired into the mcb which is 1mm

Posts: 47
Joined: Fri Feb 22, 2008 11:35 pm

Postby ericmark » Sun Jun 15, 2008 10:05 pm

Cable rating are not easy 1mm² flex is rated at 10 amp and should be exposed throughout it’s length but 1mm² flat twin and earth can vary from 8 amp to 16 amp depending on where the cable is run which could be an insulated wall or a highly conductive bit of cable tray but I would have thought the MCB will need changing to either a 6 or 10 amp version. You say radial but not what the radial feeds for lights we would normally limit the MCB to 6 amp as that is maximum rating of ceiling roses we would not normally feed sockets with 1mm² cable 1.5mm² is the minimum normally used. Where cable size is reduced one normally has to limit the current either with a fuse or MCB there are a few exceptions for example where feeding a single item where the item has some current limiting built in for example a 13 amp socket which will always have a fuse in the plug so a spur off a ring in 2.5mm² is allowed.
You must realise we can’t see what you have but I would have thought there are two ways to go forward one is fit a fused spur (Most likely one with a switch as we so if door bell gets stuck you can turn it off) and the other is change the MCB for smaller one.

Yours Eric

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