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I'm an industrial electrician with no real domestic experience, although I did get a 17th edition certificate a few years back. I've got a crap 4 inch extractor fan in the bathroom that I want to swap out with a 6" inline fan I have. I have a cooker style switch for my shower on the outside of the room. Is there any reason I can't drop a 1mm cable from the loft and use a fused terminal block at the switch to feed a relay for the fan? Would have the relay in a wiska box or similar in the loft and could label that there are 2 circuits enclosed.
Thanks in advance!
From a strictly electrical point of view, then yes you could connect into the existing heavy current shower circuit by means of a fused terminal block.
From a practical point of view, there might not be room to install this fused terminal block in the box for the shower switch. A better arrangement would be a fused connection unit with a 1 amp fuse, adjacent to the shower switch.
A pedant might argue that you need 1.5mm cable for the circuit as that is now the minimum cable for a “power circuit”
A more common approach is to connect the extract fan to the lighting circuit since this facilitates the provision of both a fixed live and a switched live for a fan with a run on timer.
Where does the old 4 inch fan get power, and why not re-purpose that ?
If however you do decide to take a 1 or 1,5mm feed from the shower cct, to supply a fused spur or switch-fuse or similar for the fan, try and keep the length of thin cable on the shower circuit that is on the side before you fuse down to a minimum.
4 inch fan is supplied from the lighting live and switched live as described by broadgage. The 6 inch fan I've got doesn't have a timer but you're right, it'd probably be easier to just get a fan timer unit and wire into that.
Talk about making hard work of a simple job 🙄
There are several options that are a lot more practical to install and for the users, all supplying the power from the lighting circuit.
The fan manufacturers may require the installer to install a 3 Amp fan supply fuse. Please read the instructions. 1.5mm2 is overkill for such a small load. A model with a timer overrun circuit inside it is desirable. The fan can be room light activated so continue running for a period to remove steam or smells after the room is vacated.