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[Wallace2017]

I am looking to replace a previous in-line extractor fan; located in my loft however, it was clearly fitted by a cowboy DIYer before as there is only 2 core and earth.

The problem arises with the wiring as the fan requires a switch live but there isn't one as the pull cord has a live, neutral and earth for both feed and source. The feed comes from the main fuse box and the source being the bathroom downlights.

There is a 3 pole isolator switch located on the wall outside of the bathroom which I have fitted a 3 core and earth from to the extractor fan, however, the manufacturer instructions state a 3AMP FCU is required???

It is causing some confusion as to how to wire this new extractor fan with timer into the lighting circuit. I am looking to locate the FCU with neon in the loft next to the extractor fan unit. How do I get a switch live for the extractor fan, FCU & isolator switch??

There is no ceiling rose for the bathroom lighting circuit, just the 1.5mm cable running to/from each downlight. There is a junction box on the mains cable on to the junction boxes completing the upstairs radial lighting circuit.

[ericmark]

First things first, do you need a fan, or is it just something you want, when I say need I mean to comply with building regulations?

It's all down to windows, if there are no windows then the fan can be controlled solely by lights.

If windows open it does not need a fan.

If it has windows which don't open then you must be able to switch on the fan without switching on the lights. With this scenario best option is likely a simple pneumatic push switch. To get it to switch with the lights plus have an independent method would mean a double pole switch for lights, so although same switch they are not electrical connected.

With all options likely the best option is a PIR to switch it on. Again no electrical connection to lights.

The 3 amp fuse is the preferred size, electricians carry 3 and 13 amp fuses and they are matched to wiring not the appliance. Although the fan manufacturer says 3A in real terms it will not protect the fan, the wiring is too thin in the motor so motor will burn out before fuse blows, with a 6A fuse in the lighting the 3A fuse will rupture first, but with a 6A MCB or RCBO likely they will open first so the 3A fuse is pointless. One is in theory allowed up to 16A for lights, as long as there are not any junction boxes etc which have a lower rating, and in the main ceiling roses are rated 6A so in real terms 6A is highest supply for lighting.

So in the main three pole isolator only is used, a FCU with 3A fuse is only used when supplied from power circuit.

The idea of three wires is the permanent line keeps fan running after lights are switched off, where a pneumatic time switch is used this is not required.

If when the house was built there was no fan, you don't need to fit one. Unless that is you change something like fitting a non opening window instead of a opening one. How one knows what happened in past I don't know?

The problem today is bathrooms require any new items to be RCD protected, this can make a simple job of fitting a fan more expensive, although a RCD FCU would seem easy way out.

If the pull cord does not work lights then reasonable easy, but if it also works lights crossing all t's and dotting all i's is not that easy. The guy before may have not been the cowboy he may have been following the rules.

Personally having a fan in bathroom is OK, but in the toilet it's a pain as you wake up house hold when you turn on loo light, same with PIR. So although we were forced to fit a fan by building control, as soon as we got the completion certificate the isolator was switched off and it was never used again.

If on leaving the toilet you could push a button and let fan run for 15 minutes that would be great, clears the steam without putting you in a draft, same would be good for under floor heating 1 hour after leaving to dry floor. Both are pointless while having the shower.

[Wallace2017]

Thanks for your reply.

The extractor fan is needed as there is a shower cubicle which is enclosed and no where for the stream and condensation to go until the shower cubicle doors are opened, obviously this doesn't help when the shower is full of steam during use.

Ideally, I would like to have the extractor come on with the shower use which is operated by a separate pull cord switch than the light circuit. The shower pull cord powers the shower and the downlight within the shower cubicle.

The confusion comes as to how to wire in the extractor fan, 3 pole isolator switch, and 3amp FCU especially when a switch live is required for the fan timer???

[ericmark]

There is really no need for three pole isolator. Permanent line and neutral into FCU from FCU to pull switch, the pull switch then splits the permanent line into permanent and switched. Normally you would use a momentary pull cord switch so it does not need switching off and set timer at 20 minutes instead of 15 allowing for 5 minutes in shower.