Is a 3 amp FCU for a Fan required on a lighting circuit?

Whimsical · 24 Mar 2013

Hello all,

I'm in the progress of fitting back boxes and extending the lighting wiring to the bathroom to include the fitting of extractor fan (im not doing it all myself just saving where I can on what I can do myself)

The diagram with the fan suggest that the feed into the bathroom is protected by a 3 amp FCU. However there is no specific information on this.

3 amp FCU > Light > Isolator switch > Fan

Is the 3 amp FCU only a requirement if I am coming off a ring main (rather than lighting circuit)? Is there any benefit to putting one in if not needed?

Many thanks

Many thanks

EFLImpudence · 24 Mar 2013

Whimsical said:
Is the 3 amp FCU only a requirement if I am coming off a ring main

That would depend on what cable you used.
The fuse/mcb is there to protect the cable - not the appliance.

Is there any benefit to putting one in if not needed?

By definition, no.

3A fuses for extractor fans are needed if the cable requires such protection.
This is not usually the case.

Unfortunately one must be fitted if the manufacture demands it because they have not designed their product sufficiently well enough.

Whimsical · 24 Mar 2013

Many thanks,

The cable is 1.5 mm. If the diagram suggests the feed is protected by a 3am fuse (no other information other than a wiring diagram, Axia silhouette T 100) would you therefore suggest I put one in anyway, even if not normally required?

Not sure if this is important but there will also be a lighted bathroom mirror.I assume that the feed for this will come from the ceiling rose?

Spark123 · 24 Mar 2013

If the manufacturer instruction says a 3 amp fuse is required then unfortunately you have little choice in the matter (if you want it done to regs that is)

Whimsical · 24 Mar 2013

OK, Many thanks

sparkwright · 24 Mar 2013

In practice, it seems people don't actually fit a 3 amp fuse, and rely on the lighting 5 amp fuse or mcb.

Is this a timed fan?

If so, the wiring to introduce one 3 amp fuse to serve both the permanent live and switched live is a bit involved. The chances are this may baffle your electrician who will be connecting it up - as many don't bother with a 3 amp fuse.

The easier way is to fit an unswitched fused spur (with 3 amp fuse) which serves the bathroom light, and therefore in turn, the timed fan as well.

Much more easier to wire.

fireman22 · 24 Mar 2013

well to do it correctly a 3 amp fused spur should be fitted, not to protect the cable, but to comply with the fan man. recommendations. on new builds we install a dual box with 3a spur one side and the three pole fan isolator on the other.

EFLImpudence · 24 Mar 2013

Nevertheless, electrically they are not necessary and only required because the manufacturer has -

not made the product satisfactorily to be connected to a 6A circuit and -

not fitted the product with its own integral fuse.

Spark123 · 24 Mar 2013

The manufacturer would have to fit more than one fuse for a timed fan as there are two inputs (live and sw live) hence probably the reason for taking the fuse offboard.

EFLImpudence · 24 Mar 2013

Manufacturers of other products do it if it is necessary.

Do they not require fusing only for the permanent live?
The switched live cannot be overloaded by the motor failing.

Even if two are needed, they should do it.

Spark123 · 24 Mar 2013

Some manufacturers do, others don't!
Eterna have a habit of making their maintained emergency lights a pain in the backside to switch!

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Is a 3 amp FCU for a Fan required on a lighting circuit?

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