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Whilst wiring a new extractor fan, I’ve ran out of twin and earth but I have some left over three core and earth from the lighting (1mm^2). The fan doesn't have an over-run and only requires 2+e. It will be on a 6A MCB circuit with a 3-pole isolator with 3A fuse regardless of the cable used.

I don’t fancy buying a new reel of 2+e for such a short length, so besides the fact it’s a bit wasteful, is there any reason I can’t use the 3+e and just leave one core unused?

I can’t think of any reason why it would be a problem as long as the end is properly trimmed so it’s not touching anything.

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  • 1mm wire sounds small, though. Does it meet requirements for the breaker size?
    – isherwood
    Apr 1 '19 at 15:29
  • @isherwood regs say to use either 1.0 or 1.5 mm^2 so confident the core thickness isn’t a problem, it’s only a 3A circuit.
    – Notts90 supports Monica
    Apr 1 '19 at 15:32
  • Ah. Here we almost never see breakers under 15A.
    – isherwood
    Apr 1 '19 at 15:34
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    @Harper twin and earth is required for single switch in UK, three and earth for two way switching.
    – Notts90 supports Monica
    Apr 1 '19 at 16:53
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    Sure. I'm saying if you make a habit of bringing true neutral (i.e. 3-core) to switches, you'll thank yourself later when you are trying to install a smart switch. I don't know how easy wires are to change in the UK, in the US it is usually rather hard.
    – Harper - Reinstate Monica
    Apr 1 '19 at 17:03
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The best thing to do with an unused wire is to mark it 'unused' at each end and then wire it into the earth connection at both ends. That way there won't be any loose wire which could possibly touch a connection and become live accidentally, and if it did the protective device would open.

Lighting circuits are usually 5A fuses or 6A MCBs.

Running 3+e to the fan (live, switched live, neutral, earth) allows you to use a fan with a built-in over-run timer.

If running 3+e you may need a triple pole isolator for the fan, eg

https://www.tlc-direct.co.uk/Products/MKK4859.html

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  • The fan doesn't have an over-run and only requires 2+e. It will be on a 6A MCB circuit with a 3-pole isolator with 3A fuse regardless of the cable used.
    – Notts90 supports Monica
    Apr 1 '19 at 21:32
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    I like the suggestion of wiring to earth to protect from accidentally becoming live.
    – Notts90 supports Monica
    Apr 1 '19 at 21:35
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Sure. It's done to future-proof bedroom ceilings all the time, allowing for fans to be installed later. Just put wire nuts (or your local equivalent) on each wire termination and tuck them into the box.

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    Hopefully as part of that future-proofing they also install a fan-rated junction box. Finding /3 on a switch-fed lamp run is a shibboleth for "this is fan-ready".
    – Harper - Reinstate Monica
    Apr 1 '19 at 15:51
  • Wire-nuts are strictly forbidden in the UK. Choc-block in an "accessible" location, or Wago connectors anywhere.
    – Martin Bonner supports Monica
    Apr 1 '19 at 16:59
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    @MartinBonner "or your local equivalent" :)
    – ThreePhaseEel
    Apr 2 '19 at 1:10

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