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It'll be about 15m long. No idea on voltage...
The heater / humidifier will only be there on a thermostat to keep the place above the dew point so hopefully not on for long periods.
I was planning on it being not much more than a glorified extension cord from an outside socket that's already in place (and wired to circuit breakers on the main fusebox). Is it reasonable to do this?
christianSpaceman-
Not really sadly. Garden electrics all need signing off these days.
If you were to use armoured cable, which you should, then you score around the cable with a hacksaw and the wire (armour) can be broken off quite neatly. The terminations are quite pricey. Don't even think about doing it with conduit but also don't do it if you don't feel comfortable breaking the law.
Airhead
How long will the cable be?
Calculated the voltage drop?
Part P may seem quibblesome, but a qualified Electrician can sign off the works to the local Building Control.