When I wanted about 15m of it, easiest way was to use standard
1mm T&E, short the far end, and feed it with an electronic 12V halogen transformer. I bought a 250W transformer and connected it to the cable, using a mains power meter to make sure it was within the load range (IIRC, I could make it 150W or 230W, depending if I connected the earth or not). This solution was orders of magnitude cheaper than mains heating wire. It's taped to an underground water main which I could only get 500mm deep rather than the recommened 750mm because the end connections were nowhere near that deep (and the original lead pipe was only a few inches deep at one end).Note you cannot work out the power rating by measuring the resistance of the wire and using ohms law. An electronic halogen transformer will be at least 20kHz output, and the skin effect comes into play, and the impedance will be significantly higher than the DC resistance, which in this case is a big bonus.
I've been controlling the humidity in a large shed for someone for
5 years now. Originally I was thinking I would need a dehumidifier, but I didn't have a spare one so started with a heater. That works so well, I stuck with it. It's an old oil-filled 1kW radiator, running at half power with a half-wave rectifier.You are right that the absolute humidity (dew point) is low at low temperature even when the relative humidity is 100%. This means that a very small temperature rise is required to drop the relative humidity and protect against condensation.
However, without protection, that moisture is dumped out on the contents of the shed when the temperature drops any further, and that's exactly what you don't want.
I keep the relative humidity to no higher than 80%, and the contents of the shed are on palets and spaced away from the walls so there's circulation all around (otherwise you will get cold spots which drop below the dew point). It also has frost protection which kicks in at 5C (and bumps radiator up to 1kW at 3C, although I just searched through the logs and that's never happened - coldest shed has ever got is 4.5C when it was -4.9C outside on Tuesday).
I monitored the costs in the first year, and I think it was around £50 for the electricity. Previously it was costing £200/month to store the stuff in a warehouse, so that's an enormous saving.