Why do I have to uncoil an extension reel before using it?

Every current-carrying conductor generates heat. Laid out flat, that heat disperses into the air along the whole length. Wound onto a drum, the inner turns are surrounded by other warm turns and by plastic, so the heat accumulates. That is not a marginal effect: it is the reason your reel carries two ratings that differ by a factor of three or four.

Safety

Step by step

1. Read the two figures on the reel.Almost every UK reel is labelled with a rating when fully wound and a rating when fully unwound — commonly 750W or 1000W coiled and 3000W unwound. Those are not marketing figures; they are the safe current limits.
2. Unwind fully for anything with heat or a motor.Heaters, kettles, pressure washers, lawnmowers, tile cutters, vacuum cleaners and any power tool. All of them are well above the coiled rating.
3. Unwind fully even if you only need two metres.The temptation is to pull off just enough. The remaining coil on the drum is still carrying the full current and is still the hottest part. Length used has nothing to do with it.
4. Do not trust the thermal cut-out as permission.Better reels have a thermal cut-out that trips when the drum overheats. It is a last-resort safety device, not a licence to run coiled. It also resets when cool, so people learn to work around it, which is exactly the wrong lesson.
5. Keep the drum out of the sun and off insulating surfaces.A reel sitting on a carpet, in a bag, or in direct sun starts hotter and has less margin. Stand it on a hard surface in the open.
6. Feel the drum during heavy use.On a long job with a heavy tool, put a hand on the coil occasionally. Warm is a signal to check that it is fully unwound and that the load is within the rating.
7. Coil it loosely for storage, not tightly.Tight coiling stresses the flex at the drum entry, which is where reel flexes usually fail. Wind evenly, and never leave a load connected to a stored reel.

Tips

• A reel used for a 2kW heater in a garage all winter is exactly the scenario the coiled rating exists for. That is a job for a proper socket, not a reel.
• If the reel has a cut-out that has tripped, unplug everything, unwind fully and let it cool completely before resetting.
• Reels with 1.5mm flex carry more than the thin 1.25mm ones. Check the flex size if you routinely run tools.

Common mistakes

• Pulling out just the length you need — The current flows through the entire flex including the part still on the drum, and that part is the one that cannot lose heat.
• Assuming the plug fuse protects the coiled cable — The fuse protects against the current the flex can carry when unwound. At a coiled rating four times lower, the flex can reach damaging temperatures at currents the fuse is entirely happy with.
• Leaving a heater running on a coiled reel because it has been fine before — Insulation damage is cumulative and invisible. A reel that has been overheated repeatedly has degraded insulation before it fails.

If it doesn't work

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