A sparky plugs a 2kW fan heater into a cable reel on a cold site, leaves most of the cable wound on the drum, and comes back to a reel that is too hot to touch and a smell of scorched plastic. The reel was rated 13A. So what went wrong?

Why a coiled reel overheats

Any cable carrying current gets slightly warm, because the conductor has resistance and resistance turns some of the current into heat. When the cable is stretched out, that heat escapes into the air along its whole length and the cable stays cool. Wind the same cable tightly onto a drum and the picture changes completely. The inner turns are buried under the outer ones, the heat has nowhere to go, and it builds up in the middle of the coil.

That trapped heat softens and eventually degrades the cable's insulation, which is how a wound reel goes from warm to smoking to a fire. It is not a fault in the reel. It is basic physics, and it is why every cable reel carries two different ratings, not one.

What the wound rating actually means for the job

Put those numbers against real loads and the risk becomes obvious. Most power tools are fine, but anything with a heating element is not. Here is roughly where common site loads sit against a wound reel's 720W ceiling.

The practical rule on site is simpler than the table: if you are drawing more than a couple of hundred watts, or running anything for a sustained period, unwind the drum fully first. It takes a few seconds and it is the single habit that prevents nearly every reel fire.

Thermal cut-out: the feature that earns its place

A good trade reel does not rely on the operator remembering the physics. It has a thermal cut-out built into the drum, which trips and kills the power if the coiled cable starts to overheat, then resets once it cools. On the Masterplug Pro-XT range this sits alongside a localised power switch and an LED indicator, and the reels are built to BS EN 61242, the standard for cable reels for household and similar use. When you are choosing between a cheap reel and a trade one, the thermal cut-out is the difference that matters, because it protects against exactly the mistake that is easiest to make.

The switch itself is worth a look too. A double-pole switch, as fitted to the Pro-XT reels, isolates both live and neutral when you turn the reel off, which is the safer arrangement for something dragged around a working site.

Open, cassette or metal drum: which reel for the job?

Once safety is settled, the format is about the environment and the cable length. Open-frame reels are the lightest and cheapest for a given length and suit general site and garden use. Cassette reels enclose the drum, which protects the cable better in storage and transport and suits a van that gets knocked about. Metal-drum reels are the toughest and take the heaviest sustained duty, which is why they turn up on the busiest sites. Cable length is a straight trade-off: longer reaches further but weighs more and, crucially, generates more heat when wound, so do not buy far more length than the job needs.

For outdoor and wet work, check the socket protection. A reel used in a garden or on an exposed site wants weatherproof sockets or covers rather than bare 13A outlets. If your job is really about fixed or semi-permanent power rather than a portable drum, our guide on which Masterplug extension lead you need covers the non-reel options. And whatever the source, respect the maximum rating and add up your loads, in line with HSE guidance on electrical safety at work.

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