The rating plate on a cable reel carries two different power figures — 1000 W and 3500 W, for instance. This is neither a misprint nor a marketing device. They are two real limits: one for the cable while it is coiled, one for the cable fully unwound.

The physics of a coiled cable

A cable under load generates heat. Unwound, it sheds that heat freely into the surrounding air across its whole surface. Coiled on a drum it forms a tight winding: the layers heat one another, and the heat trapped inside the coil has nowhere to go. Temperature at the core of the roll climbs far faster than at the visible surface — and it is there, out of sight, that the insulation ages fastest.

The result is softening and loss of insulating properties, and in extreme cases a melted jacket and a short circuit. The damage rarely shows immediately; it accumulates over many such cycles.

The two figures on the plate

• Power with the cable coiled — usually three to four times lower. That is the ceiling if you do not unwind the reel.
• Power with the cable fully unwound — the full figure, determined by the conductor cross-section.

If the device you are connecting draws more than the “coiled” figure, unwinding is not a recommendation. It is a condition of safe operation.

A thermal cut-out is a safeguard, not a permission

Better reels include a thermal cut-out that disconnects the supply on overheating. Understand what it does: it protects against fire, not against the gradual destruction of the insulation. A reel that trips regularly is being worked outside its range — that is a reason to change how you use it, not to wait for it to cool down.

Rules for everyday use

• Unwind the full length, even when you only need a few metres.
• Do not cover the unwound cable with tarpaulins, mats or building materials.
• Do not rewind the reel straight after prolonged work under load — let it cool first.
• Inspect the cable end to end after every season, paying attention to where it enters the drum.