# Cable on a drum: why unwind it fully?

> Wound on a drum, a cable can no longer shed its heat: conversion factors by number of layers, a worked example and good practice at an event.

Source: https://www.darkside-energy.com/en/did-you-know/cable-on-a-drum/ · Dark Side Energy · Updated: 2026-11-03

3 November 2026 · 3 min read

- A wound cable can no longer shed its heat: the hottest point is at the core of the drum, invisible from outside.
- On its drum, a cable loses a large part of its capacity: 20% with a single layer, about half with three layers.
- As soon as the load is significant, unwind the cable fully: the socket's protective device does not protect a cable left wound.

A cable carrying current always heats up a little: this is the Joule effect. Unwound, it sheds this heat into the air. Wound on a drum or a cable reel, the turns heat each other and the heat builds up. The hottest point is inside the drum, between the first layers: where you cannot see it.

The insulation can then soften, and melt: short circuit, fire, or a live protective conductor. And the damage sometimes only shows when the cable is unwound.

## How much capacity is lost?

The German standard DIN VDE 0298-4, used by cable manufacturers, gives the factor to apply to the current rating of the unwound cable:

| Layers on the drum | 1 | 2 | 3 | 4 | 5 |
| --- | --- | --- | --- | --- | --- |
| Factor | 0.80 | 0.61 | 0.49 | 0.42 | 0.38 |

As long as a single layer remains on the drum, the cable has already lost 20% of its capacity.

## An example

An H07RN-F 3G2.5 cable, the size of our 16 A cables, is rated at 26 A unwound, in free air at 30 °C. Left in three layers on its drum, it can only carry 26 × 0.49 ≈ 12.7 A: less than the 16 A of the socket supplying it. An appliance drawing 16 A will not trip the 16 A circuit breaker, but it will heat this cable beyond its limit.

Manufacturers say so: the data sheet of a 25 m professional cable reel in 3G2.5 gives 1,000 W with the cable wound, against 3,500 W with the cable unwound. If the reel has a thermal cut-out and it trips, do not reset it: unwind the cable.

## Good practice

- Unwind the cable fully as soon as the load is significant, and lay it so that nobody trips over it.
- Do not leave it in a tight heap: it heats up for the same reason.
- Respect the power ratings given by the manufacturer on the reel.
- A hot drum or a smell of hot rubber: switch off, then unwind.

> A cable left on its drum cannot carry the current of its socket: unwind it before loading it.

Unwinding cables before loading them is part of our core business: [event power distribution](https://www.darkside-energy.com/en/power-distribution/).

Which socket and cable for your equipment? The calculator suggests them, with the voltage drop over the length.

[Open the electrical calculator](https://www.darkside-energy.com/en/electrical-calculator/#conversion)

[Why doesn't a generator like running almost unloaded?](https://www.darkside-energy.com/en/did-you-know/generator-light-load/)

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