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.