A cable reel can have a much lower permitted load while coiled than when fully unwound. The difference mainly reflects how the cable sheds heat. Read both ratings on the exact reel and follow its unwinding instructions before powering a substantial load.
Current heats the conductors along the entire cable
Real copper conductors have resistance, so current produces heat according to I²R. The cable remaining on the reel is still part of the electrical path even if the appliance is only a short distance away. Pulling out a few feet does not remove the unused-looking length from the circuit. For an illustrative total conductor resistance of 0.4Ω, 10A produces 40W of cable heating. This example explains the mechanism; the actual resistance and allowable load depend on the particular cord and installation.
Coiled layers make it harder for that heat to escape
Cable packed against other warm layers has less effective cooling than a fully deployed length with suitable exposure to surrounding air. Heat can accumulate inside the reel where it is difficult to see or feel directly. The key concern in an ordinary multi-conductor extension reel is resistive heating and restricted heat dissipation, not treating it as an ideal single-wire electromagnet. Covering the reel, placing it near a heat source, or using it in a hot environment can worsen the thermal conditions further.
The two label values describe different permitted configurations
A manufacturer may give one maximum power or current for the fully coiled state and another for the fully uncoiled state. Those figures belong to that specific product and voltage; they are not universal limits for every reel. Brennenstuhl explicitly directs users to the product label and advises complete unwinding for high loads. If the label is damaged or unreadable, obtain the correct instructions before use. A partially unwound reel should not be assumed to qualify for the fully unwound rating unless the manufacturer says so.
Thermal protection is a backup rather than a routine control
Some reels include a thermal cutout that interrupts power when the assembly gets too hot. Repeated trips mean the operating conditions need attention, not that the reel is successfully regulating a normal workload. Disconnect and investigate according to the instructions, allowing the equipment to cool where required. Do not bypass the protection or keep resetting it to finish a task. Melting, discoloration, damaged insulation, or a burning smell require taking the reel out of service. A breaker upstream may not detect every localized overheating condition.
Unwinding does not remove the other extension-cord limits
The appliance instructions may prohibit extension cords or specify particular cable sizes and lengths. The reel also needs appropriate environmental protection and suitable connectors for its location. Avoid pinching cables, creating trip hazards, or leaving connections where water can collect. Check the combined demand of all connected equipment against the applicable rating. Do not daisy-chain reels to reach a distant load. If the required setup cannot meet the equipment instructions safely, use a properly located supply or have an appropriate installation arranged instead of stretching a temporary connection beyond its purpose.
What to check before you act
- Read both coiled and fully unwound ratings.
- Follow the exact reel’s deployment instructions.
- Add the demand of every connected appliance.
- Stop using a reel that trips repeatedly or shows heat damage.
Common questions
Does only the cable pulled off the reel carry current?
No. The full connected conductor length remains in the circuit and can generate heat.
Does fully unwinding allow any appliance to be connected?
No. The reel, supply, environment, and appliance instructions still impose limits.
The practical takeaway
The coiled rating exists because cooling changes with the cable arrangement. Deploy the reel as instructed and stay within the complete setup’s limits rather than relying on its largest printed number.
References and further reading
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
