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This question already has an answer here:

I picked up an extension cord today, and I noticed that it had a different value for its wattage and its ampage, based on whether the cord is wound around its base or not.

enter image description here

(I apologize that the picture is blurry. In the left box it says, ”Max current wound - 4 Amps (960 Watts),” and in the right box it says, “WARNING - Max current unwound - 13 Amps (3120 Watts).”)

  1. Why does this make a difference? Assuming that the wire is properly insulated, shouldn’t it transfer the electricity just as efficiently wound as unwound?
  2. Is there a formula that I can use to calculate a cord’s wattage/amperage when only part of it is wound, or when all of it is wound, given its wattage/amperage when straight, how much of it is wound, and how tightly it’s wound, and maybe some other variable I can’t think of?

Obviously my question assumes that it has nothing to do with a liability concern or the like.

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marked as duplicate by Transistor, SamGibson, The Photon, Sparky256, pipe Nov 16 '18 at 12:08

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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The answer is "heat".

When an electrical cord is in free air, heat generated by power wasted in the cord's resistance is able to dissipate, keeping the temperature relatively lower. When the cord is spooled up, by contrast, the heat accumulates and the temperature rises higher, for the same level of wasted power.

I don't know if there is a formula for this, or if it is modelled in simulation, or if it's just measured.

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  • \$\begingroup\$ Is the issue that it’s in the casing and so the heat gets trapped, or is the issue that it’s wound up and piled on top of itself, so there’s less surface area by which the heat can dissipate? \$\endgroup\$ – DonielF Nov 15 '18 at 23:07
  • \$\begingroup\$ Well, anything that traps heat will have some effect. But I believe it's mostly the second one. \$\endgroup\$ – Glenn Willen Nov 15 '18 at 23:23
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A cable has resistance and so when a current is passed through it, it dissipates heat. When the cable is coiled up, this heat builds up as it cannot be transferred to surrounding air so well. If you overload this sort of extension cable, it can get hot enough to melt the insulation. I know because I did it once!

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