I am talking about the extension cord of length 10 meter or more as follows.

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The clerk said that it is better to unroll it when using. So what is the difference? I know for alternating current there will be inductive impedance for the wound cord. How to quantitatively calculate the difference? Empirical approach is also welcome!


closed as off-topic by David Z Aug 21 '18 at 10:09

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The reason it is better to unroll the cable is because it improves its ability to dissipate heat, which could be important for heavy loads, i.e., when the cable could potentially become hot.

The role of the inductance here is minimal, since the current in the cable is flowing in both directions and the net current is zero.

  • $\begingroup$ Does it means that the unrolled cords have greater resistance than the rolled ones? $\endgroup$ – Friendly Ghost Aug 20 '18 at 15:57
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    $\begingroup$ @FriendlyGhost The resistance and thermal power, $I^2R$, will be, initially, the same for both rolled and unrolled cords, but the unrolled cord has more contact with open air and will cool (dissipate the heat into the air) faster. If the rolled cord got hot, its resistance would increase, which could affect the current, etc., but that is beyond the point. $\endgroup$ – V.F. Aug 20 '18 at 16:28
  • $\begingroup$ This is the correct answer. Many cords have a thermal cut-out for the same reason. $\endgroup$ – Ben Aug 20 '18 at 18:49
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    $\begingroup$ I have a 100 foot outdoor extension cord that is housed in an enclosed reel. The design is such that the entire cord must be unreeled (pulling out the female plug end) until a door pops open to reveal the male plug end. The cord cannot be rolled up again until the male end is unplugged from an outlet and replaced in its little compartment with its door shut. They really don't want current passing through the coiled cord $\endgroup$ – DJohnM Aug 20 '18 at 21:54

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