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extension cord on a reel

Discussion in 'Electronic Basics' started by John Shamblin, Oct 31, 2004.

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  1. I have a 50 ft. 3 wire extension cord for use with an electrical leaf blower
    or weed eater outside. I normally keep the cord wound around a plastic reel
    whose inside diameter is 6 inches and there are about 50 turns of the cord
    on the reel when full. A caution on the side of the reel states the "cord
    should be removed from the reel before use". I assumed that was because of
    the inductance (and impedance of the 60 Hz current flowing through it) when
    wound on the reel would impede the current flow in such a coil. Is this
    truly significant or are there safety factors to be considered?
  2. Gareth

    Gareth Guest

    Another factor could be heat dissipation.

    The cable will have some small, but non zero, resistance. This will
    cause some of the electrical power to be converted to heat in the cable.
    If the cable is coiled up the heat cannot escape as easily as if the
    cable were uncoiled. Therefore the temperature rise in the coiled cable
    will be higher for the same current flow. The coiled cable could
    therefore overheat and catch fire even though the current was less than
    the rated current for an uncoiled cable.


  3. John Larkin

    John Larkin Guest

    The inductance will be the same, wound or not. They're probably
    concerned with cooling.

  4. Rich Grise

    Rich Grise Guest

    It's probably a cheap cord, and it needs to be exposed to air along
    its whole length, or it will overheat from I^2*R losses.

    The amount of inductance in a coil like that at 60 Hz is negligible,
    and it's a bifilar winding, wired parallel opposing, so will cancel
    out anyway. Boy, that really sounds like I know what I'm talking about,
    doesn't it? ;-)

  5. Joe

    Joe Guest

    A buddy of mine had one of these extension cords powering a window air
    conditioner while coiled up. By the end of the summer the plastic
    reel had melted to the carpet.
  6. But the issue isn't first that it was rolled up. One doesn't want to
    use an extension cord without very careful consideration with something that
    uses power like an air conditioner. And the fact that he had at least
    some of the cord rolled up means that it was a lousy choice, since he
    could have used a much shorter extension cord.

  7. Don Kelly

    Don Kelly Guest

    Overheating- the extension cord is (thermally) rated on the basis that it is
    extended and not enclosed. For example, codes would not allow the use of
    extension cords in a wall in place of proper wiring. The reel is handy but,
    in my mind, its usefulness for storage and winding is outweighed by the fact
    that one should completely unwind the cord when using it.
    As Rich has said, it's not the inductance. If your blower is like mine, you
    should really have a cord rated at 12A (unwound).
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