Power Cord & Platic Cable Covers


I have some of the long plastic cable covers that adhere to the wall/baseboard to conceal cables. I read on one of the (many) audio sites that power cords should not be in contact with plastic. Any truth to this?
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I have my subwoofer cable under a vinal cover in front of a doorway,have tried it with and without.No difference.As Al said,running two in the same cover could be detrimental.
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all my cables are made just from copper,cotton,paper and air(spacers)....sounds the best for me....