Almost a guess, in the absence of ANY context!! However, cables are "rated" according to how much current they can carry, and are "de-rated" as a safety factor.

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Note added at 58 mins (2009-08-03 15:37:10 GMT)
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Thanks for clarification above. I should have added that the de-rating factor also depends on ambient temperature, what the medium is where the cables are (eg in water, oil, air, concrete, etc), which determines the heat transfer coefficient and hence the rate at which internally generated heat can dissipate.