Ted - I'm only an electrician part of the time, so I can't quote the exact code for you. The issue is heat build-up if a portion of the cord is still on the reel during use. We have all grabbed the end of a plug and found it warm/hot after heavy use, be it a power tool or vacuum cleaner. Now picture heat build up in the wire, while it is confined on the reel, next thing you know the whole reel is on fire. Derating it would limit the amperage demand, thereby reducing the chance for heat retention.
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