What the NEC says about extension cords?

Extension cords aren’t permanent wiring, so the National Electrical Code doesn’t apply to them per se, but it does lay out some guidelines. The NEC would prefer that you use a close-by receptacle, but that’s a little unrealistic if you’re running an electric lawnmower, for example. The code recognizes that extension cords are meant only for temporary use for portable loads that aren’t fixed to one specific location.
What about a table lamp and clock radio by your bedside connected to a small-gauge extension cord because the only receptacle on the wall is beyond the length of their appliance cords? The NEC would like you to install a new receptacle, but sometimes this isn’t practical. If you’re renting, you would have to convince your landlord/landlady to accept this additional expense. For such a small load, the use of an extension cord on a regular basis isn’t a big deal. Problems occur, however, when extension cords are used to run large loads on a more or less permanent basis. Small extension cords become a bigger problem when they are installed under a rug or are in any way covered over so they retain heat.