The electric code reads:
NEC ARTICLE 400 Flexible Cords and Cables General 400.1 Scope.
This article covers general requirements, applications, and construction specifications for flexible cords and flexible cables.
400.8 Uses Not Permitted.
Flexible cords and cables shall not be used for the following:
(1)As a substitute for the fixed wiring of a structure
(2)Where run through holes in walls, structural ceilings, suspended ceilings, dropped ceilings, or floors
(3)Where run through doorways, windows, or similar openings
(4)Where attached to building surfaces
Exception: Flexible cord and cable shall be permitted to be attached to building surfaces in accordance with the provisions of 368.8.
(5)Where concealed by walls, floors, or ceilings or located above suspended or dropped ceilings
This is becoming problems with people installing Tv's on their walls and just running the power cords behind them in the wall along with their AV cables. Now the AV cables are aloud just not the power cable. That's why the use power bridges when doing it now..
This is what a power bridge is:
http://www.powerbridgesolution.com/home.html