The NEC does not prohibit a receptacle outlet from being installed above a
suspended ceiling, typically for maintenance purposes to plug in hand held portable tools. However, it should be noted that where the space above the suspended ceiling is considered other spaces for environmental air as specified in 300.22(C), the wiring methods and equipment must be installed as required by that section.
Therefore, even in a residential application where NM cable typically would be permitted, because it is a space used for environmental air 300.22(C) prohibits the use of NM cable.
Section 400.8(5) prohibits flexible cords and cables from being installed above suspended or dropped ceilings whether used for return air or not for any type of premises or application. These rules limit the use of materials that would contribute smoke and products of combustion during a fire in an area that handles environmental air.
Additionally, cords are prohibited from being installed above a suspended ceiling in all cases to comply with 400.8(5) and with 400.8(1) which prohibits the use of flexible cords and cables as a substitute for the fixed wiring of a structure. Restrictions on the use of flexible cords through and above suspended ceilings were first introduced in the 1999 NEC. Part of the reason for this revision is in concert with the language that has been in the NEC for several editions in 410.30(C) then 410.62(C) that permits cord-and-plug connection of electric-discharge luminaires only when the flexible cord is visible for its entire length. Any cord installed through and or above a suspended ceiling is not visible for its entire length. Flexible cord located out of site such as behind suspended ceiling panels is subject to damage or fatigue which would notbe readily detected. Some suspended ceilings are made of metal thereby increasing the potential for damage.