300.11(A)Support wires and associated fittings that provide secure support and that are installed in addition to the ceiling grid support wires shall be permitted as the sole support. Where independent support wires are used, they shall be secured at both ends. Cables and raceways shall not be supported by ceiling grids.
Frodo an old work box in the tile will not work. It will cause the ceiling tile to sag and will not even be secure enough to mount a fixture. There are bars for evey kind of recessed light and ceiling box. These allow for proper installation. Voltron, here in Mass (not sure if its NEC too) the ceiling holds the fixtures however if they contain a ballast of some type they must be chained to the building structure. This way they remain hanging if the ceiling comes down.
I don't think Caddy makes a T-Bar hanger for 4x4 tiles. (Or do they?) These fixtures must be independently supported, but the support fastened to the grid. We use support wires fastened to the structure above, and EMT screwed to the housing/box and tie wired to the grid to achieve this on a 4x4. (But then again I'm in SoCA=earthquake) You might want to check the Caddy IDS hanger if this is a surface mounted fixture. Don't depend on the tile for any support at all. It wouldn't hurt to call your AHJ and have them tell you what they want
[This message has been edited by electure (edited 04-10-2002).]