See the Definition of a "Plenum" in Article 100; and then see Section 300-22, and Articles 640; 800; and 820 in the 1999 NEC.
For those who ask these types of questions on this BB help is always available with specific references to the NEC.
If you don't have a code book you can order one here from the ECN Bookstore.
In the end the rules you really need are in Section 300-22(c)and in each Article that includes references to this section.
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