8.) The improper supporting and securing of branch-circuit wiring methods, located above suspended ceilings, that supply light fixtures or equipment.
The most common problem with this type of installation is the mis-application of the flexibility allowances for the particular wiring method that is being used. Typically, the branch-circuit wiring methods most commonly used in installations above suspended ceilings are NM, AC or MC Cables. Section 333-7 (b) (3), allows AC cable to run unsupported in lengths up to 6' from an outlet for connections, located within an accessible ceiling, to light fixtures or equipment. Article 334-10 (b), has similar language for MC cable. In the 1996 NEC, there was no language for this type of flexibility for NM Cable in Article 336. However, the 1999 NEC, Section 336-18, Exception # 3 will allow for NM Cable to run up to 4-1/2' unsupported between an outlet in an accessible ceiling and light fixtures or equipment.
Example: a recessed fixture has been installed in a suspended ceiling and it has been supplied by either MC, NM or AC cable. The cable has been run from the wall switch to the first fixture or from fixture to fixture. The cable runs parallel with a metal bar joist and is properly supported. The cable then runs from the bar joist, unsupported, directly to the fixture. This cable must be installed in accordance with the supporting and securing methods described in the applicable Article. The flexibility methods described in the exceptions above would not apply because the wiring method is not supplied from an outlet within the accessible ceiling. For example, a 12/2 MC Cable that runs directly from the bar joist to the fixture would have to be supported every 6' and within 12" of the fixture as required by Section 334-10 (a).