Does the NEC require clips on lay-in light fixtures?

Yes. Section 410.36(B) requires luminaires supported by suspended ceiling framing members to be securely fastened by bolts, screws, rivets, or listed clips identified for that ceiling framing member and luminaire.

Does the NEC require four support wires at the corners of a 2x4 troffer?

Not directly. The four independent corner wires come from the building code and seismic standards such as ASTM C636 and the IBC, plus many local amendments, rather than the NEC.

Can I support an MC whip from a ceiling grid support wire?

No. Section 300.11(A) prohibits using ceiling assembly support wires to support raceways, cables, or boxes. Independent support wires secured at both ends and distinguishable from the ceiling wires are permitted under 300.11(B).

How do 2 by 4 lay-in fixtures have to be supported in a T-bar ceiling?

Section 410.36(B) is the controlling rule. Framing members of suspended ceiling systems used to support luminaires shall be securely fastened to each other and shall be securely attached to the building structure at appropriate intervals. Luminaires shall be securely fastened to the ceiling framing member by mechanical means such as bolts, screws, or rivets, or by listed clips identified for use with the type of ceiling framing member and luminaire involved. Simply dropping the fixture into the grid and walking away does not satisfy that section.

The two-part obligation

First, the grid itself has to be capable. The grid members must be securely fastened to each other and to the building structure. In practice that is the ceiling contractor's work, but 410.36(B) makes the luminaire installer responsible for verifying it before hanging weight on it.

Second, the luminaire has to be mechanically attached to the grid. The common field methods are two or four listed grid clips, sometimes called earthquake clips or hold-down clips, screwed to the fixture housing and snapped over the tee. Screws through the fixture flange into the tee work as well. The key word in 410.36(B) is listed and identified, so a bent piece of strap or a tie wire around the tee does not qualify as a clip.

Where the four independent support wires come from

Crews often say the code requires four slack wires at the fixture corners. That requirement is generally from the building code and seismic standards, ASTM C635 and C636 and the IBC seismic provisions, along with many local amendments, not from the NEC itself. In Seismic Design Categories C through F the building code typically requires independent support wires at each corner of a lay-in luminaire, sized and slack-tied per the standard. Check your local amendment before you argue with the inspector, because in a lot of jurisdictions those wires are absolutely required.

Do not hang the wiring on the ceiling wires

Section 300.11(A) is where most inspection failures come from. It requires raceways, cable assemblies, boxes, cabinets, and fittings to be securely fastened in place, and it specifically states that support wires used for supporting the ceiling assembly shall not be used to support raceways, cables, or boxes. Section 300.11(B) then permits independent support wires that are secured at both ends to be used, provided they are distinguishable by color, tagging, or other effective means from the ceiling support wires. That is why you see whip support wires painted or taped a different color. Section 300.11(C) allows the fixture whip itself, 6 feet or less of flexible cable or raceway, to be run without independent support in an accessible ceiling.

Field takeaway

Clip or screw every lay-in fixture to the grid with listed hardware per 410.36(B), verify the grid is actually anchored to structure, add the four independent corner wires if your seismic category or local amendment calls for them, and never tie an MC whip to a ceiling support wire.