Here, on some larger jobs the means of support is spelled out by the engineer as part of the Building Code requirements.
In every day practice, on the smaller jobs, the requirement varies a little from jurisdiction to jurisdiction with most requiring 1 independant support tied to corners of 2x2 and smaller fixtrures and 1 independant support on each end of 2x4 fixtures.
Often times the E/C will contract out the anchoring and support wires to the ceiling installer.
Take a look at 300.11 for some help.
Here is a typical requirement from specs:
V. LIGHT FIXTURE SUPPORTS
[FONT=Arial,Arial][FONT=Arial,Arial]– Due to seismic conditions, only "Heavy-Duty" ceiling systems may be used.
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[FONT=Arial,Arial]All light fixtures shall be attached (with approved screws or clips) to the suspended ceiling grid systems at two opposing sides.
B. Wires required to support the fixture:
1. Fixtures less than 10 lbs: One #12 gage hanger wire from the fixture housing to the structure above. This wire may be slack.
2. Fixtures between 11 to 55 lbs: Two #12 gage hanger wires connected from the fixture housing to the structure above. These wires may be slack.
3. Fixtures 56 lbs or more shall be supported directly from the structure above without using the ceiling suspension system for direct support.
4. Pendant Hung Fixtures shall be supported with # 9 gage wire without using the ceiling suspension system for direct support
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