I'm installing 8 ft plug in fixtures in a large retail commercial building (28 per row). Is there any spec as to the spacing of supports?

Best answer may be:
Check mfg specs
or
Apply common sense.
8' fluor channel fixtures are surface mounted at 3 points; suspended at 2 points (each fixture)
It's a 'no definite' answer as the layout is vague by your description above.

The only rule I know of is 110.3 (B)...well that and 110.12

With hanging 8' strip lights I usually support the first one at both ends and the far end of each succeeding one.

Here is a job I finished last year, almost 600 fixtures.



That said I recently hung some 8' custom linear lights in offices and that jobs foreman had me do the same as I described above. IMO the fixtures being cast where to week for that and should have had three supports a piece for each fixture as they had been designed.

I guess what I am saying is follow Johns advice.....apply common sense.

Bob:
NICE....
BTW, quite a while back...like you said, 2 on the first; chase nip/ln thru end ko's, one on each additional.
I didn't want to write a novel on the many ways....
Now the 'old' 8' INdustrial type with the full reflector shade...nice old heavy mag ballast, and a housing made from real live steel, not recycled Pepsi cans (man's fixture), that was a definite 'two chainer'.

Back to my serious mode....any shot to get good old common sense into the NEC??

Stay safe

Thanks for the input everyone. I agree on the 2 on first fixture then one a foot or so from the end of all the rest; however, bar joists don't fit that senario. I guess my actual question was can you miss a hanger on a fixture or must you have a support on every fixture? Checked manufactures specs, told me nothing. The store is a Wal-Mart Superstore.....many many fixtures.

Thanks for the pics...nice job

Bar joist locations not 'matching' a place for a support are solved by bridging b'twn bar joists with strut, or another suitable alternate.
What about the plans/specs? Most, if not all will provide a detail on the REQUIRED method of installation by the Architect/Engineer of record.

Your 'client' is not a small operation, so you should have details available, or submit an RFI.

I would not skip a support, as John mentioned you need to come up with a way to get a support where you need it.

In my picture above out of over 600 supports only a few actually land on a truss. If you notice the fixtures are diagnal...a major pain in the rear.

Anyway we used these from Caddy...





To install most of them you use this tool...



But before we got started I sent an RFI to the engineer asking if this method of supporting from the deck was OK. He approved it and away we went.

I can tell you these are very strong.

BTW, also the fixtures in the above photos are joined with factory couplers with 4 tek screws added per coupling to ensure the fixture could not pull apart and drop.

How does this grab the deck?