I think that you've mixed up a few things.
Of course, that's no surprise; the NEC does not directly address the issue, despite there being some imcomplete references. The UL "White Book" does a better job.
Any securely attached metal box with a metal plaster ring - of any style - is good for supporting fixtures to 50 pounds.
The 6 pound limit you refer to is for 'old work' boxes, where the box is supported by the drywall, rather than by being attached to a framing member.
Reno,
That's what I was hoping to here, but would really like to have something in writing just in case it ever comes up as an issue on a job.
I have searched the 2008 UL White Book up and down and can only find the following statements posted below regarding this.
I suppose it could be argued that since The UL White Book only references device boxes and device plaster rings, not ceiling boxes and round plaster rings, then by default they are allowed to be used in ceiling applications for fixture support.
It just seems that there should be some entry that directly addresses this to prevent any possible misinterpretation of the matter.
Metallic Outlet Boxes (QCIT)
FIXTURE SUPPORT
A box, with or without a bracket or bar hanger, intended for support of a fixture weighing 50 lbs or less is marked "FOR FIXTURE SUPPORT" on the carton to indicate that the box is intended for fixture support. A box, with or without a bracket or bar hanger, intended for support of a fixture weighing more than 50 lbs is marked with the weight of the fixture to be supported.
Metallic device boxes and device plaster rings have not been investigated for support of a ceiling fixture unless marked for use in ceilings, walls, and with the weight of the product to be supported. Metallic device boxes or metallic device boxes intended to be installed in an existing structure have been investigated for the support of fixtures, smoke detectors and carbon monoxide detectors weighing not more than 6 lbs.
Nonmetallic Outlet Boxes (QCMZ)
FIXTURE SUPPORT
A nonmetallic box, with or without a bracket or bar hanger, intended for support of a fixture weighing 50 lbs or less is marked "FOR FIXTURE SUPPORT" on the carton. A nonmetallic box, with or without a bracket or bar hanger, intended for support of a fixture weighing more than 50 lbs. is marked with the weight of the fixture to be supported.
Nonmetallic boxes and device plaster rings have not been investigated for support of a
ceiling fixture unless marked for use in ceilings, walls, and with the weight of the product to be supported. Nonmetallic device boxes or nonmetallic device boxes intended to be installed in an existing structure have been investigated for the support of fixtures, smoke detectors and carbon monoxide detectors weighing not more than 6 lbs.
TKX
