Screw weight capacities

[Frank Neri]

I am looking for documentation on the load capacity of a 6-32 screw. I maintain a production facility and the GC that guilt it is installing the finish in our Lobby area. I am refusing to let him install a 50# and 90# chandelier onto a 4-S box with a single gang P-Ring. he claims the screws are rated for 550#. When is topped laughing I let him know that he was wrong. just looking for documentation showing to provide.

 

[junkhound]

minor diameter = 0.099"m area = 0.0077 in sq.
softest cheapest 'steel' screw you can find = 20 ksi yield, so yield of 6-32 = 153 pounds.

breaking strength about twice that.

Now a NAS601 aircraft 6-32 screw is capable of about 1200#, in which case the limiting factor is the 4-s box.

So, think UL rating for standard boxes is 50 pounds for light fixtures, 30 pounds for fans.

What does the installation data for the 90# chandelier say. It likely has a 3/8" dia tube or rod or 1/4" bolts mounting?

Thus, the GC is correct for the 50# fixture, but likely wrong for the 90# fixture.

 

[electrofelon]

Why not at least use a round mud ring and thus use 8-32 screws?

 

[480sparky]

Why not at least use a round mud ring and thus use 8-32 screws?

Or a fan box and use 10-32s?

 

[LarryFine]

There's more than just the tensile strength of the screw; there's also the thread interface between the screw and the hole, especially in the straight pull-out direction.

A lot depends on the relative fit (machining tolerances) between the screw and the hole. Plus, the larger the screw, the greater the contact area. I'd want at least 8-32.

My first choice would be a fan box. Better yet, a chandelier lift.

 

[steve66]

Isn't there a specific code section for fixtures that weigh that much?

Something about not being supported by the jbox unless its rated for the weight?

 

[Frank Neri]

I found a Nema Bulletin saying that an 8-32 minimum should be used
Found a code in the NEC but it referred to bonding. I suggested to the GC that it should be a 3-O or 4-O box which would give it at least an 8-32. They are trying to backpedal because it is a completed job and they do not want to have to cut open the ceiling to make the changes.
Chandelier is a custom made fixture. The chandelier designer also needs to try and fit a transformer in the decorative housing to drop the voltage from 277 to 120.

 

[Frank Neri]

Thanks for all the advice. I feel better about saying NO now.

 

[growler]

Isn't there a specific code section for fixtures that weigh that much?

Something about not being supported by the jbox unless its rated for the weight?

2017 NEC Article 314.27 (2)Ceiling Outlets.

 

[growler]

I suggested to the GC that it should be a 3-O or 4-O box which would give it at least an 8-32.

The thing is the box will need to be marked by the manufacturer with the weight it can support. A fan box is probably the only one that will be marked and rated for more than 50 pounds. Other than that he would have to have independent support.

 

[zbang]

For your pleasure-
https://www.fastenal.com/en/84/load-calculator

Proof Load Tensil Strength For Grade 2, 5, & 8

Tensile Strenght: Proof Load Tensile Strength for Grade 2, 5, & 8,

www.almabolt.com

(the only go down to 1/4")

http://www.fasnetdirect.com/refguide/Machinescrewthreads.pdf

tells me that a steel (presumably generic metal) 6-32 machine screw has a tensile of 550#, so a working load would usually be 1/10th of that. (tensile strength of 8-32 is 850# and 10-24 is 1050#)

 

[retirede]

There's more than just the tensile strength of the screw; there's also the thread interface between the screw and the hole, especially in the straight pull-out direction.

A lot depends on the relative fit (machining tolerances) between the screw and the hole. Plus, the larger the screw, the greater the contact area. I'd want at least 8-32.

My first choice would be a fan box. Better yet, a chandelier lift.

As a rule of thumb, threads machined to ANSI standards will have a pullout strength equal or greater than the tensile strength of a grade 2 screw as long as the thread engagement is at least a great as the nominal diameter of the screw.

FWIW.....

 

[drcampbell]

I'm sure the data & calculations (above) about #6-32 screws are reasonable, but they're less than half the answer.
What about the screwholes? Non only are plaster rings made of mild steel, but the threads provide a generous loose fit and the material is only thick enough to provide one full thread.
How is the box secured to the building? How much weight will pull the box out of the ceiling?

You might improve matters slightly by carefully retapping the screwholes for a precisely-fitted #8-32 or #10-32, but the thinness of the material and the proximity to the edge still worry me. A pair of fully-threaded speednuts?

 

[growler]

How is the box secured to the building? How much weight will pull the box out of the ceiling?

What did they use? Tech screws or drywall screws?

 

[tom baker]

A device box has 6-32 screws and is intended for devices, IE receptacles. A device box can be used for a luminaire but the weight limit is 6 or 10 pounds (I can't find the section) Outlet boxes are rated to 50 pounds using 8/32s.
Would it be possible to sawzall off the existing box and put in a retrofit fan box?
And 90# should have independent support

 

[texie]

I found a Nema Bulletin saying that an 8-32 minimum should be used
Found a code in the NEC but it referred to bonding. I suggested to the GC that it should be a 3-O or 4-O box which would give it at least an 8-32. They are trying to backpedal because it is a completed job and they do not want to have to cut open the ceiling to make the changes.
Chandelier is a custom made fixture. The chandelier designer also needs to try and fit a transformer in the decorative housing to drop the voltage from 277 to 120.

Sounds like you have more issues than just the weight. Light fixtures must be listed per 410.6. And the 277 supply to a 120 fixture is going to be a difficult issue to resolve in a code compliant manner.

 

[synchro]

Chandelier is a custom made fixture. The chandelier designer also needs to try and fit a transformer in the decorative housing to drop the voltage from 277 to 120.

I think more details about how the base of the custom fixture is built is needed to make sure that support is properly applied. For example, you could have aircraft quality high tensile screws but if the support they provide is not distributed properly at the fixture base then they could pull out of the base.
I'm bringing this up because the screws themselves, electrical box, etc. may not be the only limitations, and the problem should be viewed in a larger context especially since this is a custom fixture.

Sounds like you have more issues than just the weight. Light fixtures must be listed per 410.6. And the 277 supply to a 120 fixture is going to be a difficult issue to resolve in a code compliant manner.

A field evaluation and certification would be necessary unless otherwise allowed by the AHJ.