Fixture screws for Luminaires

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Timotay

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san diego
What does the NEC say about the 32 threads per inch on mounting screws for fixtures? We have used a #8 philips head self tapping screw because it works very good for cleaning the hole from drywall and stucco. Also we have had a lot of trouble from stripping out the head when there's junk in the hole. Its like a #8-24 thread. I have seen several referances to the 32 per inch, but no NEC reference.

Any help.
Thanks
 
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GoldDigger

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Timotay said:
What does the NEC say about the 32 threads per inch on mounting screws for fixtures? We have used a #8 philips head self tapping screw because it works very good for cleaning the hole from drywall and stucco. Also we have had a lot of trouble from stripping out the head when there's junk in the hole. Its like a #8-24 thread. I have seen several referances to the 32 per inch, but no NEC reference.

Any help.
Thanks
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One indirect reference is in 250.8. It refers to engaging at least two full turns of thread when the screw serves a grounding or bonding function.
To engage two turns of 24 tpi requires a thicker piece of metal than engaging two turns of 32 tpi.
 
kwired

kwired

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a bonding jumper between the luminaire and box should eliminate any issues with not enough threads for bonding purposes.

Are you just cleaning the holes with this screw or using it to hang the luminaire? Outside of bonding issues it really comes down to is this strong enough to support the weight of the luminaire and can vary depending on the actual weight of the luminaire.
 
kwired

kwired

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mgookin said:
I always thought the 32tpi gave you greater holding strength for a ceiling fan subject to continuous vibration. No?
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True to an extent. There is more surface contact between surfaces involved, therefore most likely more potential friction to hold it.

A 10-32 screw and a 10-24 screw of the same material should have about same shear and tensile strength, but the 32tpi should be easier to gain more tightening torque and therefore should also be easier to overtighten to the point of breaking the screw.

There are fan boxes with 10-24 studs for fastening the fan - but they usually have some kind of locking nut that comes with the box also.
 
don_resqcapt19

don_resqcapt19

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If the box is already tapped for a 8-32, using a 8-24 self tapper will reduce the holding power of the screw.
 
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Timotay

Member
Location
san diego
Thanks All,
Really, the issue at hand is that these are plastic and or fiber boxes used in residential wiring like a Carlon brand and there is no tapped hole. they just have a hole sized for #8 screw with no threads for fixture mounting. Typically the hole is a little tight, and you end up stripping the Phillips head on the machine screw when driving it in. i just wanted to know if there was a clear code that described using a screw with machine threads for fixture installation. i am not sure why you would need to have a bonding issue here as it is a light fixture and is grounded to the system via fixture ground wire to Romex ground wire.
 
1793

1793

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Louisville, Kentucky
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One thing to keep in mind with plastic boxes in the ceiling for luminaires is the heat generated by the bulbs will cause the screw to heat up and melt the plastic and the screws can pull out due to the weight of the luminaire.
 
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Gregg Harris

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Location
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Electrical,HVAC, Technical Trainer
Timotay said:
What does the NEC say about the 32 threads per inch on mounting screws for fixtures? We have used a #8 philips head self tapping screw because it works very good for cleaning the hole from drywall and stucco. Also we have had a lot of trouble from stripping out the head when there's junk in the hole. Its like a #8-24 thread. I have seen several referances to the 32 per inch, but no NEC reference.

Any help.
Thanks
Click to expand...

Take a look at 314.27 through (E)
 
LEO2854

LEO2854

Esteemed Member
Location
Ma
Timotay said:
What does the NEC say about the 32 threads per inch on mounting screws for fixtures? We have used a #8 philips head self tapping screw because it works very good for cleaning the hole from drywall and stucco. Also we have had a lot of trouble from stripping out the head when there's junk in the hole. Its like a #8-24 thread. I have seen several referances to the 32 per inch, but no NEC reference.

Any help.
Thanks
Click to expand...
Welcome to the forum:)

It sounds like you're trying to ground the fixture,just use a 10/32 tap and a green ground screw and you should be fine.
 
jaylectricity

jaylectricity

Senior Member
Location
Massachusetts
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licensed journeyman electrician
LEO2854 said:
Welcome to the forum:)

It sounds like you're trying to ground the fixture,just use a 10/32 tap and a green ground screw and you should be fine.
Click to expand...

I'm sorry if I sound like a jerk but how does it sound like he's trying to ground a fixture when he explicitly mentioned mounting screws and plastic boxes?
 
kwired

kwired

Electron manager
Location
NE Nebraska
jaylectricity said:
I'm sorry if I sound like a jerk but how does it sound like he's trying to ground a fixture when he explicitly mentioned mounting screws and plastic boxes?
Click to expand...

Based on OP (which he quoted) his response may be credible, but he must not have read the later post that does mention that these are non metallic boxes.
 
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