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  1. #1
    Join Date
    Aug 2012
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    Question Threaded hole for grounding screw - upgrading from 2-prong to 3-prong outlet

    Hi, folks.

    I'm upgrading an outlet in my living room from 2-prong to 3-prong. The box has a number of holes in it, but none of them appear to be threaded, and none appear to be the right size (I was told that the grounding screw will be an 8-32, and the holes are much bigger than that). The box is definitely a ground - I tested for that.

    Any advice on how to proceed?

    Are grounding screws really 8-32s? What size are they?

  2. #2
    Join Date
    Jul 2010
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    SoCal
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    Default Re: Threaded hole for grounding screw - upgrading from 2-prong to 3-prong outlet

    The ground wire must go from the receptacle to the ground in the panel, not from a box to the panel.

  3. #3
    Join Date
    Jan 2011
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    Columbiana, Alabama
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    Default Re: Threaded hole for grounding screw - upgrading from 2-prong to 3-prong outlet

    Some jurisdictions mandate we must not only ground the receptacle but also the metal box.

    The 8-32 screw means it is a #8 screw having 32 threads to the inch and Code does not require it to be green. It can be self tapping only if the box manufacturer supplies it.

    Since tapping an old metal box can get to be a chore for a Home Owner, why not use a grounding clip, available at Home Depot?
    Good Luck from Columbiana, Alabama
    Maurice Turgeon, Hidden Content

  4. #4
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    Aug 2012
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    Smile Re: Threaded hole for grounding screw - upgrading from 2-prong to 3-prong outlet

    Quote Originally Posted by The Semi-Retired Electric View Post
    Since tapping an old metal box can get to be a chore for a Home Owner, why not use a grounding clip, available at Home Depot?
    Hi Maurice,

    Thanks for that reply. I know what screw designations mean. I was wondering if the pre-threaded hole that supposedly exists really is for an 8-32 screw, or if it was for some other size screw. The holes I see are larger than a number 8.

    I'm not familiar with grounding clips. I'll look into that.

  5. #5
    Join Date
    Jan 2011
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    Columbiana, Alabama
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    727

    Default Re: Threaded hole for grounding screw - upgrading from 2-prong to 3-prong outlet

    Quote Originally Posted by lj3141 View Post
    Hi Maurice,

    Thanks for that reply. I know what screw designations mean. I was wondering if the pre-threaded hole that supposedly exists really is for an 8-32 screw, or if it was for some other size screw. The holes I see are larger than a number 8.

    I'm not familiar with grounding clips. I'll look into that.
    Yes the screws are actually what you would expect. The holes in real old metal boxes may be larger than what's needed for a #8 screw but if you have the proper dril & tap the Code allows any size (#8 or larger) as long as two full threads grip the box. Code also allows a #8 or larger bolt and nut.
    Good Luck from Columbiana, Alabama
    Maurice Turgeon, Hidden Content

  6. #6
    Join Date
    Aug 2011
    Location
    Northern Indiana
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    70

    Default Re: Threaded hole for grounding screw - upgrading from 2-prong to 3-prong outlet

    Quote Originally Posted by lj3141 View Post
    Hi Maurice,

    Thanks for that reply. I know what screw designations mean. I was wondering if the pre-threaded hole that supposedly exists really is for an 8-32 screw, or if it was for some other size screw. The holes I see are larger than a number 8.

    I'm not familiar with grounding clips. I'll look into that.
    Standard grounding screws are 10-32.

  7. #7
    Join Date
    Jan 2011
    Location
    Columbiana, Alabama
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    727

    Default Re: Threaded hole for grounding screw - upgrading from 2-prong to 3-prong outlet

    True brrichter, the standard junction box is drilled & tapped for 10-32. But the Code does not specify a size and they’re not required to be listed, different size (smaller and larger) bolts are often supplied in listed equipment:


    250.8: Revised to include specific reference to grounding electrode conductors and to delete reference to grounding conductor.
    (A) Permitted Methods. Equipment grounding conductors, grounding electrode conductors, and bonding jumpers shall be connected by one of the following means: See related ROP UL Now states one or more of the following methods.
    (1) Listed pressure connectors
    (2) Terminal bars
    (3) Pressure connectors listed as grounding and bonding equipment
    (4) Exothermic welding process
    (5) Machine screw-type fasteners that engage not less than two threads or are secured with a nut
    (6) Thread-forming machine screws that engage not less than two threads in the enclosure
    (7) Connections that are part of a listed assembly
    (8) Other listed means
    Good Luck from Columbiana, Alabama
    Maurice Turgeon, Hidden Content

  8. #8
    Join Date
    Aug 2011
    Location
    Northern Indiana
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    70

    Default Re: Threaded hole for grounding screw - upgrading from 2-prong to 3-prong outlet

    Quote Originally Posted by The Semi-Retired Electric View Post
    True brrichter, the standard junction box is drilled & tapped for 10-32. But the Code does not specify a size and they’re not required to be listed, different size (smaller and larger) bolts are often supplied in listed equipment:


    250.8: Revised to include specific reference to grounding electrode conductors and to delete reference to grounding conductor.
    (A) Permitted Methods. Equipment grounding conductors, grounding electrode conductors, and bonding jumpers shall be connected by one of the following means: See related ROP UL Now states one or more of the following methods.
    (1) Listed pressure connectors
    (2) Terminal bars
    (3) Pressure connectors listed as grounding and bonding equipment
    (4) Exothermic welding process
    (5) Machine screw-type fasteners that engage not less than two threads or are secured with a nut
    (6) Thread-forming machine screws that engage not less than two threads in the enclosure
    (7) Connections that are part of a listed assembly
    (8) Other listed means
    No argument here.

  9. #9
    Join Date
    Aug 2012
    Location
    NY, USA
    Posts
    18

    Default Re: Threaded hole for grounding screw - upgrading from 2-prong to 3-prong outlet

    Tried taping with 12/24, but this is a bit too big. I have been using 9/16" self-tapping sheet metal screws for bonding wires, but then I read that this is against the NEC. I'm all for sticking with the code.

  10. #10
    Join Date
    Jan 2011
    Location
    Columbiana, Alabama
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    727

    Default Re: Threaded hole for grounding screw - upgrading from 2-prong to 3-prong outlet

    Quote Originally Posted by Paula Mera View Post
    Tried taping with 12/24, but this is a bit too big. I have been using 9/16" self-tapping sheet metal screws for bonding wires, but then I read that this is against the NEC. I'm all for sticking with the code.
    True. maybe a grounding clip would be faster and they only cost a dime.
    Good Luck from Columbiana, Alabama
    Maurice Turgeon, Hidden Content

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