Home>Discussions>ELECTRICAL & LIGHTING>Threaded hole for grounding screw - upgrading from 2-prong to 3-prong outlet
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lj3141
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?

dj1
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.

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?

lj3141
Re: Threaded hole for grounding screw - upgrading from 2-prong to 3-prong outlet
The Semi-Retired Electric wrote:

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.

Re: Threaded hole for grounding screw - upgrading from 2-prong to 3-prong outlet
lj3141 wrote:

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.

Brad
Re: Threaded hole for grounding screw - upgrading from 2-prong to 3-prong outlet
lj3141 wrote:

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.

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

Brad
Re: Threaded hole for grounding screw - upgrading from 2-prong to 3-prong outlet
The Semi-Retired Electric wrote:

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.

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.

Re: Threaded hole for grounding screw - upgrading from 2-prong to 3-prong outlet
Paula Mera wrote:

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.

Fencepost
Re: Threaded hole for grounding screw - upgrading from 2-prong to 3-prong outlet
dj1 wrote:

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

Something you may be missing here... it sounds like in the OP's case, there is no ground wire in the box; ground is provided by the conduit. So the ground screw on the receptacle needs to be bonded to the box.

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