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n0c7 08-26-2008 12:06 AM

Grounding light box
 
Theoretical grounding question. If a light box only has one grounding screw that has the incoming ground wrapped around it and the grounding wire is snipped after it was wrapped(can't pig tail), what is the best way to attach the ground wire from the light fixture?

1) Use the green grounding screw off of the fixture strap, as the strap is attached to the grounded box and is conductive

2) Insert a self tapping screw into the box and wrap the light fixture ground to it

3) Wrap the light fixture ground around the same screw that already has the incoming ground on it


I've read through my electrical code book and they only seem to care about bonding receptacles, nothing on light switches or fixtures. The diagrams all show only light fixtures without grounding wires.

Termite 08-26-2008 01:03 AM

Switches installed in plastic boxes must be grounded. Switches in metal grounded boxes don't have to be. Light fixtures and mounting hickeys should be grounded in accordance with their listings.

If I were in your situation, I'd unscrew the box ground, and would add some length to that ground wire with a Wago Walnut or a conventional wire nut. Then I'd re-attach to the box ground and continue on to the fixture ground wire.

220/221 08-26-2008 02:45 AM

I'd use a self tapper

n0c7 08-26-2008 02:27 PM

Alot of people I've talked to seem to like the self-tapper idea, but really, whats the difference between that and using the ground screw off of the mount strap?

Stubbie 08-26-2008 02:48 PM

I'm not sure I am completely following the question.... but if the equipment ground wire has been cut where it enters the box you would have no choice but to access the wiring and get a connection to the equipment ground wire in that supply cable. Otherwise all other grounding is pointless. The grounding screw on the light fixture hanging strap is fine so long as the fixture box is metal. Regardless.. the box must be bonded to the incoming equipment ground, if not and the incoming equipment ground remains cut then there will be no way for a breaker to trip on ground fault.

220/221 08-26-2008 05:14 PM

Quote:

if the equipment ground wire has been cut where it enters the box ......

It enters the box, hits the screw but there isn't enough to pigtal ar attach to. It's cut right at the g screw.




Quote:

whats the difference between that and using the ground screw off of the mount strap?
220/221....whatever it takes.

I would attach to the box because it is more permanent. The fixture strap may be removed someday when the fixture gets changed out.

As long as the strap is mounted tight to the box/mud ring you will have a good grounding path. If it is dangling or insulated by drywall/plaster...not so good.

n0c7 08-26-2008 05:43 PM

Quote:

Originally Posted by Stubbie (Post 152010)
I'm not sure I am completely following the question.... but if the equipment ground wire has been cut where it enters the box you would have no choice but to access the wiring and get a connection to the equipment ground wire in that supply cable. Otherwise all other grounding is pointless. The grounding screw on the light fixture hanging strap is fine so long as the fixture box is metal. Regardless.. the box must be bonded to the incoming equipment ground, if not and the incoming equipment ground remains cut then there will be no way for a breaker to trip on ground fault.

The grounding wire is there, just snipped after it wraps around the screw. Here is a diagram:

http://i62.photobucket.com/albums/h8.../grounding.jpg

Termite 08-26-2008 06:49 PM

You need to extend that wire. Take it off the screw and add a length of bare copper wire with a wire nut. Now wrap it around the screw and run it on up to the fixture's ground and nut them together. The fixture's ground should also be grounded to the mounting hickey or mounting plate.

That is the most bulletproof, safest way to do this.

n0c7 10-06-2008 03:38 PM

Funny thing, I was at the hardware store the other day looking at some fixture displays. They had a couple ripped off the wall and I happened to notice that they wired the incoming ground(14AWG solid) and the light fixture ground(18 AWG stranded) around the same screw.

InPhase277 10-06-2008 04:02 PM

Quote:

Originally Posted by n0c7 (Post 169000)
Funny thing, I was at the hardware store the other day looking at some fixture displays. They had a couple ripped off the wall and I happened to notice that they wired the incoming ground(14AWG solid) and the light fixture ground(18 AWG stranded) around the same screw.

Yeah, that's been done, but not properly. Here's the skinny. A self tapper, unless it has exceptionally fine threads, is a no-no for grounding. The reason being, a screw must engage two or more threads in the material in order to be used for bonding purposes. In the case that started the thread, you can either extend the original wire, as has been suggested, or you could drill a hole in the box and tap it to accept a machine thread, or if you have clamps built into the box that aren't being used, you can take the clamp out of it and use that tapped hole for another screw.

Once you have a ground, then I'd wrap that ground around the fixture strap, and attach the fixture ground to what's left with a wirenut.

n0c7 10-06-2008 04:20 PM

InPhase277,

The only thing I'm debating is why there would be an issue with wrapping both a 14 solid and an 18 stranded on the same screw would be an issue. It tightens down very well. I know that two solid 14's on a screw wouldn't work(well it can if you're careful but I would avoid it in this case). By doing this, you can avoid alot of extra work...


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