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I have some existing switch and outlet boxes installed in walls dating from the 1940's to the 1960's. The newer ones have threaded holes in the back of the box for a ground wire. Older ones have a hole which is not threaded -- but easilly threaded with a 10-32 tap. Very old boxes sometimes don't have any appropriate holes.

If I am running a new circuit (requiring the installation of a ground to the box) to one of the boxes with no ground hole, is it legal to drill a new hole and tap for the 10-32 ground screw or should I use a different method?

How about re-using an existing 10-32 threaded hole originally intended for a wire clamp or other device as a grounding point if such a point exists.

Also, what about the use of spring-loaded grounding clips which pinched the wire to the side of the box and which were used in the 1960's and 1970's. I have a partial box of these left over from an electrician's estate. Can these still be used? I have run across a few of these from time to time in use and wondered if the spring became weak and would let the ground wire eventually slip away.
 

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You can remove an unused clamp and install a 10-32, you can drill and tap or use the ground clips.

I do not like the ground clips on the older painted boxes.
 

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Not an electrican, but I have seen these types of boxes in my home when doing remodeling and was in the same situation myself. Before I'd go through the trouble of drilling, tapping, and fishing wire into in impossible to reach hole I'd ask myself this. Does the box has the appropriate volume to meet box fill requirements? If it does then march on and proceed with plan. If not or if it's questionable you might be just as far ahead to replace the box. I imagine you have plaster not drywall so I know you don't want to mess with that more than you need to. I've used these in the past with good results. http://www.arlcatalog.com/Fan_Specialty/ONE-BOX Non-Metallic Outlet Boxes.htm you can't find them at your big box stores though. You would have to get these from your local electrical supply or maybe a mom and pop shop might have them. On these boxes they have that plastic pull strain relief. Because of that I find they are hard to use particularly when using in an old work situation (it seems you always pull too much through the strain relief and then gotta fight to get some slack back through it so you can get the box mounted flush. So I usually just install a regular metal strain relief and you tighten the screws on. I just install it upside down so I can tighten the wire once I get the box in place. There is another box like it called a smart box. http://www.smartboxinc.com/ This fellow uses the typical tabs that most old work boxes use. Best of luck however you are gonna tackle it.
 
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