At the moment, I have a receptacle that has no ground but is using three prongs. I plan on fixing this by installing a ground screw in the electrical box and routing a copper wire from that to the receptacle. This would probably only be a few inches long. The box itself is grounded already as it's metal.
Do I need some kind of permit to legally do this? I know about a code that says something along the lines of needing a permit to create or extend a wire. Local code also says permit is needed to install/replace/repair electrical wiring with certain exceptions (such as replacing receptacles/switches)
Best Answer
Is the box grounded?
Just because it's metal doesn't mean there's a valid grounding path back to the service panel. They often used metal boxes simply because they're more durable and better able to contain thermal energy from a short.
If the box isn't grounded, don't install a 3-prong plug, unless it's a GFCI with "No equipment ground" label.
You don't need a permit to swap a receptacle
And attaching the ground wire is part of replacing a receptacle. No big.
If that box is grounded, it may have a single hole tapped for #10-32. They make special ground screws for that (with or without pigtails), or any 10-32 screw will do. You can tap your own hole but it must be #6 or larger and -32 or finer thread pitch, in order to engage enough threads. Using a sheet metal screw is right out.
There's a chance the receptacle is already grounded through the yoke.
The yoke is the metal outer frame, that the mounting screws go through. If the box is grounded, AND the yoke is run down hard against the steel box (with no plastic bits in there), that's a legal ground path.
It's not a legal ground if the ears are holding the receptacle above the box e.g. against drywall, and there's some length of screw thread still unbottomed.
It's also not legal if paper, plastic or paint is trapped between the box and yoke. This often happens from the square of paper/plastic they put the screws through to "capture" them.