I need to ground my switches by connecting the grounding wires from switches onto an electrical twist nut and pig tailing it it to the box. Does Home Depot or other stores sell little pieces of copper to complete the pig tail or do I need to buy a big roll of copper? Does the gauge of the copper matter?
Grounding tails are available (thanks @batsplatsterson), but you could also buy some copper wire; either on a reel or by the foot, and make your own.
As a quick rule of thumb, you should use the same size grounding conductor, as the largest ungrounded (hot) conductor used in that circuit. So you're probably looking at using 14, or 12 AWG wire for switches.
You'll want to use either bare copper, or green insulated wire. Solid or stranded makes no difference, as long as it's the proper size. Some will argue one way or the other about connecting solid to stranded, stranded to stranded, solid to solid, stranded to screw terminals, solid to screw terminals, etc. In reality, if done properly, it really makes no difference. Follow the manufacturer's documentation on all the equipment you're using, and you should have no problems.
As for the actual procedure of grounding the switches and box.
- Connect a short length of grounding wire to the ground terminal of each switch/device in the box.
- Connect a short length of grounding wire to the metal box, using a screw in the threaded hole in the back of the box.
- Using an adequate connector, connect together the grounding wire from the box, the switches/devices, and all other grounding conductors in the box.
You should match the gauge of the ground to the wires you are pigtailing. Your local home improvement store will carry single stranded THHN wire which you can use to make pigtails with.
Find out what gauge wire you are working with and buy some green THHN wire of the same gauge. Green wire is coded as ground in the US. It is usually available both by the foot, and in different sized spools.
They probably also sell premade pigtails which are made out of THHN wire that have crimped on terminals. In your case, it is probably better to just buy it in bulk and make your own.
Is the pigtail the easiest way to ground the switch? I'd say so, if there's a threaded hole available, and it's a properly grounded metal box. These pigtails from Ideal Industries: pigtails
bond your box to whatever you terminate that stripped end on.
If you attach the pigtail with its ground screw into a threaded hole in a metal box,
and terminate the stripped end of the pigtail on the ground terminal on your switch,
AND the box is grounded, then you've grounded the switch. (If it's not a metal box, you can't ground the switch this way.)
How can you tell if the box is grounded? If you see a ground wire from one of the incoming wires attached with a ground screw or ground clip, it's probably OK - it depends on that ground wire being properly connected back to the panel.
If it's a plastic box, or there's no hole available for the ground screw, or etc., you will need a plan B. Maybe there are other ground wires in the box bound up in a wire nut. You could add your pigtail to the switch ground terminal to that bunch. Wirenuts are fine, but the push in connectors
are more straightforward to use.
Beyond that - as long as there is some ground wire in there, there's a way to get everything grounded, but it's hard to say what's the way to go without seeing it.
You may need to use what is called a "greenie". It is a wire nut with a hole in the normally closed end to allow for a single wire to pass through for connecting to the ground screw. These are sold at Lowes and HD.
Some devices, like Leviton M52-RS115-2WM (found through Home Depot web site a moment ago), have a little brass springy piece connecting the device yoke to the mounting screw at one end. When this brass bit is present the ground can be terminated just to the conductive junction box. The brass piece ensures good-enough contact from the box, through the mounting screw, to the device. The usual ground screw is also present on the yoke and would be used in case a non-conductive box is used, the brass piece is damaged, installer preference, etc.