Wiring Advice – Correct Ground Connections Inside a Metal Box

Masonry boxes are metal because concrete can get wet and conduct electricity. The box can be grounded to prevent this becoming a problem. Code allows you to use items that are MORE safe than is required. For instance, if code requires 14 gage wire, you're OK using 12 gage (but not 16). The same holds here. You're proposing using a more safe box than is required. As long as it physically fits, you will be OK.

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.

1. Connect a short length of grounding wire to the ground terminal of each switch/device in the box.
2. Connect a short length of grounding wire to the metal box, using a screw in the threaded hole in the back of the box.
3. Using an adequate connector, connect together the grounding wire from the box, the switches/devices, and all other grounding conductors in the box.