Q: Am I required to use a grounding screw when working with metal electrical boxes? And what about grounding when using plastic boxes?
A: David Herres, a licensed electrician in Clarkesville, N.H., responds: Using a metal grounding screw is a convenient—and arguably the most reliable—method of grounding a metal wall box or light-fixture enclosure, but it isn’t the only method. Certain metal raceways, such as the familiar electrical metallic tubing (EMT) that fastens to couplings and fittings by means of set screws (or compression fittings for outdoor work), also qualify as equipment-grounding conductors.
If a metal box is being used, best practice is to insert a green grounding screw into the threaded hole in the back of the box or enclosure. The equipment-grounding wires then connect to the screw, making the metal box part of the grounding system.
An alternative is to use a ground clip, which is an approved piece of hardware that slides onto the edge of a metal box and anchors the equipment-grounding conductor tightly against the metal. Conversely, the trunk slammers’ old trick of folding back the bare ground wire so that it touches the inside of the Romex connector as the cable enters the box is not a reliable or an acceptable way to make a durable low-impedance grounding connection, and most electrical inspectors will flag it as a violation.
Plastic boxes cannot be grounded in the same way. But it’s still necessary to bring the equipment-grounding conductor into the enclosure to ground devices such as switches and receptacles. Connect the bare or green wire directly to the green screw on the device. If another cable exits the box to feed downstream devices, connect a pigtail to the equipment-grounding conductors in both cables to attach to the grounding screw. The pigtail ensures that grounding continuity would be maintained even if the device were to be removed for some reason.
