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Is it acceptable to ground outlets or switches through a metal device box (not pig tailing)?

3 switch box ground

The switches are connected to a long ground wire going to the light which are also screwed to the back of the box. The Ground form the lead (cut short) is screwed to the back of the box as well. My question is if the metal box is an acceptable method for connecting all the ground wires together completing the chain to the earth. The dark green dots in the diagram are screws fastening the ground wires to the box. (This is in Ontario Canada)

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    Not all devices (switches and outlets) are rated "self-grounding", be certain the devices you select are rated for "self-grounding". (Personally I think "self-grounding" is bad naming because it implies something other than "rated for ground path to a grounded metal box") – Tyson Nov 21 '16 at 21:00
  • Yes if the device is listed as a "self grounding" device. – Tyson Nov 22 '16 at 13:44
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Everyone seems to be answering a question that was not asked by the OP. He is asking if using the bonding screws without also pigtailing the bonding conductors will suffice for bonding.

Yes, the picture you have shown is an appropriate way to bond the devices.

Edit: Additionally, there is no requirement to bond the switches in the CEC with a separate bonding conductor, unless doing so is stated in the manufacturer's instructions. The screws used to mount the switch to the box suffice as the bond.

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As to the configuration in the diagram, it is allowed by 250.148(C):

(C) Metal Boxes. A connection shall be made between the one or more equipment grounding conductors and a metal box by means of a grounding screw that shall be used for no other purpose, equipment listed for grounding, or a listed grounding device.

However, what I would use instead here is called a "self-grounding" switch or outlet -- they have a spring clip on each of the yoke ears that ensures that the receptacle makes solid contact with the box even if the mounting screws loosen. This is explicitly allowed by NEC 250.146(B):

(B) Contact Devices or Yokes. Contact devices or yokes designed and listed as self-grounding shall be permitted in conjunction with the supporting screws to establish equipment bonding between the device yoke and flush-type boxes.

Note that using plain metal-to-metal contact between the yoke and box for yoke bonding is only allowed in surface mounted metal electrical boxes (i.e. a conduit box sitting on the garage wall), not on boxes that are mounted flush with the wall -- the verbiage in NEC 250.146(A) that allows metal-to-metal contact between an ordinary yoke and a box to bond the yoke only applies to surface mounted boxes:

(A) Surface-Mounted Box. Where the box is mounted on the surface, direct metal-to-metal contact between the de vice yoke and the box or a contact yoke or device that complies with 250.146(B) shall be permitted to ground the receptacle to the box. At least one of the insulating washers shall be removed from receptacles that do not have a contact yoke or device that complies with 250.146(B) to ensure direct metal-to-metal contact. This provision shall not apply to cover-mounted receptacles unless the box and cover combination are listed as providing satisfactory ground continuity between the box and the receptacle. A listed exposed work cover shall be permitted to be the grounding and bonding means when (1) the device is attached to the cover with at least two fasteners that are permanent (such as a rivet) or have a thread locking or screw or nut locking means and (2) when the cover mounting holes are located on a flat non-raised portion of the cover.

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The metal box must be grounded. U ground outlets and switches in a metal box do not require the ground be on the switch or outlet itself but if there is an insulating washer it needs to be removed.

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I did this once in an exterior metal box. It was a double-gang with explicit ground screws (one on each side you need to connect to). My understanding (I am not an electrician here, nor do I know NEC, FWIW) is that you can pass it through the box. Here's an image (source) depicting doing just that

For most interior junction boxes this is riskier because they have no native junction points explicitly for this purpose. Generally, you want the bare wire to contact the box in some fashion. The risk with the method you're talking about is that all the wires must contact the box in some secure fashion. Just sticking them in is crossing your fingers that they stay connected to the box. Without some way to explicitly connect them to the box, I would be leery of doing it this way. It seems like grounding clips would be the way to go if you choose to do this.

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