Home Tweakers' Asylum

Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ.

RE: Wiring outlet box

In your examples the boxes are fed with metallic conduit. The metallic conduit is the equipment grounding conductor. The box is bonded, connected, to the equipment grounding conductor by the conduit box connector.

Also notice the boxes are surfaced mounted.If the box is flush mounted a grounding jumper is required from the ground terminal on the receptacle to the box. Exception: If the receptacle is an approved auto grounding type
receptacle the grounding jumper is not required.

Say the box is a steel 4"X4" surface mounted box and a steel 4"x4" raised duplex cover will be used to support the duplex receptacle. Even though the steel raised cover will be fastened to the box by the two 8/32 supporting screws NEC code requires a bonding ground jumper wire to be installed from the ground terminal on the duplex receptacle to the 4"x4" steel box. Why? Because if the raised cover is removed from the box and the jumper is not installed there is the risk the cover could become energized and create a shock hazard to the person who removed the raised cover. Yes the person should have turned off the circuit first. NEC tries to make the code as idiot proof as possible.

1900 raised duplex cover
http://www.garvinindustries.com/images/itemimages/g1938.jpg


As for the OP's situation. IF Romex is used to feed the new floor flush mounted aluminum box the electrician could have simply stripped the outer jacket longer than needed then half curled the bare ground wire around a 10/32 green ground screw in the back of the box and fastened the remaining end of the bare ground wire to the ground terminal of the duplex receptacle.

The electrician could have also pig-tailed two ground wires to the Romex bare ground wire and connected one to the box and the other to the ground terminal on the duplex receptacle.

Why not just connect the bare ground wire from the Romex to the duplex receptacle ground screw and then let the yoke, supporting back strap, bond the equipment ground to the box that way? Because NEC code says that does not meet code. Why? What if someone pulls the receptacle from the box? The metal box is no longer grounded. It is no longer idiot proof.

Note: If the aluminum box does not have a tapped 10/32 hole in the back of the box and the electrician does not want to take the time to drill and tap the hole or drill a hole and fasten a ground pigtail with a machine screw and nut he could have simply used a green drive on wire ground clip on the side of the box and fastened a ground jumper pigtail wire there.

http://www.homedepot.com/catalog/productImages/1000/e1/e144b5ab-fc7b-4530-a3b2-b6d5944a2da9_1000.jpg


Note: Even if the aluminum box will be surfaced mounted to the floor the box still must be bonded directly to the bare equipment grounding conductor of the Romex. That is if the OP wants to meet bare minimum NEC safety electrical standards.




Edits: 04/14/17 04/14/17 04/14/17

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