Is grounding an outlet only to its box enough?

Question

There's an outlet in the front hall of my house near Atlanta, GA. I noticed scorch marks on the face, so I decided to investigate it a bit. My outlet tester indicated an open ground.

When I opened up the outlet, I was surprised by what I saw. The outlet's hot and neutral terminals were connected to two individual 10 AWG wires, and the outlet's ground terminal was connected to a ground screw on the metal box. I had never seen an outlet wired with anything other than Romex before.

The two individual 10 AWG wires ran to a another outlet on the other side of the wall where I confirmed they were connected to the hot and neutral wires of a circuit.

The outlet was a NEMA 5-20.

Was this outlet installed in a code-compliant manner?

Answer 1

No, it's not installed in a code-compliant manner. For one thing, the conductors are passing through box mounting holes! That particular kind of conductor belongs in conduit, which should be entering through one of the circular knock-out holes in the box.

We can see the box is mounted to the drywall with simple sheet metal mounting tabs, and based on the hack with the two supply conductors, we can be almost certain that box isn't actually grounded.

As for your title question: yes, it's sufficient to ground an outlet only to its box -- if, and only if, the box itself is properly grounded.

Answer 2

To add to Greg's answer, it's important to note when and where you can ground to the box.

Metal boxes are grounded when

1. You've bonded the box to a ground wire that connects back to the main panel neutral (directly or through a grounded subpanel). If this is done via cable, you'll have a NM clamp where the cable comes in
2. You're using grounded metal raceway or conduit

Neither of these apply to your photo. The easiest way to tell is none of the knockouts are removed. It looks like they have THHN wires fished through the back of the box. This raises a deeper question for you: how was that THHN fished to this box? Normally, THHN is run inside conduit (there's only the outer sheath of insulation between you a 120v). If this isn't conduit, you'll need to replace it with NM cable by boxing it off somewhere upstream and re-running it. Or, worst of all, it should have been in conduit (i.e. you live near Chicago) and someone cheated badly.