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I live in a home that was built in the late 1930s with some old electrical wiring and some new wiring. As I was identifying which outlets and lights are controlled by which wires I discovered a brainteaser:

A garage outlet that my outlet tester identified as properly wired and grounded:

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However, it only had 2 wires going into it: a red one and a white one:

enter image description here

Furthermore, for some reason, the box that holds the outlet is attached to another box with thicker gauge wires that are black, red and white which are not connected anywhere.

enter image description here

I suspect the other box with thicker (12AWG) wires used to have the outlet for a dryer, while the first outlet pictured used to power the washer (they have since been moved to another location).

But my question remains -- how can my tester (which has been pretty good at showing missing ground in other outlets) be showing this outlet as ALL GOOD in terms of ground even though it only has 2 wires going in?

P.S. Making sense of the source of these wires at the panel is not an option for me -- it's a mess.

Kat
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Vlad Gurovich
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3 Answers3

37

The conduit is the grounding path

Note that the wiring in your case is run not using sheathed cables, but as individual wires inside a metallic conduit (aka the pipe-like stuff you see heading off to the left in your picture). As a result, the conduit is a serviceable grounding path in its own right, connecting the receptacle grounds and boxes to the grounded panel enclosure without any need to run an extra ground wire through it.

ThreePhaseEel
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The weak link in the grounding may be the cover. Your fully-raised cover only contacts the (grounded) box at the edges and through the screws. A proper ground requires a cover with flattened corners, to make solid contact with the corners of the junction box. The flat contact between the receptacle's strap and the cover completes the ground path.

crushed-corner cover

David Lasher
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I've come across a few boxes that were like this, have accidentally contacted live wires to them (with enough oomph behind the arc to weld it to the box) and I will say wire everything securely and insulate or trim down any exposed wire if you do any work in them. In my experience, the discharge to the box didn't trip the breaker and it was blind luck that I jumped back fast enough to not eat a lethal dose of mains current. e: Insulated tools!

Netflixandkrill
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