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