# “Ground” to “ground” testing 120V, plus circuit measuring 30V?

I was replacing a duplex outlet in my basement and discovered 1) a circuit carrying 30V and 2) testing 120V between two bare "ground" wires from separate circuits.

Two pairs of black/white/bare wire enter an outlet box and are pigtailed to an outlet. I traced it using a Breaker Finder to Circuit 4 (15A), which I turned off at the breaker. I tested with a non-contact tester and found voltage. I shut off power at the main, disconnected the pigtailed wires, then turned it back on at the main (with power from Circuit 4 still off) so I could find out why there was voltage.

I discovered that Pair B from the bottom of the outlet box is Circuit 9 (15A), and it carries 30V of electricity. Pair A comes from a ceiling light in my laundry. Pair B comes from outside where it is attached to a GFCI with three pairs of Romex entering the box.

Here is a table showing voltages when measured from each wire to each other wire, testing both common and red (?) when using a multimeter.

Outlets nearby are correctly wired with a socket tester. Home built in the mid 80's.

My questions are, "Why does the circuit measure 30V?", "Why would bare wire to bare wire from two separate circuits measure 120V?", and finally "Is this safe?"

• Can we have a photo of the inside of the GFCI box? – ThreePhaseEel Feb 11 '17 at 14:59
• You're seeing phantom voltage from capacitive coupling. Do the tests again with an incandescent night-light also plugged in and turned on. – Harper Feb 11 '17 at 16:24
• Are you sure that chart is correct? One measurement says Black-A to Black-B is 0V, the other says Black-B to Black-A is 30V. White-A to Black-B is 1.5V, and Black-B to White-A is 118.7V. – Tester101 Feb 11 '17 at 16:47
• @Harper How can I test with the wires separated and an incandescent plugged in and turned on? When the outlet is assembled, it tests 120V, not 150V. – MinnesotaDIY Feb 11 '17 at 19:30
• @ThreePhaseEel I can't get a photo of the GFCI right now, what are you wanting to see? Come to think of it, if three pairs come in and are pigtailed together, and all are attached to the line screws, I imagine it won't work correctly since anything operating as load should come off of load. Perhaps I should replace it with a new GFCI, put line onto line alone, and pigtail jumpers with the neutrals and hots to attach to the load screws. – MinnesotaDIY Feb 11 '17 at 19:32