Using an extension cord (like new, great condition), I checked a disconnected circuit attached to the same service.
First, I checked the disconnected circuit:
Hot -> Ground ~0V
Hot -> Neutral ~0V
Neutral -> Ground ~0V
All is good…
Then, using an extension cord ran from an isolated (only box on circuit – identified hereafter by (1)), I checked again:
Hot(1) -> Ground(2) ~120V
Hot(1) -> Neutral(2) ~120V
Hot(1) -> Hot(2) ~120V (I don't think this is acceptable)
Can someone explain this to me?
Okay, so based on Johnny's answer, I went and checked a few circuits nearby. Apparently, I have a lot more work to do on this. There is a third circuit involved now.
To recap
Circuit 1: Extension cord from a single duplex immediately terminated at the panel.
Circuit 2: Remote circuit with an exterior load. Supposedly on its own circuit (per the electrician that just upgraded service and load center). This circuit did operate when engaging the breaker and disengaging the same breaker terminated the load.
Circuit 3: Adjacent to Circuit 2 (well, near proximity) with a small load on a timer (as suggested by Johnny). Disengaging this circuit breaker did previously perform as expected. Engaging the breaker did reconnect service to the load and timer(as expected), however, did not supply service to the load on Circuit 2.
After reading Johnny's answer, I went and disconnected this timer and load from Circuit 3. Checking Circuit 2 again gave me the following:
Hot(1) -> Hot(2) ~0
Hot(1) -> Neutral(2) ~0
Hot(1) -> Ground(2) ~0
Neutral(1) -> Hot(2) ~0
Neutral(1) -> Neutral(2) ~0
Neutral(1) -> Ground(2) ~0
Still confused.
Also, not sure exactly how this was missed by the electrician, but I digress.
Best Answer
The first two, hot to ground and hot to neutral are normal and what you'd expect.
Voltage showing on from the hot to hot leg may not be a sign of any problem -- Voltmeters have a very high impedance, so any load at all on the other circuit can provide a path to neutral that will appear as 120V to the meter. This could be a lamp, a "smart switch" or outlet, a small night light, a cell phone charger, pretty much any load at all.