We have several outlets on a circuit that test "hot/ground reversed" with a plug in receptacle tester - with the breaker turned off. With the breaker still turned off, we tested the one of the outlets with a multimeter, and read no voltage white to black, no voltage black to ground, and 120 volts white to ground.

With the breaker turned on, all three LEDs on the tester light up, including the red one indicating a reversal. With a multimeter, we read 120 volts black to white, 120 volts white to ground, and 240 volts black to ground.

The home is about 55 years old. The wiring in the subpanel is old 12-2 Romex with no ground. Some, but not all, of the wiring in the walls is newer, yellow 12-2 Romex with a ground that somebody (probably the homeowner) added recently. The wiring in this place really is quite a mess.

Our job is to install a bathroom fan on the circuit described above - not to correct all the obvious electrical deficiencies. Does anyone have any ideas what might be causing the condition described above? The 240 volts from hot to ground has us especially puzzled (and concerned).

- Mike M.

You should be concerned about the 240 volts to "ground".

Along with a neutral and hot reversed, there is a serious miswire, or a broken shared neutral to cause the 240 volts.

I have to assume if you know how to trace this condition you would not need to ask questions here.

I need to strongly recommend that you hire an electrician to trace and repair these conditions you have.

Tk's nailed it, you can't get 240 volts to true ground, you can get 240 volts between both hot legs,
That's where he mentions "a hot neutral reversed".
Go to electrical panel and verify both hots are hot and 240 volts between them, and 120 volts between each hot to neutral and ground.
It seems like maybe someone may have changed recepticles and didn't keep white wires on white screws, plus the open neutral as in a multicircuit.
Has this problem been like this, or just started.

I'm onboard with the OP hiring an electrician :D