Electrical – Ceiling lamp wiring problem

You have some pretty weird wiring to the light fitting. It looks like at the light fitting (fed from top-right romex at box) you would find

• the "ground" wire (green or bare copper in the US?) is being used as a neutral return from the light bulb.
• the "hot" (black) is a switched hot to the light bulb.
• the "neutral wire (white) is being used as a permanent hot for some other purpose.

So I'd expect an ordinary light fitting to be using the false "ground" and switched hot wires only.

The permanent hot (false neutral, white) might have been used (now or previously) to power some kind of device that needs live power. For example a remote light control or a ceiling fan that is operated by a pull-cord independently of the light bulb itself.

I would check the wiring in the light fitting controlled by that switch. Then I'd label the wires at each end to make clear what each wire is really used for. Alternatively, you could rewire it properly - that would be best if the light fitting is a normal conventional simple light only. I'd double check that something else (an extra outlet?) isn't already wired from the light fitting end - disconnect the white wire from the three black and see if an outlet or something stops working.

If you can't figure it out it might be best to call an electrician.

What you are describing is called a Multi-wire Branch Circuit, and it is completely legal (given that it's done properly).

The first problem, is that it doesn't should like the breakers feeding this circuit are handle tied together. It should be feed by a double pole breaker, or the two breakers handles should be tied together. You should not be able to shut off only half the circuit (red only or black only).

As for the voltage. I'd suspect it's phantom voltage (inductive coupling), and so should not cause any problems (especially if the breakers were properly tied together).

If the breaker for the "red" conductor is off, and current is flowing on the "black" conductor. Because the conductors are parallel, and in close proximity, a voltage may be induced on the "red" wire. There should be no current, however. So if a load was introduced, the voltage would drop to 0.

You can determine if it really is phantom voltage, simply by using a low impedance voltmeter. The low impedance meter will read 0 volts, while a high impedance meter will read the phantom voltage. An analog meter is one example of a low impedance meter, though some digital multimeters are able to operate in both modes.