Why would there be a voltage on the ground wire of an LED light, that varies according to how the dimmer is set?

When at full brightness, the ground wire reads ~30VAC, when it is mostly dimmed, it reads ~60VAC. So changing the brightness changes the voltage on the ground wire.

I am taking the reading between the ground wire of the fixture and the chassis of the metal electrical box. The house uses old style fabric-insulated 12-2 MC cable, so there is no standard ground wire to speak of.

  • there is no standard ground wire to speak of ... you are asking why there is a voltage reading between LED light and an unknown wire – jsotola Dec 27 '19 at 5:56
  • That's old-style AC (BX) you're looking at, not MC, which is a rather different beast altogether. What make and model is this fixture, by the way, and what's the resistance from neutral to ground with the breaker for the circuit open? – ThreePhaseEel Dec 27 '19 at 15:31

In order to try and answer your question I have to make two assumptions. First the ground wire at the fixture is not grounded to the chassis of the metal box. Second the BX cable is either not grounded or poorly grounded which is one of the reasons why this particular type of system was outlawed in most jurisdictions and different systems were employed.

What this causes is a difference of voltage potential between the neutral and the ground. It's really a simple example of a circuit with both series and parallel loads made a little more complex because the conductors need to identified as a part of the load.

If you properly connect the ground of the fixture to the box which can be done with what we refer to as a "stinger" then your readings will go away, there is still could be a shock hazard since the system is at best poorly grounded. The NEC provides us with a method of protection of ungrounded circuits which is to replace a standard breaker with a GFCI breaker or device.

If you do this and the GFCI trips then you probably have a bad dimmer or fixture, or a compatibility problem with dimmer and fixture. The really bad news is that it could also be a problem unrelated to the dimmer and fixture but some other device or fixture on the same circuit. This would require someone to investigate the problem on site due to the many variables.

Hope this helps.

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