Strange lighting issue

[mhunter]

Had 2 new lighting groups installed on a multi wire branch circuit that used to be used for kitchen outlets.

The 12-3 cable enters the switch box where the neutral is tied to the 2 neutrals from the lights. Hots go to 3 way switches with travelers sent to 3 way dimmers in another box. Hots are to a 2pole breaker with each on separate phase. These lights work flawlessly.

When one or both of these lights are on other lights on other circuits flicker. It's not consistent which lights will flicker and when but it happens all the time and shutting off the new lights stops the flickering.
Any ideas?

 

[Lucien Nunes]

It sounds like the combination of specific models of lights and dimmers is one that is just marginally stable in its own right. When one dimmer is in use, the interference and waveform distortion it creates across the circuit resistance affects the operation of other dimmers that share all or part of the circuit. Do you have any identical pairings of dimmer and light fixture in use elsewhere? What are the specific models and numbers of fixtures in use? There might be a work-around by subtle alterations in the load, increasing or decreasing it or adding capacitive load, etc.

I don't think the use of a multi-wire circuit is particularly significant. Certainly, in the event of a failed neutral, peculiar interactions can occur as the two groups of lights and dimmers would be in series on 240V and failure would be likely. However, the fact that lights on one hot operate correctly without the others, indicates that the neutral is OK and is not the key to the interaction. I suspect the same behaviour would occur if they were all fed from the same hot on a 2-wire circuit.

For UK readers: This type of USA multiwire branch circuit is a 3-wire split-phase final circuit with two lines (hots) and one neutral, providing 120/240V but often used as 2 x 120V. The phase difference is thus 180° not 120° but the same scenario of the neutral current not exceeding the highest phase current applies (ignoring harmonics.)

 

[Megawatt]

Had 2 new lighting groups installed on a multi wire branch circuit that used to be used for kitchen outlets.

The 12-3 cable enters the switch box where the neutral is tied to the 2 neutrals from the lights. Hots go to 3 way switches with travelers sent to 3 way dimmers in another box. Hots are to a 2pole breaker with each on separate phase. These lights work flawlessly.

When one or both of these lights are on other lights on other circuits flicker. It's not consistent which lights will flicker and when but it happens all the time and shutting off the new lights stops the flickering.
Any ideas?

I think your problem is that you have 2 different circuits from 2 different phases with each it’s own neutral and then you hook up all the neutrals together. That’s a problem