I have a weird wiring fault.

The kitchen and the dining room each have dimmer switches that work fine individually. However, if the dining room dimmer is on and within a certain range of positions (about halfway), the kitchen dimmer dims both sets of lights (not fully, but to some extent).

I can't imagine this being by design. What type of miswiring could explain these symptoms?

edit: Both lights are on the same circuit.

Looking inside, the kitchen dimmer looks uninteresting:

kitchen dimmer

The back box in the dining room is, frankly, a mess:

dining room back box

Suspiciously, the dining room dimmer has two wires connected to the L1 terminal:

dining room dimmer

I've not yet attempted to trace what goes where, but would love to hear your thoughts on what might be going on here.

P.S. The dining room has a single LED bulb which is well below the 60W minimum the dimmer is rated for. I doubt that's relevant, but am mentioning for completeness.

  • Are these dimmers on the same homerun circuit back to the consumer unit, or on different homeruns? – ThreePhaseEel Sep 13 '18 at 15:06
  • @ThreePhaseEel: They're on the same circuit. – NPE Sep 13 '18 at 15:36

In case someone runs into a similar problem, here is what I found and how I fixed the fault.

In the dining room, it turned out that the L1 terminal was connected to line and C to the load. This is the opposite to what I'd expect but I think is fine electrically (do correct me if I'm wrong). Anyway, swapping them round made no difference.

What did make a difference was replacing the dimmer in the dining room with one that's rated for the load (a single LED bulb). That made the problem go away completely.

(I'd still love to understand how the dimmer was picking up what was going on in another part of the circuit though.)

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