When I turn my bedroom light switch on, the ceiling light turns on and seems fine. (Older North American home.)

When I then turn the switch for the hallway light on, the bedroom light becomes brighter.

Is the bedroom light somehow now getting 240v instead of 120v? And what would cause this?

  • Cross posted here: electronics.stackexchange.com/q/518202/152903 – Solar Mike Aug 24 at 19:44
  • Set your microwave to defrost, and heat a cup of water. On defrost, which will make the microwave cycle the magnetron on/off every few seconds. While that runs watch the bedroom light. Does it change brightness as the microwave cycles? – Harper - Reinstate Monica Aug 24 at 21:08
  • Start by measuring the AC voltage wherever you can, perhaps at the bedroom switch itself. And as always: is this a new phenomenon? Did you change anything else? What do you mean by "older" house? Some people would call a 2000 construction "older," while I would think "pre WW2" . – Carl Witthoft Aug 25 at 15:30

I'm willing to bet that there's a split circuit (Multi wire branch circuit) improperly wired here.

So when both switches are on, you're getting 240v between the two phases rather than the expected 120v between a single phase and neutral.

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    This is called a lost or floating neutral, and results in uneven division of the 240 VAC, e.g. 80 VAC on one circuit with low resistance (e.g. a refrigerator), and 160 on a ight load (e.g. a few LED and incandescent lamps). It can destroy equipment and start fires. – DrMoishe Pippik Aug 24 at 23:48
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    How would this be resolved? Or is a certified electrician the only route? – jordan Aug 25 at 0:08
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    I'm a bit surprised that even 160 VAC wouldn't cause incandescent bulbs to pop immediately. – Carl Witthoft Aug 25 at 15:28
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    With a mwbc the 2 legs are 180 out of phase With the return on the neutral there will always be 240 between the 2 legs but the max voltage to ground is 120v. – Ed Beal Aug 25 at 19:47

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