In an older North American home, when I turn my bedroom light switch on, the ceiling light turns on and seems fine. When I then turn the switch for the hallway light on, then the bedroom light becomes brighter. Is the bedroom light somehow now getting 240v instead of 120v? And what would cause this?
\$\begingroup\$
\$\endgroup\$
10
-
\$\begingroup\$ If it were 240V the light bulb would pop! \$\endgroup\$– AaronCommented Aug 24, 2020 at 19:36
-
1\$\begingroup\$ Cross posted here: diy.stackexchange.com/q/201869/97780 \$\endgroup\$– Solar MikeCommented Aug 24, 2020 at 19:43
-
1\$\begingroup\$ @CharlesCowie is there any indication of an open neutral? \$\endgroup\$– jordanCommented Aug 24, 2020 at 20:27
-
1\$\begingroup\$ @manassehkatz-Moving2Codidact if it were an LED bulb it wouldn't change brightness based on voltage variations. They are either on or off once you pass the on/off voltage threshold. \$\endgroup\$– AaronCommented Aug 24, 2020 at 21:21
-
1\$\begingroup\$ @CharlesCowie, an open neutral means loss of connection to the center of the split 120-0-120 VAC of the transformer. This is not uncommon, and it causes the mains to be divided unevenly across each side. Putting a load on one circuit causes an increase in voltage on the opposite circuit. This is dangerous and requires immediate attention! \$\endgroup\$– DrMoishe PippikCommented Aug 24, 2020 at 22:31
|
Show 5 more comments
1 Answer
\$\begingroup\$
\$\endgroup\$
1
I think you have a multiwire branch circuit (also called shared neutral). Instead of running two separate cables from Phase A and Phase B at the panel to the branch circuits, the electrician runs a single 3-wire cable carrying Phase A Hot, Phase B hot, and Neutral (shared) to power two branch circuits. Saves a cable run, and therefore labor and copper costs.
The downside is that if the loads are unbalanced on these two branches, there can be enough voltage drop in the neutral conductor to cause slightly unbalanced voltages at your devices. Perhaps that's why the bedroom light brightens when the hallway light comes on. But it should take a bigger unbalance than that, so I suspect you may have a corroding connection in the neutral line somewhere. Get a voltmeter and do some testing.
answered Aug 25, 2020 at 4:22
-
\$\begingroup\$ No the legs are 180 out of phase and have no affect on each other for a mwbc \$\endgroup\$– Ed BealCommented Aug 25, 2020 at 19:35