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just installed 2 new separate ceiling lights each with its own wall switch. The 2 wall switches sit beside each other. With analogue voltmeter I noted when both circuits are switched off there is 0 voltage across black/white wires on both circuit 1 and 2. But when circuit 1 is switched on and showing 120v, circuit 2 even though switched off, shows about 12 volts. Same occurs when circuit 2 is on and circuit 1 off. I presume this is just phantom voltage and not too worry? or could it be something that needs repairing? House built in 1975, aluminum wires.

also, I installed new lights to replace the integrated LED units, one of which had broken down twice. The first time the LED unit lasted 4 yrs and started flashing and then went dim. Replaced with same unit from Lowes, it lasted 4 months and went dim. Still don't know why LED units failed, wondered if related to voltage issues. I replaced with standard fixtures.

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  • Do the cables for the two circuits run next to each other? Nov 21 '19 at 5:21
  • Hello, and welcome to Home Improvement. You're right to note the type of voltmeter; I'd expect yours not to show phantom voltages. And, you should probably take our tour so you'll know how best to participate here. Nov 21 '19 at 10:51
  • @ThreePhaseEel I suspect that cables for the two circuits run next to each other behind the wall and for ~4 feet in ceiling
    – Ben Tunite
    Nov 22 '19 at 20:59
  • @DanielGriscom voltmeter is an old radioshack Micronta 22-211 indicating 10,000 Ohms / Volt AC, not sure if this is a low enough impedance
    – Ben Tunite
    Nov 22 '19 at 21:05
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This is called phantom voltage. When a cable is powering a device a field is generated, this field then induces a voltage on any wires in close proximity to the live wire. There is no problem here, this is related to how transformers work. I have seen phantom voltages close to the line voltage where there is a long run of multiple cables in parallel. The higher impedance the meter, the higher the voltage will be displayed. Their are low-Z or impedance meters that load the circuit enough to dampen the phantom voltages. But low-Z meters are not as sensitive for accurate measurements of electronic circuits so they are somewhat uncommon. I would not worry.

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  • Which suggests this can be verified by sticking a low-value resistor across the Switch2 feed and seeing if the voltmeter now reads zero. Nov 21 '19 at 19:25
  • Note that some modern DMMs have a low-Z function (such as my Brymen BM235, for one) Nov 22 '19 at 1:21
  • I have a fluke that can do low z also but most home owners or diy really won’t pay even close 100$ for a meter with low z when they can get all the other specs for 50$ Or less. Recently I have to really have long conversations with apprentices they want to go cheap and buy a new one every other year. But yes they are out there.
    – Ed Beal
    Nov 22 '19 at 2:58

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