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I have a small, octagonal junction box in my basement with one 2-wire-no-ground cable going in and a standard porcelain lightbulb fixture attached to the bottom. In the process of investigating an unrelated issue on a different circuit, I discovered that my non-contact voltage tester would go off anywhere in the vicinity of this box. I pulled out the multimeter, touched one lead to the box, and the other to the ground socket of an extension cord plugged in elsewhere. It showed about 5V. When I switched the circuit off, that 5V went away. I opened the box up and checked to see if there was any worn insulation or bare wire contacting the metal, but I couldn't find anything that looked fishy.

There are other similar junction boxes down there, but none of them set off the voltage tester and none of them show any voltage on the multimeter. Any idea what could cause this? The number 5 in particular is confusing to me. Contact with the hot wire would give 120V, but how can you get 5? All the components here (box, wires, fixture) are probably about 60 years old.

  • Why wouldn't you expect the detector to go off near a live circuit? Maybe I'm misreading something. Is the metal box not grounded perhaps? – JPhi1618 Dec 8 '15 at 22:09
  • @JPhi1618 In other cases, it will go off from being near the hot wire(s), but not just from being near the box itself. But no, the box isn't grounded--it's on an old circuit without ground wires. Does the voltage on the hot wire slightly raise the box's potential even if there's no metal-to-metal contact? Is this just physics? – dlf Dec 8 '15 at 22:16
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    If you're comparing the voltage on an un-grounded box to a known-grounded extension cord, having some voltage is not surprising. I can't speak in detail about the physics, but at a minimum, the non contact detector can sense the electric field generated by the wires, and that transferring to a nearby metal object isn't that much of a stretch. – JPhi1618 Dec 8 '15 at 22:24
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    inductive coupling is how transformers work. If this box has a larger loop of wire or the 2 wires enter from different holes there will be a voltage generated. With 3 phase if you split a phase and enter a separate hole you just made a heater out of the metal box, so this makes since to me. – Ed Beal Dec 9 '15 at 1:03
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This sounds like a capacitive coupling issue -- the electric field radiated by the wires can and will couple to nearby metal objects, and a voltage-detector or high-impedance multimeter will pick it up. If you can get a multimeter with a low-impedance (LoZ) voltage range, then that can be used to discriminate between capacitively coupled "phantom" voltages and voltage sources that have some "oomph" behind them.

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