I picked off 120v from an outlet (parallel) to power a dual switch that inturn will switch on a light and the other to switch on a bathrom fan using a 3/12 with grnd. The circuit is connected through the dual switch by providing the black "hot" wire to the "com" terminal of the dual switch. The other side of one switch will go to the light (wht) and the other side of the second switch will go to the fan (red) and all the grounds and nuetrals are continuously connected respectively. The open wires are waiting to be connected to their respective fixture. With my multimeter on the open wires I turn the switch on and I get 120volts but when I turn the switch off I get a 15volt reading. Is this normal because I am reading through a switch that doesn't have a load on it?
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2The neutral does NOT connect to the switch. You need to re-explain exactly what wires you have connected to where. Better yet, provide a picture of this box and the box you took power from.– Speedy PeteyCommented Nov 21, 2014 at 12:11
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What this sounds like to me is that you're reading across a (low) resistance, which -- depending on what you're measuring -- may or may not be an issue. Or you may just be picking up induced hum. We need details of exactly what you're probing.– keshlamCommented Nov 21, 2014 at 14:29
1 Answer
Is this normal
It is most likely a capacitatively coupled voltage from "hot"/"live" wires running adjacent to the wire being measured using a high-impedance multimeter. If your meter has a "low-Z" setting, try that. If the voltage is zero using a low-Z setting, there is nothing to be concerned about.
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Due to the high impedance of measuring instruments, a voltage reading may be detected on open conductors where there is no hard electrical connection to a voltage source. Conductors that are installed in close proximity to one another, and are capacitively coupled to each other, can cause this a.c. voltage reading. Such a reading could be 2 or 3 volts, or it may be as high as the voltage on the adjacent conductors. This is what is referred to as a “phantom” voltage. ... NEMA recommends the use of a Listed low impedance multimeter in place of a high impedance multimeter or other high impedance measuring device for testing on open conductors where there is no hard electrical connection. Without a low 2 impedance measuring device, a high voltage reading is an inconclusive indication of possible faults in the cable
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It's not a false reading, that voltage does actually appear across the leads of the multimeter. It just isn't usually a useful measurement when dealing with building electrical systems. By using a low-Z meter you are bleeding off the voltage because it doesn't have enough current behind it to keep it at the higher voltages. Commented Nov 22, 2014 at 4:12
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