Why do different meters show different voltages between hot and ground
multimetervoltage
I am measuring voltage between hot and ground wires on a light box.
Why does one voltmeter show 4 volts. And another shows 0 volts?
How many volts should there be between these two wires?
Best Answer
Any time you see wack-a-doodle voltages associated with safety ground, you should consider that safety ground might not be connected to anything.
It's very common in pre-1965 homes to have the original wiring laid groundless with contemporary /2 no-ground cable, and then in the 1970s and onward, extended with /2+ground cable. Typically they either connect the extension ground to nothing (preferred), or to the metal boxes (bad, since if it isn't connected back to the panel it only causes those locations to share their ground faults, energizing all their grounds if one has a ground fault).
So when you see a hot-ground reading like that, you are best off investigating why that safety ground is not properly connected back to the main panel (or maybe the hot is not properly connected). If the wiring is old enough that it simply isn't connected, then furl up the ground wire in reserve for the happy day when ground is retrofitted, and do your best to insulate it from touching the metal box.
When you see a phantom voltage like that, it's the hallmark of a sensitive meter. Though sensitivity sounds like a feature, most cheap meters have it too. Disregard.
Well, we can read a few things from the tea leaves.
You know this wire runs near a "hot" wire for a little bit of distance - enough to pick up an SLF radio signal from the adjacent wire. If you've ever built/used a crystal radio that uses a tiny earpiece and is passively powered (from the AM airwaves themselves), you understand tiny amounts of power can be induced from radio.
You know this wire is rattling i.e. not connected to any supply. You know in a North American panelboard you have 4 wires: L1 that's 120.4 volts to ground, L2 that's 119.6V to ground, neutral that's 0.4 volts to ground, and ground that's 0.0V to ground. (assuming a 0.4V neutral-ground drift; YMMV.) You know two things from the rattling/floating wire.
it is not connected to any of those (because if it was, it would read that voltage).
it is not connected to a load whose other wire is connected to any of those (because if it was, the load would "pull the wire" to the voltage of the other wire.)
Best Answer
Any time you see wack-a-doodle voltages associated with safety ground, you should consider that safety ground might not be connected to anything.
It's very common in pre-1965 homes to have the original wiring laid groundless with contemporary /2 no-ground cable, and then in the 1970s and onward, extended with /2+ground cable. Typically they either connect the extension ground to nothing (preferred), or to the metal boxes (bad, since if it isn't connected back to the panel it only causes those locations to share their ground faults, energizing all their grounds if one has a ground fault).
So when you see a hot-ground reading like that, you are best off investigating why that safety ground is not properly connected back to the main panel (or maybe the hot is not properly connected). If the wiring is old enough that it simply isn't connected, then furl up the ground wire in reserve for the happy day when ground is retrofitted, and do your best to insulate it from touching the metal box.