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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?

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  • Ok, that cable is plainly marked 12/2. to be clear, that wire you are probing is actually a bare wire, that some jackass put a black sleeve over illegally. – Harper - Reinstate Monica Apr 15 at 22:51
  • It says "12/2 with ground type NM ROMEX TJ 600V". But what do you mean? It looks like shielded ground wire and it seems that it came that way from a store. I don't see how someone could put black sleeve over the entire length of the wire that's inside the cable sheathing – dennismv Apr 15 at 23:26
  • It might be paper wadding.. But I super doubt any wire builder on earth would intentionally put black sleeving on a ground wire. I assume this was slipped on by someone who didn't want the bare ground wire hitting the hot screw. Which may have happened; note the spallation on the screw head... – Harper - Reinstate Monica Apr 15 at 23:31
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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.

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  • thanks. What could happen if the ground is touching the metal box if it's not connected to anything? – dennismv Apr 15 at 23:28
  • @dennismv If the ground connected in both boxes, it would cause a ground fault in either box to energize all the grounded things connected to both boxes. – Harper - Reinstate Monica Apr 15 at 23:33
  • That's right, the ground wire was not connected to anything & home was built in 1964. – dennismv Apr 17 at 2:59
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If one lead or the other is floating, the readings are not reliable. This is because the internal resistance in the multimeter is in series with any parasitic capacitance, inductance, or resistance from the floating line to other parts (e.g. coupling through the insulation to an adjacent hot wire, or for ungrounded gear, through timber to ground).

This forms a voltage divider where both parts are of a similar impedance - several megaohms. Changes in the impedance of either side will substantially change the measured voltage. Different multimeters will have different internal resistances, and different makeup of capacitance vs inductance as part of that.

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In layman's terms, meters have different readings because they all have different degrees of accuracy, 1 to 3.5% plus or minus. If your question is correct, the reading between hot and ground should be 120V + or -.

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