Finding a neutral with a multimeter

[Sparkyboy71]

Good day everyone. I am a first year apprentice with a quick troubleshooting question. I notice sometimes things will not be what they should so I open a 1900 box and all 12awg conductors are black. Using my DMM can someone please tell me how to find my neutral for the circuit? Any help would be greatly appreciated guys!! Thanks in advance!!

 

[charlie b]

I don't think it can be done. The best I can come up with, and I know it is not necessarily a reliable test, would be to turn off all the breakers feeding that box (good luck figuring out that you have gotten them all), disconnect all wires within the box (label them all first, so that you can reinstall everything correctly), and measuring resistance from each wire to the box itself. If the box is metal, and if it is properly bonded, then there should be close to zero resistance between the box and the N-G bond point back at the main panel. So the resistance between a neutral conductor and the box should also be close to zero. When I say "close to zero," I mean in comparison to the reading you should get between a phase ("hot") conductor and the same box. With the circuit breaker open, the resistance reading should include the resistance of whatever loads are connected to that circuit. You will be reading the resistance of the wire from the point of connection within the box to the load(s), and the resistance of the load(s), and the resistance of the neutral wire(s) from the load(s) back to the main panel's N-G bond point, and the resistance of the equipment ground wire from that same N-G bond point to the box (which, as I assumed earlier, is bonded). That value should be noticeably higher than the resistance value you would get from a neutral wire to the box.

The only really reliable way to tell is to disconnect all wires within the box and within the panel (once again, label them all first, so that you can reinstall everything correctly), and test continuity from the panel to the wires inside the box. This, of course, requires you to use a test lead that reaches all the way from the panel to the box.

Welcome to the forum.

 

[Sparkyboy71]

Thanks so much for your answer Charlie! Am I wrong to say that I was told to do a continuity test on each conductor to the grounded metal
box and when I get a ring that is the neutral?


Sent from my iPhone using Tapatalk

 

[gadfly56]

I don't think it can be done. The best I can come up with, and I know it is not necessarily a reliable test, would be to turn off all the breakers feeding that box (good luck figuring out that you have gotten them all), disconnect all wires within the box (label them all first, so that you can reinstall everything correctly), and measuring resistance from each wire to the box itself. If the box is metal, and if it is properly bonded, then there should be close to zero resistance between the box and the N-G bond point back at the main panel. So the resistance between a neutral conductor and the box should also be close to zero. When I say "close to zero," I mean in comparison to the reading you should get between a phase ("hot") conductor and the same box. With the circuit breaker open, the resistance reading should include the resistance of whatever loads are connected to that circuit. You will be reading the resistance of the wire from the point of connection within the box to the load(s), and the resistance of the load(s), and the resistance of the neutral wire(s) from the load(s) back to the main panel's N-G bond point, and the resistance of the equipment ground wire from that same N-G bond point to the box (which, as I assumed earlier, is bonded). That value should be noticeably higher than the resistance value you would get from a neutral wire to the box.

The only really reliable way to tell is to disconnect all wires (once again, label them all first, so that you can reinstall everything correctly), and test continuity from the panel to the wires inside the box. This, of course, requires you to use a test lead that reaches all the way from the panel to the box.

Welcome to the forum.

Aren't loads connected across the hot and neutral in parallel? The only resistance on the neutral should be the wire back to the neutral bar plus what little additional resistance there is at each screw connection or wire nut connection in the outlets (assuming residential as an example). You're not actually going through the load (let's say a portable electric heater) because the hot is open. You're only going back to the panel on the other side through the box, pipe, connectors, etc.

 

[K8MHZ]

How do you know there is a neutral?

What if the box is switch legs only or something like that?

 

[jumper]

Sniff, sniff.

This smells like a homework/ school question.

 

[charlie b]

Thanks so much for your answer Charlie! Am I wrong to say that I was told to do a continuity test on each conductor to the grounded metal
box and when I get a ring that is the neutral?

I think you are wrong. Again, I am not completely certain, as I don't work with wiring as part of my job. But I would think you would read continuity between a hot conductor and the metal box, due to the circuit path I described earlier. It's just that that path would have a higher resistance than the path from a neutral wire to the box.

 

[charlie b]

Aren't loads connected across the hot and neutral in parallel?

Not quite sure what you mean. Any given load is wired in series from the hot conductor, through the load, and to the neutral conductor. Any two loads are in parallel with each other. But I don’t see how that bears on this discussion.

The only resistance on the neutral should be the wire back to the neutral bar plus what little additional resistance there is at each screw connection or wire nut connection in the outlets (assuming residential as an example).

The resistance between a neutral wire’s screw connection inside the box to the box itself would also include the resistance of the EGC (whether that be a wire-type EGC or the metal conduit or both). But as I said, that value should be fairly low.

You're not actually going through the load (let's say a portable electric heater) because the hot is open. You're only going back to the panel on the other side through the box, pipe, connectors, etc.

If you open the breaker, there is no path that goes back to the panel via the hot conductor. But there is a path that goes from the hot conductor’s screw connection inside the box, through the load, via the neutral wire back to the panel, then to the N-G bond wire, and finally via the EGC to the box itself. That path would have a higher resistance than you would get between a neutral wire and the box. That was my point.

 

[gadfly56]

Not quite sure what you mean. Any given load is wired in series from the hot conductor, through the load, and to the neutral conductor. Any two loads are in parallel with each other. But I don’t see how that bears on this discussion.
The resistance between a neutral wire’s screw connection inside the box to the box itself would also include the resistance of the EGC (whether that be a wire-type EGC or the metal conduit or both). But as I said, that value should be fairly low.
If you open the breaker, there is no path that goes back to the panel via the hot conductor. But there is a path that goes from the hot conductor’s screw connection inside the box, through the load, via the neutral wire back to the panel, then to the N-G bond wire, and finally via the EGC to the box itself. That path would have a higher resistance than you would get between a neutral wire and the box. That was my point.

Ahhh, now I see what you mean. Yes, of course you're right on that. I think I was looking at the question too narrowly.

 

[GoldDigger]

If you also open all of the other connections in the box, there is a chance that you would be disconnecting any wired load, either by disconnecting the hot going in their direction or by disconnecting the neutral coming back.
That would leave only one wire showing continuity to ground (and the outbound neutral would be identified by having originally been connected to the one you identified.

But if the box is not at the beginning of all of the circuits passing through it, there will be, as charlie said, more than one wire with continuity to ground and it may be possible to rule most out by their higher resistance value.

 

[MD84]

Is this real or hypothetical?

Why do you need to find a neutral in a JB?

And neutral wire should be white or grey in color.

What type of source is this? What is the load?

I would look at either the source or the load and see if a black wire is landed on the neutral bar or the device neutral connection.

If there is in fact a black wire being used for a neutral there are some ways to determine it from the others. You could use a dmm or some other tools.

 

[gar]

160217-2000 EST

Sparkyboy71:

I believe I have a techbique that using a high impedance meter, such as a Fluke 27, that I could identify a neutral vs a 120 V wire without disconnecting anything.

This technique makes use of the concept of a capacitive voltage divider. If I have a small conductive plate attached to the measuring end of a high impedance meter and the other meter lead is connected to neutral at the main panel by some means. This means can be an extension cord to the main panel, or if you know that the EGC is good to the main panel then it can sever as the grounded test lead to the meter.

Initially I tried a copper strip about 0.5" x 1.5" as the plate, but this is sort of dangerous in a main panel. So I changed to just using a Fluke alligator clip laid parallel to the wire being tested. With this as a probe I got about greater than 1.5 V from a hot 120 V lead, and less than 0.8 V from a neutral lead. If I had a shielded probe, then the differentation between hot and cold would have a greater ratio.

The capacitor plate needs to be close and parallel to the wire under test. The wire under test should be away from other wires and straightened to get maximum coupling to the wire under test to the test probe and minimum coupling from the other wires.

If everything is completely insulated, then wrap some aluminum foil around the wire under test and measure the voltage of the foil to ground.

Using a #18 rip cord and with a 1.3" length of aluminum foil wrapped around only one conductor I read 5.6 V from the 120 V hot wire and 1 V from the neutral. If I were to shield the meter and the lead to the foil probe, then 1 V reading would be considerably less.

A different question is how to determine which neutral is the desired one if more than one is in the box. You figure that out.

.

 

[junkhound]

If you include clamp on ammeter within 'multimeter' definition, it is easy.

clamp onto black wire, read current. If zero note position and move to the next.

If non zero, clamp onto white wires and look for same current, double check by turning off breaker and verifying white wire current goes to zero.

For circuits with no black wire current, go add a load to that circuit and continue.

Of course, easiest way is to see which conduit or NM both wires come out of, eh ?

That makes it sound like a homework question to me also.

Also, for remote locations, if there is current in the black wire the reading with both white and black in the clamp on should be zero* - unless white wires got bussed together downstream, now ya got bigger problems.

* or exactly double if you got the directions 'lopsided'.

 

[K8MHZ]

If you include clamp on ammeter within 'multimeter' definition, it is easy.

clamp onto black wire, read current. If zero note position and move to the next.

If non zero, clamp onto white wires and look for same current, double check by turning off breaker and verifying white wire current goes to zero.

For circuits with no black wire current, go add a load to that circuit and continue.

Of course, easiest way is to see which conduit or NM both wires come out of, eh ?

That makes it sound like a homework question to me also.

Also, for remote locations, if there is current in the black wire the reading with both white and black in the clamp on should be zero* - unless white wires got bussed together downstream, now ya got bigger problems.

* or exactly double if you got the directions 'lopsided'.

OP said all wires were black.

 

[kwired]

Thanks so much for your answer Charlie! Am I wrong to say that I was told to do a continuity test on each conductor to the grounded metal
box and when I get a ring that is the neutral?


Sent from my iPhone using Tapatalk

Depends on circumstances. If there are line to neutral loads connected you will read continuity through the connected loads and will throw you off with this thinking.

If you are certain there are no potential misleading paths, maybe some spare conductors that haven't been put to use yet, then this method will work.

If conductors in question are energized then a voltage test to a reliable ground should tell you which one is at or near ground potential, if your neutral is carrying current it will have some voltage drop on it and that will give you "near zero" instead of zero.

 

[roger]

The OP's question was similar to a question on an exam from 35 + years ago.

It went something along the lines of "in a box containing two black conductors measuring 120V across them and using only a voltmeter how can you determine which conductor is the grounded / neutral conductor", I don't remember the multiple choice answers but the correct answer was "impossible".

Of course bringing other items or methods into the scenario changes things but with nothing more than a voltmeter it can't be done.

Roger

 

[kwired]

The OP's question was similar to a question on an exam from 35 + years ago.

It went something along the lines of "in a box containing two black conductors measuring 120V across them and using only a voltmeter how can you determine which conductor is the grounded / neutral conductor", I don't remember the multiple choice answers but the correct answer was "impossible".

Of course bringing other items or methods into the scenario changes things but with nothing more than a voltmeter it can't be done.

Roger

:?

The one with low voltage to ground isn't necessarily completely determined to be the grounded conductor but that chance is pretty high.

I have found the grounded conductor of K&T circuits a few times while in an attic or other area with no grounded surfaces around by simply testing with a DMM with one probe on conductor and other probe in my bare hand. The one(s) with higher voltage reading (it won't be very high) are ungrounded conductors. Switch in the ungrounded will throw you off if switch is open, but that conductor is not really grounded when the switch is open either.

 

[K8MHZ]

:?

The one with low voltage to ground isn't necessarily completely determined to be the grounded conductor but that chance is pretty high.

I have found the grounded conductor of K&T circuits a few times while in an attic or other area with no grounded surfaces around by simply testing with a DMM with one probe on conductor and other probe in my bare hand. The one(s) with higher voltage reading (it won't be very high) are ungrounded conductors. Switch in the ungrounded will throw you off if switch is open, but that conductor is not really grounded when the switch is open either.

I was thinking the same thing. I haven't tried it, but I would think that by using a volt meter (assuming high impedance) with one lead in my hand and the other lead on a conductor, the hot would show a higher voltage than the neutral.

 

[don_resqcapt19]

I was thinking the same thing. I haven't tried it, but I would think that by using a volt meter (assuming high impedance) with one lead in my hand and the other lead on a conductor, the hot would show a higher voltage than the neutral.

Never tried that with a meter, but used that method the neon type voltage testers when working on knob and tube systems.

 

[roger]

:?

I have found the grounded conductor of K&T circuits a few times while in an attic or other area with no grounded surfaces around by simply testing with a DMM with one probe on conductor and other probe in my bare hand.

I was thinking the same thing. I haven't tried it, but I would think that by using a volt meter (assuming high impedance) with one lead in my hand and the other lead on a conductor, the hot would show a higher voltage than the neutral.

Note that I said the question was on an exam around 40 years ago and there wasn't an abundance of high impedance meters being carried in tool pouches in those days. I don't remember the exact wording of the question but a wiggy was probably what was being referred to as a voltmeter and it may have even said a solenoid type tester in the question.


Roger