Why low voltage (40 Volts) at outlet

[ktnwin]

First, I have to apologize for the following really long description. I guess if I draw a diagram, most of the text below will disappear. I will draw diagrams to attach to this topic.

I need advise on this situation (40 Volts at outlet).

When I plug Xmas light in one outlet near the patio door of my living room, the lights do not lit up.
Plugging into a different one , Xmas lights lit up. All outlets in my living room are on same circuit, same breaker. I knew this when I replace all the (40 yrs) old receptacles with new ones and verify all are done properly with 3 prong receptacle tester.
The problematic outlet is NOT on the same circuit,, only get 40 Volts between hot and neutral.
The outlet is near the floor and above it at shoulder level, there is a switch for outdoor light and this works. \I tracked and found the 40 Volts come from the box containing the switch.

I expect to see 3 Romex: an active Romex goes in to bring in electricity. The second Romex goes down to the outlet below. And the third Romex goes to the light outdoor. And the connection should be as below:
The active black should be connected to the black going to the outlet and connected to the switch, the second terminal of the switch is connected to black wire for the light, All the 3 white wires are connected together and so are the 3 ground wires. Pretty straight forward. Right ?

However, what I see is something very abnormal: there are actually 3 white wires connected together, 2 of them have been stripped off in the middle about 1 inch exposing bare copper, the intact one goes to the outlet below. The 4-th white wire goes to the outdoor light and is connected to the black wire going to the outlet below !!! This wire also goes into a terminal of the switch, the other terminal is connected to the black wire going to the light outdoor.

Sorry for the long long description.

OK, I went ahead to take a picture of the connection and disconnect ALL at the switch box.
To my surprise, no wire carry electricity EXCEPT the white wire that goes to (or comes from) the light outdoor. I measure the voltage of this white wire to the other 3, I got 40 volts. That how the outlet below gets 40 Volts.

I have searched the Internet and many people have this same issue, but no one seems to have a solution. All just described the problem briefly.

I greatly appreciate any help I could get. (pictures will be added)

 

[Maji236]

Does the voltage change when you turn the switch on?
It might be that it's using the neutral as a switch leg.

 

[Lucien Nunes]

Check the outside light to see whether there are more cables there. If there is a power feed direct to the light, one possible reason for this odd setup is that what was just a switch at first has now been modified to add outlets. The black permanent hot supply yo the light would correctly be looped through the light to the switch via the white wire that you say has voltage on it. The switched hot would return to the light via the black. Then someone has come along and picked up a neutral from somwhere and brought it to the switch box, so that the white permanent hot from the light cable, and the white neutral from a separate run of Romex provide power to the outlet.

In that case the likely cause of it stopping working is that there is a bad connection in the light or before. As Maji236 says, if when you apply some load at the outlet or light the voltage collapses, it's 99% certain a broken connection.

 

[ktnwin]

Check the outside light to see whether there are more cables there. If there is a power feed direct to the light, one possible reason for this odd setup is that what was just a switch at first has now been modified to add outlets. The black permanent hot supply yo the light would correctly be looped through the light to the switch via the white wire that you say has voltage on it. The switched hot would return to the light via the black. Then someone has come along and picked up a neutral from somwhere and brought it to the switch box, so that the white permanent hot from the light cable, and the white neutral from a separate run of Romex provide power to the outlet.

In that case the likely cause of it stopping working is that there is a bad connection in the light or before. As Maji236 says, if when you apply some load at the outlet or light the voltage collapses, it's 99% certain a broken connection.

Thanks for your input. I replaced the light fixture there myself about 1 month ago and there are 3 standard wires, black, white, ground. I did verify when all these 3 wires are disconnected that: black is hot when switch is on, white and ground has no electricity. So I hooked up as with any other new light fixtures I installed (or replaced). About 6 months ago, I replaced the receptacle in this outlet with new one (I actually did for all outlets inside the house, except garage), and did verify that it works properly with a receptacle tester. Possibly, when wiring the new light fixture, something went wrong, but the light works so I did not pay attention until the outlet cannot feed my Xmas lights.
I do intend to take out the light fixture and disconnect these 3 wires again. Then check the outlet voltage.
I like to identify at the switch box which black wire is the main live feed. That must be the key to solve this issue.
Few weeks before, I have added outlet below a switch in my garage and knew what to expect.

 

[ktnwin]

Thanks for your input. I replaced the light fixture there myself about 1 month ago and there are 3 standard wires, black, white, ground. I did verify when all these 3 wires are disconnected that: black is hot when switch is on, white and ground has no electricity. So I hooked up as with any other new light fixtures I installed (or replaced). About 6 months ago, I replaced the receptacle in this outlet with new one (I actually did for all outlets inside the house, except garage), and did verify that it works properly with a receptacle tester. Possibly, when wiring the new light fixture, something went wrong, but the light works so I did not pay attention until the outlet cannot feed my Xmas lights.
I do intend to take out the light fixture and disconnect these 3 wires again. Then check the outlet voltage.
I like to identify at the switch box which black wire is the main live feed. That must be the key to solve this issue.
Few weeks before, I have added outlet below a switch in my garage and knew what to expect.

Today, I spent 2 hours to understand what is there in the switch box and the outdoor light box.\
Then spend another hour to draw the 2 diagrams, one each.

The light box contains 2 ROMEX cables, upper and lower as shown in the diagram below.
Voltage measurements (all wires are disconnected, light fixture removed), are also included in this diagram.
Much better than 1000 words.
The lower ROMEX appears to be the feed line.
MYSTERIES: 17 Volts between neutral and ground
40 Volts between hot and ground.

Connections are presented by yellow lines.

And the switch box also contains 2 ROMEX cable, upper and lower.
It is interesting to note that only 40V is measured between neutral and ground of the upper cable.
No voltage (0 Volt) everywhere else. The diagram clearly shows why the outlet has only 40 Volts between hot and neutral, same for hot and ground.
Pls note: crossing yellow lines without the yellow DOT, mean no connection.

FOR NOW: I have wiring information of these two boxes, still have two mysteries mentioned above.
That is where I am. Will continue to study the diagrams to see if they are done correctly or not.

Any help & idea are greatly appreciated. THANK YOU.

.

 

[Lucien Nunes]

The wiring in the light box appears to be normal and correct (although the voltages are not). The black power feed hot goes to the white of the switch leg (ideally there should be a black tape marking this as hot) and the switched hot returns, goes through the lamp and back towards the panel via the white neutral. The ground is connected through. The wiring in the switch is correct as far as the switch leg is concerned, again the white being the hot should preferably be marked. The hot arrives on the white, passes through the switch and returns to the light via the black.

The problem seems to be a broken ground wire upstream of the light, which will not affect the operation of the light, but due to the way the outlet is wired, it could be particularly dangerous. The outlet gets its black hot from the white hot of the switch leg, but then it uses the ground of the switch leg as both its neutral and its ground, since there is no neutral available at the switch position. The voltage suggests that the ground wire is broken / disconnected upstream of the light, therefore current from the outlet neutral trying to return to the panel could make exposed metal that is supposed to be grounded, live instead. It is a more serious issue than a broken wire that is just a ground. THERE IS A HIGH RISK OF ELECTRIC SHOCK AND I RECOMMEND DISCONNECTING THE OUTLET, AND HAVING THE CIRCUIT CHECKED BY A PROFESSIONAL.

I don't know whether using the ground wire of a general purpose branch circuit as a neutral was ever acceptable under the NEC, but in the event of a cable or connection failure as it seems you have here, it causes an increased risk of electric shock. There have been exceptions such as 3-prong 240V dryer outlets but they are deprecated now. Perhaps a US-based contributor can confirm whether this configuration ever complied with code? Combined neutral/ground conductors in branch circuits have at times been permitted in some countries but not here in the UK with normal house wiring systems like Romex.

 

[ktnwin]

Hi Lucian Nunes,

"but due to the way the outlet is wired, it could be particularly dangerous"

The outlet is wired based on wire color. However, as you said, I have disconnected the wires in the switch box that go into this outlet. So the outlet has NO POWER at all, and marked it as such. I do not like to see an outlet with wrong voltage.

And the wiring at the light box appears weird to me but after checking, it is correct as you said. I found a picture on the Web showing wiring diagram when the power source is at the light box. It is exactly as you said. Will add black tape on both ends to show that white wire is hot.
I will locate the upstream outlet/light and continue to troubleshoot this issue for my own learning experience. My career was in electronic and software design and I still have the patience to work on this. I have retired by the way.

 

[ktnwin]

OK, I have identified all the outlets and switches related to this issue. At these outlets:
hot to neutral is 120 V
hot to ground is about 40 V
neutral to ground is about 18 V
Pretty much what I see at the light box.
One of the outlets has a power strip plugged in. I unplug it and measure again at all outlets in the same circuit:
hot to neutral is 120 V
hot to ground is about 2 V (should be 120V)
neutral to ground is about 2 V
This power strip has good continuity in all its hot, neutral and ground, no short. But I will leave it unplugged for now to continue my troubleshooting.
In both cases the receptacle tester shows OPEN GND on all these outlets and that makes sense to me.
At the switch box of the same circuit that is closest to the circuit breaker, I got ~2 V between hot and gnd.
So the issue appears to be at the panel. It's already dark so I will check this tomorrow morning.

 

[ktnwin]

One small update: I plug a different power strip into the outlet and the voltage measurements are identical as if no power strip is plugged in.
hot to neutral is 120 V
hot to ground is about 2 V (should be 120V)
neutral to ground is about 2 V

 

[Lucien Nunes]

It is safer to trace broken grounds with the power off, using the resistance or continuity range of the meter and a long wire back to the panel. Hopefully you will find the bad connection right at the panel but if it is more involved, I strongly recommend doing that rather than looking for voltage with the circuit live. I know US and UK practice are different here, in the UK we have a rule that a circuit must not be switched on until the resistance of its ground is measured and recorded as satisfactory.

 

[ktnwin]

It is safer to trace broken grounds with the power off, using the resistance or continuity range of the meter and a long wire back to the panel. Hopefully you will find the bad connection right at the panel but if it is more involved, I strongly recommend doing that rather than looking for voltage with the circuit live. I know US and UK practice are different here, in the UK we have a rule that a circuit must not be switched on until the resistance of its ground is measured and recorded as satisfactory.

Lucien,
thanks for your suggestion on how to detect bad ground connection. At present time, I feel relieved as the mysterious 40V and 17V went away after unplugging the (questionable) power strip. Other power strips do not exhibit this problem when plug in the same outlet. I hope open ground is easier to trouble shoot and fixed. When I started my career in USA, I worked as electronic technician, so I am very familiar with electricity. I learned about residential electricity recently to know about circuit breakers, outlets, 2-way and 3-way switches, GFCI, junction box and how to install them up to code. I join this forum to get advise from experts before I do any electrical work. I wish I joined much sooner.
That is why I feel comfortable working on this issue.
My home is 40 yrs old and there are quite a few things that are not up to code which I plan to upgrade one by one. For example: no GFCI in kitchen countertop outlets, no GFCI in garage outlets, these can be fixed easily by using GFCI breaker for these circuits. Another example is: garage is required to have one dedicated line 120V, 20A. My garage has 3 lines, two of them are 120V, 15A, one line is dead, no power and I have to troubleshoot this issue later. I am hoping this is a 20A circuit (yellow ROMEX instead of white).

 

[ktnwin]

Here is the updated wiring diagram, much easier to understand. Please disregard the diagrams in previous posts. I am also creating a video showing why I got 40 Volts at outlet.

From previous updates, I identified the issue is mainly caused by open ground circuit in my bedroom, this circuit also feed the outdoor light which is on the wall outside my bedroom. The mysterious 40V and 18V goes away when I unplug a power strip from the outlet in my bedroom. Other power strips do not exhibit this symptom. And the power strip above do not cause any voltage anomalies when plugged in a good outlet.

Here is some updates on open ground troubleshooting:
1) I open the breaker panel, track the black wire (hot) at the target circuit breaker until I reach the ROMEX cable that enter the panel, I can see the bare ground wire here
2) measure voltage between this black wire and its corresponding ground, got 120V
So there is no open ground in the panel.

The panel is near the door going in the hall way that goes straight to my bedroom.
There is a switch near the door, I opened and measure voltage between hot and ground: 120V, no issue
This switch controls two lights on the ceiling. I remove the clear cover, remove the light bulb (which is ON), and measure the voltage between hot (middle part of the light socket) and neutral (internal side of the socket: 120V, then between hot and the metal box: 120V. No problem with these two lights.

Next is the bedroom entrance, near the door there is a switch to control one ceiling light (of the bedroom hall), This is a 3 way switch. I noticed the circuit detector shows the ground to be hot whether it is connected to the ground terminal of the switch or not !!! So the wiring (which is on the attic above) must be wrong. At the light controlled by this switch, the metal box (ground) is also hot !!!

I will have to climb up the attic (been there before to run Ethernet cable) to track down the bad wiring.
I feel that I am closing on the culprit J-BOX in the attic.

 

[ktnwin]

Today, I verified at outlets in my bedroom (that have open ground), the ground wire does turn my circuit detector turning from GREEN to RED. that is the same as the ground wire at the switch. I feel better as there is only one problem to deal with.
I climbed to the attic and reverified the voltage between hot and ground of the light in the hallway (120V so good, no open ground) and the light on my bedroom hallway (about 2V meaning open ground).
I use a ROMEX cable between the junction boxes of these two lights, only connect the ground wire, leaving the black/white not connected. With this change, all outlets in my bedroom tested to be good per outlet tester.
I posted a youtube video showing details of this issue covering: description of the problem, wiring diagrams, all the steps I took to locate the culprit, and once found, how I fix it.
Here is the youtube link

 

[Lucien Nunes]

I think what you have now is a functioning circuit that is still potentially dangerous and not code-compliant. As I understand it, the original outlet in question is running its neutral current down the ground wire of the cable to the nearby light, which is now going across your link to the other light and some unspecified route of ground wires back to the panel.

Could @Megawatt, @Maji236 or someone US-based please comment?

 

[ktnwin]

Hi Lucian,

I totally agree with you on this outlet that has no neutral.
Connecting neutral to ground is only done at the main panel, and not allowed in any other place.

I will remove the receptacle and put a blank cover on this outlet. The ROMEX cable from the switch box above to this outlet box is not connected to anything at present time. Can I leave it this way or must do something about this cable ?

Ideally, I could remove this cable, remove the outlet box, do some drywall work to patch up the outlet opening then paint it. This is much more work but still within my DIY abilities.

I have removed more than a dozen low power devices embedded in the walls (Radio , Intercom, Alarm systems, their power adapters, alarm sensors) of this home and patched up the wall openings when my home went thru completely remodeling / painting, etc ... 9 months ago. Those are state of the art 40 years ago and now become state of the lost art.