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docaudio
12-01-2008, 08:12 AM
I'm trying to sort out an intermittent issue with a bedroom and bathroom feed.
The home was built in '84 - Big Bear Lake, CA. Two downstairs bedrooms and a small bath are feed from one 20A run. One bedroom is fine, receptacles test OK.
Second bedroom and bathroom receptacles show 25v neutral to ground, 90v neutral to hot, and 120v hot to ground. I also have lighting in a hall and the bathroom that is not working (bulbs are good) The problem comes and goes, but have not found any pattern. It's a vacation home, so my time is a bit limited to troubleshoot.
I thinking I have a neutral that is floating, but I'm not sure.
Thanks for any ideas!
-Bill

jar546
12-01-2008, 08:18 AM
The circuits need to be verified with a megohmeter or "megger". That is the better and safer course of action for diagnosing a potential problem.

jwelectric
12-01-2008, 08:23 AM
I'm trying to sort out an intermittent issue with a bedroom and bathroom feed.
The home was built in '84 - Big Bear Lake, CA. Two downstairs bedrooms and a small bath are feed from one 20A run. One bedroom is fine, receptacles test OK.
Second bedroom and bathroom receptacles show 25v neutral to ground, 90v neutral to hot, and 120v hot to ground. I also have lighting in a hall and the bathroom that is not working (bulbs are good) The problem comes and goes, but have not found any pattern. It's a vacation home, so my time is a bit limited to troubleshoot.
I thinking I have a neutral that is floating, but I'm not sure.
Thanks for any ideas!
-Bill

If the grounded (neutral) was open then the circuit would not work at all.
Without being there to actually see and test everything there is no way that proper advice could be given on how to find the problem.

My most sincere advice is to hire someone to fix the problem. Chances are that you could stumble on the problem and even cure the problem but without more information such as the lay out of the circuit anything said would be only guess work.

Start with what you think is the last junction be it a light or receptacle that is working properly and work downstream from that point if everything is on the same circuit.

beekerc
12-01-2008, 09:45 AM
<quoted from quote>Second bedroom and bathroom receptacles show 25v neutral to ground, 90v neutral to hot, and 120v hot to ground.

JW,
for the benefit of those of us who don't have formal electrician's education, what should we expect to see on a properly wired 120v circuit?
this would be my (un)educated guess - please correct as needed.
hot to neutral - 120v
hot to ground - 120v
neutral to ground - 120v (for a receptacle) or 0v (if hot is switched off up-line)
or if i'm way off, please clarify.
Thanks
BeekerC

in general, what can you say about potential causes for a voltage drop on any of the three wires?

Thatguy
12-01-2008, 10:05 AM
25v neutral to ground,
90v neutral to hot, and
120v hot to ground.
not working (bulbs are good)


There is cable capacitance, from each conductor to each other conductor. The 90/25 represents the ratio of capacitance reactance, at 60 Hz, from N to H and from N to G (but, I'd be more comfortable with these measurements if the N measured 60v to H and 60v to G).

Assuming you used a high input impedance meter (what make and model is your meter?) to measure these, I think the neutral wire to this outlet is floating.

To confirm, if you switch on the non-working bulbs the N should then read 120v w/respect to G. Alternately you could connect an incandescent lamp from N to G and have the 25v go to 0v.
If this doesn't happen, more testing is req'd with a known good outlet and an extension cord.

Cookie
12-01-2008, 10:30 AM
There is cable capacitance, from each conductor to each other conductor. The 90/25 represents the ratio of capacitance reactance, at 60 Hz, from N to H and from N to G (but, I'd be more comfortable with these measurements if the N measured 60v to H and 60v to G).

Assuming you used a high input impedance meter (what make and model is your meter?) to measure these, I think the neutral wire to this outlet is floating.

To confirm, if you switch on the non-working bulbs the N should then read 120v w/respect to G. Alternately you could connect an incandescent lamp from N to G and have the 25v go to 0v.
If this doesn't happen, more testing is req'd with a known good outlet and an extension cord.

Thank you for replying to the OP, I hope with that information the OP will be able to solve his problem.

jwelectric
12-01-2008, 10:48 AM
JW,
for the benefit of those of us who don't have formal electrician's education, what should we expect to see on a properly wired 120v circuit?
this would be my (un)educated guess - please correct as needed.
hot to neutral - 120v Yes except it would be from ungrounded to grounded
hot to ground - 120vYes except it would be from ungrounded to equipment grounding
neutral to ground - 120v (for a receptacle) or 0v (if hot is switched off up-line)The voltage down strean of the service should be -0-volts between the grounded and equipment grounding conductor no matter where the reading is taken
or if i'm way off, please clarify.
Thanks
BeekerC

in general, what can you say about potential causes for a voltage drop on any of the three wires? The votlage drop across the entire circuit will equal the applied voltage.
In the problem above there are several reasons that he is getting the voltage readings he is getting. It would take the rest of the day to cover each and every one of them here which I do not have the time for.

For ease in answering your question I answered each one beside each using red font

docaudio
12-01-2008, 11:03 AM
Hey 'Thatguy', that makes a lot of sense. BTW, the meter is a Triplett 9000.
I have most of the walls opened up (drywall replacement) so I'll try to find the problem neutral and trace the run where it goes bad. I know it's not enough info to know exactly, but it seemed to me that it was a flaky neutral somewhere.
Thanks to all for the responses!
-Bill

jwelectric
12-01-2008, 11:08 AM
There is cable capacitance, from each conductor to each other conductor. The 90/25 represents the ratio of capacitance reactance, at 60 Hz, from N to H and from N to G (but, I'd be more comfortable with these measurements if the N measured 60v to H and 60v to G).

Assuming you used a high input impedance meter (what make and model is your meter?) to measure these, I think the neutral wire to this outlet is floating.

To confirm, if you switch on the non-working bulbs the N should then read 120v w/respect to G. Alternately you could connect an incandescent lamp from N to G and have the 25v go to 0v.
If this doesn't happen, more testing is req'd with a known good outlet and an extension cord.

Would not the capacitance between two insulated conductors be different than between one insulated conductor and one uninsulated conductor?
If it is capacitance between the conductor that he might be reading with a digital meter it would all go away with a analog or solenoid meter would it not?

If he is reading 120 volts between the grounded (neutral) and the equipment grounding conductor on a non working light then there would be a wire lose somewhere in the circuit is that not true. Remember that the grounded and the equipment grounding are bonded together in the service equipment assuming that the system is installed correctly. If this is the case then it wouldn’t work at all would it?

I agree with your last statement, “more testing is required.”

Thatguy
12-01-2008, 02:02 PM
but since it's a digital meter I assume it's at least 10 MΩ.


Would not the capacitance between two insulated conductors be different than between one insulated conductor and one uninsulated conductor?
Yes. The capacitance between the two insulated ones would be lower because the spacing is higher and so the voltage would be higher, if the dielectric constant of the insulation is about equal to that of air. If higher spacing cancels out higher dielectric constant, then there'd be no difference.
I don't have a C meter so I measured just once between two of the three conductors; I don't remember which. With 120v applied, the current to ground should be about 120/500 of a mA (see below).
If it is capacitance between the conductor that he might be reading with a digital meter it would all go away with a analog or solenoid meter would it not?
The capacitive reactance for 50' of Romex is about 500kΩ at 60Hz.

With analog meters of low sensitivity, like 1000Ω/volt, the reading changes depending on what scale you're on, with the higher voltage scales reading higher voltages. You keep going to a lower scale, and the reading keeps getting smaller. It's because this is acting more like a "current source" than a "voltage source."
If a solenoid meter has an input impedance of ~5kΩ it would read 1% of the voltage that was there before the meter was connected.

A working outlet looks like a voltage source with source impedance at about 1/4 Ω.

I prefer using a 4w or 7-1/2w incandescent to load the circuit. If things get weird, I also use a 100w, 100 Ω rheostat.

If he is reading 120 volts between the grounded (neutral) and the equipment grounding conductor on a non working light then there would be a wire lose somewhere in the circuit is that not true.
?
I meant that if the neutral was floating, the voltage at the neutral should be pulled up to 120v w/respect to G because the lamp has much lower resistance than the cap reactance that is supplying voltage to the floating neutral.
It's a voltage divider, and the cold resistance of a 100w incand. lamp is about 10Ω, way less than the reactance.


Remember that the grounded and the equipment grounding are bonded together in the service equipment assuming that the system is installed correctly.
That is the first place I'd look for this loose connection. Maybe day/night temp. changes cause these connections to loosen over time.

If this is the case then it wouldn’t work at all would it?
?
The test or the circuit?

I agree with your last statement, “more testing is required.”

I think the fact that there is more than 2v or so between N and G (see page 5 of the link below)
http://www.idealindustries.com/media/pdfs/products/instructions/61-164-165_instructions_v4.pdf
means the neutral is not connected, for starters. Hopefully that is the only problem.
Usually I put a non-neon test lamp between H and N and H and G (and H and a cold water pipe if available). The lamp should light in all cases. It will trip upstream GFIs, though.
If this test gives me answers that don't correspond to my mental image of how the wiring should work, I look for a good ground to measure with respect to.

Cookie, it is my pleasure.
The taxpayers paid for some of my education, and they might as well get something for it.:p

seaneys
12-01-2008, 07:25 PM
but since it's a digital meter I assume it's at least 10 MΩ.



I think the fact that there is more than 2v or so between N and G (see page 5 of the link below)
http://www.idealindustries.com/media/pdfs/products/instructions/61-164-165_instructions_v4.pdf
means the neutral is not connected, for starters. Hopefully that is the only problem.
Usually I put a non-neon test lamp between H and N and H and G (and H and a cold water pipe if available). The lamp should light in all cases. It will trip upstream GFIs, though.
If this test gives me answers that don't correspond to my mental image of how the wiring should work, I look for a good ground to measure with respect to.

Cookie, it is my pleasure.
The taxpayers paid for some of my education, and they might as well get something for it.:p

Would a standard plug-in circuit tester also point out the problem? I don't usually use a meter to check outlets.

Steve

docaudio
12-01-2008, 10:16 PM
Would a standard plug-in circuit tester also point out the problem? I don't usually use a meter to check outlets.

Steve

Actually, that's what I started with and noticed that the center green light was dim when checking the problem receptacles.
-Bill

Thatguy
12-02-2008, 06:48 AM
Would a standard plug-in circuit tester also point out the problem? I don't usually use a meter to check outlets.

Steve
Yeah, I just don't own one.
They can correctly check about 1/3 of the problems resulting from the ~30 wrong ways to connect or not connect one, two or three wires to one, two or three terminals.

docaudio
12-03-2008, 07:18 AM
I went up yesterday to have another look at things and found a suspect receptacle with a loose neutral. Of course, it was nowhere near the problem area (upstairs, on a living room wall, instead of downstairs where the problem was) So, I have to now re-label the breaker panel with the correct circuit info. Actually, the breakers are much better divided throughout the house than the labeling would suggest.
Anyway, thanks for all your info and help!
-Bill

seaneys
12-03-2008, 03:59 PM
Yeah, I just don't own one.
They can correctly check about 1/3 of the problems resulting from the ~30 wrong ways to connect or not connect one, two or three wires to one, two or three terminals.

Ahh. So there is still a use for my Fluke!

Thatguy
12-03-2008, 05:20 PM
Ahh. So there is still a use for my Fluke!

Yes, but for household use you have to load it down with an incandescent lamp so phantom voltages don't register at all.

But with the lamp, you have to know if you're looking at 120v or 240v.

Try two 120v 4w or 7-1/2w lamps in series for all cases. Probably neither lamp will light on 120v but it still gets rid of phantom voltages.

I have a cardboard sleeve around mine so the other tools don't crush the glass envelope.