Possible loose neutral?

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All,
Short story: On a US-standard 240/120 100 A service, I am seeing 8 to 9 volts difference between phases when a 120 V 1.3 kW microwave is running. Is this excessive, or acceptable?
Long story:
For the past few mornings, the UPS (APC Smart-UPS 600) in my office has been switching on to battery power for a few minutes, even when the lights in the rest of the house hadn't flickered or gone out. I finally put a meter (Metex ME-11 DMM) on the wall outlet supplying the UPS while the UPS was running and found out it was probably tripping on _over_voltage: the wall socket was delivering over 130 volts!
After a bit of experimentation, I discovered that the voltage went up when the microwave in the kitchen was running, which is why it happened in the mornings when the other half was making tea. The microwave is on one of the 20 A kitchen circuits, while my office is on a 20 A receptacle circuit with a couple of other rooms. It's a GE microwave, about 10 years old, 120 V 1300 W per the nameplate. I also noticed that the voltage rise was less pronounced when the central air conditioner was running. The compressor is 240 V and is fused and breakered at 40 A.
Suspecting a loose neutral, I inspected the breaker panel. Nothing looked out of place. I tried tightening all of the screws on the neutral/ground bus bar. They all took a bit of tightening - less than 1/8 turn - but none were really badly loose. I made very sure that the neutral coming in from the meter was tight at the bus bar. I have the US-standard 240/120 service at, I believe, 100 amps, and a GE split-bus panel. It is original to the house (1969), although the breakers have all been replaced. To my knowledge, the kitchen circuits are two separate 20 A circuits with their own NM cable, not split like a Canadian kitchen circuit.
After I tightened the screws, I went around in the house and made sure most things were turned off. There were still a few computers running, but the TV, stereo, lights, etc were all off. The air conditioner was also shut off. I measured the voltage between the neutral/ground bus bar and each hot lug with the microwave off and then with the microwave running, heating about 8 oz (230 mL) of water in a coffee cup. I then turned the A/C on and tried again. Results...
A/C uWave left right l-r notes off off 122.5 123.2 -0.7 run 1 off on 126.3 117.6 8.7
off off 121.9 121.3 0.6 run 2 off on 126.1 116.7 9.4
on off 119.6 123.3 -3.7 on on 123.7 118.5 5.2
I know that the two sides of the service will very rarely be in exact balance, but the 8 to 9 V difference seems somewhat high. Is this considered within reasonable bounds, or should I have the power company out? Or should I chase the wiring in the house further? When I moved into the house 5+ years ago, most of the receptacles were original and back-wired. I changed all of them that are regularly used for new spec grade side-wired receptacles.
Out of curiosity, I measured the voltage drop across each breaker. This was with most of the loads shut off, so it may not be too useful, but I found no breaker over 100 mV and most under 40 mV.
Let me know if you need more information. Thanks for your help!
Matt Roberds
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The gain on the hot leg opposite the microwave is about 1/2 the loss on the microwave side. This make sense because the the neutral leg is 1/2 the circuit for the microwave.
Let me get this straight - you are measuring at the panel? If so, the gains and drop seems excessive. John

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On Sun, 05 Jun 2005 00:03:03 GMT, snipped-for-privacy@worldnet.att.net wrote:

Call the utility. This looks to be on their side.
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It is too high but I doubt the power co will help and they may cause you more problems. I'd try to find a smart, honest electrician.
N
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snipped-for-privacy@worldnet.att.net wrote:

I am not sure it is a loose neutral, although it could be. I don't believe it is just one circuit, as the change is too little for that, it may be the whole house or maybe the whole circuit with your neighbors. I would not rule out a couple or more of those back wired devices causing at least part of the problem. It may well be on the hot side as well as the neutral.
--
Joseph Meehan

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Those voltage changes are excessive. Furthermore, those voltages may have been larger than 9 volts, but for earth ground which is keeping those voltage differences from becoming excessive. IOW it may be a human safety problem. gfretwell has provided an accurate solution that should not be ignored. You have numbers and can reproduce the problem. The utility therefore would have no problem finding and fixing the problem the first time. Symptoms that are unacceptable and should not be ignored.
This post assumes those numbers were recorded at the GE panel.
snipped-for-privacy@worldnet.att.net wrote:

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Matt,
I think you have a run-of-the-mill power quality issue that your APC caught. The microwave appears to be causing the problem.
Microwave tubes fire at a very high frequency and can send AC noise back into the whole system. Your APC is seeing this and reacting.
Solutions: Put the APC or the microwave on a different phase (switch breakers around) and see if this helps. Otherwise, a more modern microwave with better filtering maybe in order.
Jake

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sounds very much like a high resistance in the neutral wire to the main panel to me. id give the power company a shout first. might be a connection gone bad in the meter or something.
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Hello Matt,
Sounds like a problem at or upstream of your panelboard. Your service voltage should stay pretty much unchanged if a smallish load like your microwave goes on or off. You could put a call in to your utility. Under the circumstances, hopefully they'd send someone to have a look for free.
130V at your outlet is pretty crazy, too. We need a second opinion but I would think that problems with a neutral connection should not show that much effect, except maybe at the end of a circuit using that neutral (ie. the circuit using the neutral or the multiple circuits using it if in fact it is shared). Again, though, a neutral problem at or upstream of your panelboard could cause this trouble.
Wait for a few more opinions though. I wouldn't take my word on this stuff.
j
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<snip>

It sounds a lot like a loose neutral. The loose connection could be anywhere from his panel up to the service transformer, including inside his service panel.

Wrong. I have seen much greater than 130V with loose neutral problems. Theoretically, you could get nearly 240V on one leg and zero on the other but realistically it doesn't get that bad.
Have a qualified electrician check the connections in your panel to make sure they are tightened to the correct torque specifications. If that does not help, contact your utility. Most will have a device that they can temporarily put in the meter base to determine if a loose neutral exists on the service drop. They remove your meter, and plug in the device. It has two voltmeters (one for each leg) and a large load that they can switch between the legs. Simple and effective.
Charles Perry P.E.
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You have it right on. I also have seen this problem. Another characteristic is that adding loads cause further unbalance. If you happen to have a couple of 100W lights on one circuit and a toaster on the other, the voltage distribution can get quite hairy as you indicate.
--
Don Kelly
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Ditto.
After a truck knocked a ground installed 240/120 transformer for my neighbor and my house, I was having trouble with lights getting bright when the furnace started. Called the utility right away, explained that they had just re-set the transformer the night before, and what my lights were doing.
Service man showed up within the hour. Cut seal on meter and read voltages there. Had me just turn on the coffee pot. That load was enough imbalance that he could see there was a problem (and that it was on the utility side of the service). Whole line-crew showed up within the next hour and found the problem. A loose neutral when they re-installed the xfmr.
While talking with the guys, mentioned that it was real nice of them to come out after 6:00 PM and all, but it could have waited. They replied that faulty wiring on their side is a big liability issue and they had to get to it ASAP. In the end, didn't cost me a dime.
daestrom
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Neutral failed inside pole transformer. Homeowner had a defective earth ground. Therefore house obtained neutral through the gas meter - until gaskets eventually failed. Fortunately no one was home when the house exploded. Neutral failures and missing/defective earth ground should not be treated lightly.
daestrom wrote:

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snipped-for-privacy@worldnet.att.net wrote:

i would check the ground electrode where the fuse box is that goes inside. make sure it has a good seat to the earth. secondly, it wouldn't hurt to run another ground rode on the other end of the house and connect it to the ground in your romix of the outlets some where but not to the neutral because that is not legal. the neutral shall only be reconnected to the earth ground in the box. unless codes are different where you live!
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ees right! run the romex to the ground on the other side of the box from where the wires come out of the box. Romix is tricky tho, so run a wire from the one wire that is hooked to the romix in the box where the other wires are.
Then hook it in somewher to an outlet somewher but not before the place where an outlet hooks to something, and make double sure you dont hook it to something that hooks to something before the box with the wires in it hooks up to the other thing.
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On Sun, 05 Jun 2005 00:03:03 GMT, snipped-for-privacy@worldnet.att.net wrote:

I had a similar situation, but even greater voltage difference with even smaller loads. Found it to be present at the panel. Sure looked like a loose neutral in the 120/240 service. I was pretty certain it was power company connections. Called the power company. Fortunately I was an electrician in a previous career and left a note on the meter telling sparky what I thought the problem was. I was real scared that they would blow me off figuring I was some homeowner nut case. Thank goodness when they came they found the neutral loose in their meter section and fixed it pronto.
Steve J. Noll | Ventura California | The Used Equipment Dealer Directory: | http://www.big-list.com | The Peltier Device Information Site: | http://www.peltier-info.com
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mrobe: You should immediately call up your Power Company and have them come out and check the service to your meter.... they are responsible for that... and may even check everything up to your breaker box if they are nice guys. I had a similar but more catastophic problem with a previous home I owned; it turned out that their Neutral connections were faulty from the power pole to the meter.... they did come into the home and check everything in the breaker box as a courtesy. I was blowing out light bulbs and eventually blew up a microwave oven. Don't let this go... it will not get better, but only worse to the point of destroying some of your appliances and equipment.... call your power company right away. electricitym - - - - -
snipped-for-privacy@worldnet.att.net wrote:

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snipped-for-privacy@worldnet.att.net wrote in
Matt, I'd call my utility company and have them check the neutral from your meter back to the transformer. Had a similar situation with increasing voltages on one leg, turned out to be a loose neutral at the transformer.
--
Anthony

You can't 'idiot proof' anything....every time you try, they just make
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On Sun, 05 Jun 2005 00:03:03 GMT snipped-for-privacy@worldnet.att.net wrote:

Go back and do this test again, but this time measure everything at the breaker box entrance. That will allow you to determine whether the cause is inside or outside your house.
Be careful!
- ----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA -----------------------------------------------
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On Sun, 05 Jun 2005 00:03:03 GMT, snipped-for-privacy@worldnet.att.net wrote:

Before jumping to any conclusions, I would beg borrow or steal an analog VOM and retake the measurements. A Simpson 260 leaps to mind.
DMMs sample the waveform and try to calculate the RMS voltage, but spikes on the waveform can fool the mathematics giving you false readings.
The power supply in the microwave might be putting spikes onto the line and the meter is sensing them.
Another thing to try would be to open *all* of the breakers on one phase so that the only return has to be the neutral. If the voltage now goes *down* with increased load, i.e. when you turn on the microwave, then the neutral is suspect.
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