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Possible loose neutral?

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

Reply to
mroberds
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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
Reply to
operator jay

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.

Reply to
Charles Perry

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.

Reply to
Don Kelly

All,

Thanks for the advice! To clarify something that was asked a few times, all of the numbers in the chart I gave were measured directly at the breaker panel. I took the cover off the panel and put one meter lead on the neutral/ground bus bar and the other meter lead on one of the two screw terminals where the hot wires from the meter are connected.

If it is a loose neutral, it almost has to be on the power company's side. My breaker panel is right on the other side of the wall from the meter... the supply wires into the breaker panel enter through the rear of the box and I presume they are coming out the rear of the meter can as well. The neutral connection to the neutral/ground bus bar in the panel appears to be tight.

I will call the utility in the morning and see what they have to say. I'll post back with further developments.

Thanks!

Matt Roberds

Reply to
mroberds

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

Reply to
daestrom

Reply to
w_tom

Short version: Utility claims they fixed something and the test results are better (3.1 V difference as opposed to 9.4 V), but I'm still not sure what exactly they fixed.

Long version:

I called the utility on Monday afternoon about 2 PM, then hung around the house, hoping to catch the lineman and hear what he found, if anything. The utility did try to call at 7:03 PM per the Caller ID. I was in another room and the machine picked up before I did; I stopped the machine and picked up and got silence. By 9 PM I hadn't heard anything, hadn't noticed the lights going out, and I hadn't retested, so I decided to call again today (Tuesday).

I called today at about 2 PM and the call center rep said that they came out yesterday and "repaired a connection at the weatherhead". To me, the "weatherhead" is the thing at the top of the conduit running up from my meter, where the wires make almost a 180 degree turn and go out through a plastic disc with holes in it. I went out and looked at it from the ground and couldn't see anything different. I can't see my connections at the pole very well, so I'm not sure if anything changed there.

I decided to retest. I measured again at the breaker panel, with most of the loads in the house shut off except as noted. The results were:

A/C uWave left right l-r notes off off 120.4 118.7 1.7 run 1 off on 120.9 117.8 3.1 on off 118.7 118.2 0.5 on on 119.0 117.1 1.9

off off 121.2 119.7 1.5 run 2 off on 121.5 119.0 2.5 on off 119.5 119.5 0.0 on on 120.0 118.2 1.8

The 3.1 volt spread does seem to be an improvement from the previous result, which was a 13.1 volt spread (-3.7 to +9.4).

I climbed up on the roof and inspected the connections at the top of the meter conduit. I have an insulated crimp splice on each hot wire, and a bare crimp splice on the neutral wire. I'm pretty sure that the hot wire splices weren't changed, as each one has a small crack in the insulation that has been there for a while, and I'm assuming the lineman would have replaced the insulation sleeve if he installed a new connector. I am not sure if the bare neutral splice was changed; even though I haven't inspected it carefully in the past, it doesn't _look_ brand new - there are some black spots (oxidation?) on it, and the engraved markings are easy to read because dirt seems to have accumulated in the grooves. There weren't any telltale bits of wire lying on the roof or shiny places on the wire next to the connector, either. The thing that holds the mechanical tension on the bare netural wire (basically a ramp on the wire and a mating ramp with a metal loop around the conduit) hasn't been changed.

I'm also pretty sure that they didn't do anything to the meter. First, my lights didn't go out. Also, even though I didn't write down the serial number on the meter seal before I called, it's at least the same type of seal, and the metal loop through the meter can tab isn't shiny as I would expect with a new seal.

It's entirely possible that the bad connection was at the pole and I'm mistaken about where the "weatherhead" is, or that the path from the lineman to the call center four states away is lossy and noisy. I'm curious to know what exactly was repaired, but the test results seem to show an improvement, so maybe I should just be happy.

Thanks to all for the advice and assistance!

Matt Roberds

Reply to
mroberds

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