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Possible loose neutral?


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


Jan 1, 1970
"[email protected]" bravely wrote to "All" (05 Jun 05 00:03:03)
--- on the heady topic of "Possible loose neutral?"

mr> From: [email protected]
mr> Xref: aeinews

mr> All,

mr> Short story: On a US-standard 240/120 100 A service, I am seeing 8 to
mr> 9 volts difference between phases when a 120 V 1.3 kW microwave is
mr> running. Is this excessive, or acceptable?

Had a recent similar circustance with the microwave popping the
breaker every so often. Turned out the electric counter had an
electrode contact that was burnt in the socket. This after a long
diagnostic where the building entrance lines were replaced and a new
higher amperage circuit upgrade for the MW oven. It was a tough one to
find because it was intermittant and only had about a 1 volt drop at
full power. Yours seems much more straightforward with 9 volts. In
fact that is pretty severe! It may be your polepig's ground that is bad.


.... Dunno if we'll get that past the CSA und UL 'owever.
In said:
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?


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.


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

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

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