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