Possible loose neutral?

[mroberds]

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

 

[JohnR66]

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

 

[gfretwell]

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?

Call the utility. This looks to be on their side.

 

[NSM]

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.

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

 

[Matt (archive)]

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.

 

[w_tom]

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.

 

[Jake (archive)]

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

 

[TimPerry]

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

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.

 

[operator jay]

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

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

 

[Jamie (archive)]

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

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!

 

[electricitym]

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
- - - - -

 

[Anthony (archive)]

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.

 

[Jim Adney]

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

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!

-

 

[Charles Perry]

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.

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.

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.

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.

 

[Don Kelly]

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.

 

[mroberds]

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?

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

 

[Art (archive)]

IMHO: Sounds that you have imbalanced circuit design within the residence also, along with the possible loose neutral. Most moderately high current devices in the kitchens and utility areas of a residence have priority circuits specified to them, and are connected to the service in a way that they cause minimal un-balancing of the overall system.

 

[NSM]

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

Perhaps you have a grow op next door!

N

 

[daestrom]

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.

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

 

[w_tom]

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.