Low household AC voltage

[Ira]

A few days ago, I noticed that the AC voltage in my home is lower than it has been. I have three UPS's that display the voltage. I noticed about 10AM one morning that all three were showing about 100 volts, instead of the usual 120 (+/- a couple) volts. It gradually increased until later that night it reached about 117 volts. It seems to drop during the day (the UPS's are showing 107 now, at 4PM, didn't look earlier), and I'm guessing they will get back close to 120 volts tonight. Is this something to be concerned about. Note that all three UPS's are on the same circuit. The only "problem" I've seen is that it seems like the light in the microwave oven is dimmer than it usually is, but the MW seems to be working fine.
 
I thought about moving one to a different circuit (to see if it's "circuit related"), but it's a pain to shut down and restart the electronics that are attached. Wish I could find my multimeter.
 
Thanks,
Ira

 

[RAL]

Use a voltmeter to check other circuits in your house.  If any are higher than normal, say 130V, while other are low, you have an open neutral.  If you are comfortable doing it, remove the cover on your breaker panel and measure the voltage between the two hots, and from each hot to neutral.  If you see a difference in the two hot-to-neutral readings at the main breaker, then it is most likely the power company's problem.  You could also make these measurements at the outlet for an electric dryer, if you have one.

The voltage may drift back to a more normal level as other things are turned on or off and the loads on each leg of your electric service become more balanced.
 
An open neutral can be dangerous to you and to all of your 120V appliances.  If you turn on a high load device like a toaster or a hair dryer, it can pull that side of the service down to say, 60V, while the other side goes to 180V.  That can destroy expensive appliances like your refrigerator.
 
Make the measurements, and call the power company and tell them you have a power outage (not an open neutral).  That will get them there in a hurry.  In the meantime, I would turn off as many things as possible.

 

[LarrylLix]

This can be caused by where water meter techs that disconnect  ground connection around the water meter, or you never had one and the meter has been replaced with the new plastic type or plastic plumbing.
This shouldn't make a difference to your electrical service, but as Ral posted above, if your neutral has a bad connection the ground can substitute for the neutral until it becomes dry earth, poor connection, or becomes disconnected.
 
However, you likely need to get your utility involved so they can check connections inside your meter base as well as the neutral connection as your street transformer. Ask if they can check your grounding connections. Your property and your responsibility inside your home but most are good enough to give help where they can. Buy them a coffee.

 

[Ira]

Once again, as it get later in the evening/night, the voltage slowly increases. Right now, about six hours from my original post, the UPS's are showing 115 volts.

 

[pete_c]

@Ira,
 
I would find that VOM you were writing about or purchase one from Amazon and take some measurements.
 
Years ago here and right after I moved in and was adding circuits to my computer room (basement) I found a split neutral circuit.  (two circuits, 2 hot leads and one neutral lead).  I never understook why the shortcut was done.  
 
A few years ago a tree root shorted one of the main house leads to ground.  
 
It happened on a Sunday morning.  Electric company tech came over and bypassed the wire using an outside plug replacing the meter to use only two main leads of three.  Electric company had to dig under the tree to reconnect the wires just as the first winter snow fell.  Recalling now it was an everygreen tree about 3 foot tall when I moved in and becoming some 30 plus feet after 10 years.

 

[LarrylLix]

IN Canada the rules are 120Vac +/- 10%. That is 108 to 132 Volts. I believe that would apply to the US N.E.C also. Outside of that, the utility is required to make a correction.
 
Mind you we inherited many farms as we had rural customers dumped on us, at the long end of pole lines (miles of #6 copper) that got replaced later that went higher and lower than the specs. and it took years to give them decent voltage.

 

[RAL]

IN Canada the rules are 120Vac +/- 10%. That is 108 to 132 Volts. I believe that would apply to the US N.E.C also.

Here (US), the standard is +/-5%, so 114 to 126V.   I thought that applied to Canada, too, but perhaps I misunderstood.

 

[LarrylLix]

IIRC the +/-5% applies to the voltage drop on household circuits for conductor sizing in a building.

Utilities would never be able to deliver that tight of a spec. to your demarcation point. 10 percent is almost impossible for long runs across all customers in a 100-500 MVA transformer feed area.
Somebody is getting high voltage and somebody is getting low voltage. Just simple physics.

Hope this wasn't dupped many times.



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[RAL]

The US tolerance of +/-5% is actually an ANSI standard, and is the normal voltage to be delivered at the customer's service entrance.   The utilities can adjust voltages up or down along the distribution network through the use of taps on the distribution transformers so that they can meet this requirement. 
 
There is a second range, Range B, which is +6% to -13% (127V to 110V) that the voltage is allowed to be in for short periods due to infrequent events.  
 
Here's a pretty good overview.
 
If the OP is seeing voltages as low as 100V, he's below even the Range B limits.

 

[LarrylLix]

We very seldom adjusted the taps on distribution transformers as it caused bigger problems with high voltage under light loading, and the feeder could never be backed up from the other end of the feed, creating a less secure system, only being capable of a single source. Distribution transformers typically came with 0, +/-2.5%, and +/-5% taps. Not enough to compensate for long multi-drop feeds in the rural areas.
 
I really doubt utilities can actually achieve a +/- 5% delivery tolerance, everywhere. Very short feeder runs would have to be used, or extreme conductor sizes. We installed inline automatic tapchangers to compensate further for extreme cases, but they still could never maintain that tight of a variance after another mile or two of line length. They also made reverse feeding a circuit even more complex and can quickly make the system very unstable with ferroresonnance problems. We saw this on a few long runs where some farms got voltages ranges from 95 volts to 160 volts due to long runs and tapping up transformers. There is definitely limits for this technique in some applications.
 
The OP definitely should not be getting 100v but if they could even tap his transformer up 14% to get 114v his lighter load voltage would be 120v x 114% = 136.8v, enough to damage some household equipment, or cause some short life. He needs to move closer to the last substation.

 

[RAL]

he OP definitely should not be getting 100v but if they could even tap his transformer up 14% to get 114v his lighter load voltage would be 120v x 114% = 136.8v, enough to damage some household equipment, or cause some short life. He needs to move closer to the last substation.

 
I agree, the OP's problem is not something to be addressed through tap changes since he sees such a variation in voltage range.  If it is a utility problem, a voltage swing that large makes me think it could be an open neutral. Or, if it is just on one leg of his service, perhaps a high resistance in the hot wire feeding that leg.   Of course, at this point we don't even know if it is occurring on anything but that one circuit within the house.  Knowing what voltages he sees in the breaker panel would be very helpful in narrowing it down.
 

 

[Ira]

By the time I found my multimeter (about 18 hours after my original post), the UPS's were all showing voltages a little over 120VAC (now showing 122VAC). I haven't noticed them showing less than 120VAC since then. I think low 120's VAC is what they usually show. I checked quite a few outlets to hopefully include some off of both busbars, and all work showing the same voltage, +/- one volt.
 
I guess unless this happens to me again, we won't know for sure what was going on.

 

[LarrylLix]

By the time I found my multimeter (about 18 hours after my original post), the UPS's were all showing voltages a little over 120VAC (now showing 122VAC). I haven't noticed them showing less than 120VAC since then. I think low 120's VAC is what they usually show. I checked quite a few outlets to hopefully include some off of both busbars, and all work showing the same voltage, +/- one volt.
 
I guess unless this happens to me again, we won't know for sure what was going on.

In the electrical grid utility world there are times when problems or maintenance must be performed and feeder circuits may need to be fed from another end temporarily while the line repair or rebuild is done. This means people at the new far end of the line may get low voltages for a time. This can't be helped at times within a reasonable budget for some customers. Not nice but the occasional inconvenience to a area of customers, sometimes doesn't outweigh the cost of beefing up the system there.
 
I had low voltage for years in my new house until a solar farm went in across the highway and now when the sun shines I get 126V++. The utility must have boosted the conductor size now, as I don't see 104V  anymore though. I had to compensate my PV system backup as it was seeing below the "flip-over" voltage and using my batteries when the grid was still feeding.

 

[JimS]

You should do some checking of other circuits.  A poor ground connection isn't going to cause the issue.  But a bad neutral connection could.  Once had an old house and the voltages started going wonky.  Some were high and some low.  The neutral connection at the weatherhead was going bad so I still had 240V from line to line.  Was lucky nothing got damaged and the power company fixed the bad connection. 
 
Check with your neighbors if they have any issue.  Is it all your circuits or just a few?  You could take a large load (about 12A) like a blow drier or heater and plug it into various circuits and measure the voltage difference between when it is on and off to see what circuits may have bad connections. 
 
Since it varies with time of day it may be a brownout of the entire area.