I live in an older house in New England. I bought an UPS (uninterruptible power supply) to plug my computer into.

This particular UPS has an input and output voltage reader on it. I notice when my window AC unit and my computer are running, the input voltage on my UPS can go down to 118 V, then back up to 122 V five minutes later, then back to 120 V. I have about 5 people living in the house. Mainly these fluctuations happen in the summertime when the power grid is already under stress.

So my overall question is, will a ±3 V fluctuation be harmful to my equipment? Is it bad to have that much fluctuation?

  • What is the nameplate ampacity on the air conditioning unit, the PC, laser printer, and everything else plugged into this circuit (i.e. that loses power when the breaker is off)? Feel free to ignore anything with a wall-wart and any LED or CFL lighting. – Harper - Reinstate Monica Jun 16 at 16:24
  • In the U.K. my UPS revealed that the voltage in my area (Nottingham) is 250 Volts. This is still within the “230 -6% +10%” standard, but it was a surprise! Mine doesn’t seem to vary as much as yours though. – Tim Jun 17 at 9:35

This is typical. Can't exactly say "normal", because if everything is sized and installed correctly then there should not be significant fluctuations. But it is really quite common, particularly in older homes. In my case, I installed a UPS due to real power outages (used to be common, now very infrequent thanks to Pepco rewiring the neighborhood a few years ago), and it kicks in due to voltage drop every time the laser printer goes on.

Most equipment should be immune to these problems, as long as the range is relatively small - e.g., between 110 V and 125 V. See ANSI C84.1 for more details (thank you @Khrrck).

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    I would say "normal, utterly" - you have a long complex system where various people (anywhere on your line, or even other lines from the same sub-station) turn loads on and off more-or less at random and the power grid has to attempt to maintain a constant voltage despite that, with very little in the way of "buffers." I happen to have a device which shows voltage (among other things) on my well pump circuit as I'm a curious sort that wants to know that, and the nominal 240V is almost never exactly 240V (whether or not it's running.) – Ecnerwal Jun 16 at 16:39
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    For additional information, the ATX specification for computer power supplies requires them to function normally at voltages as low as 90 and as high as 135. – Khrrck Jun 16 at 16:41
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    @Khrrck Agreed. I thought about including something like that. But there is plenty of other equipment that isn't quite as flexible (and some that is even more - 90 - 240 for a lot of stuff) but also plenty of non-computer stuff that is really expecting 110 - 125. – manassehkatz-Moving 2 Codidact Jun 16 at 16:52
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    Indeed. Perhaps a better citation is ANSI C84.1 which is the source for that range. – Khrrck Jun 16 at 17:16
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    Are you running the laser printer through your UPS ? That's generally considered a bad idea and often recommended against. If you must, then the UPS needs to be ~4x the size you might expect. I had one laser that would idle at 0.1A but periodically kick up to a full 9 A load at 240V – Criggie Jun 17 at 0:20

You are lucky to have variations that small.

Some variation in the grid voltage is inevitable because of how these things work. Wires have resistance, transformers are less than ideal, the load varies. The less variation one allows, the more expensive the grid will be and those expenses inevitably will land in the electric bill, so compromises are made.

I live in Europe where the electrical grid was started years after the US one and a lot of things that Edison (100v that later become 110, 115 and 120) and Westinghouse got wrong were fixed before it was too late to change anything. Our grid voltage is standardized at twice the US one exactly in order to lessen the variations because of the changing load.

The law here mandates your wall socket voltage within 10% from its nominal value (230v). It was 15% not so long ago (say, 20 years) and the nominal value was 220. The new norm (10% around 230v) fits perfectly in the old norm (15% around 220v). I am not sure how exactly it is in the US right now, but one can divide EU voltages by 2 and get (roughly) the evolution of the US grid voltage standard. In Japan, their nominal 100v allows for only 5% variation and they have their very own higher electricity bills to pay.

Most electronic devices (computers, TV-sets, modern inverter-type AC units, etc) have switching-mode power supply unit that is very efficient at changing its internal mode of working in order to keep its output voltage steady when the input voltage changes. And because many of them are marketed world-wide, they are made to accept anything between 100-240v (nominal) plus 10% above and below in order to work world-wide.

Or the market is split to US-type (including Japan so 100-120v plus variation) and EU type (200-240v plus variation) and the power supply units get a bit cheaper at the cost of complexified logistics.

That's why your 116-121 or so volts is completely a non-issue.

There is another group of devices (space heaters, incandescent light bulbs, some modern, but cheap LED lighting devices, cooking appliances, power tools, etc, ...) that don't have any internal stabilization means. Their performance varies a bit (say, +/-20% for +/-10% variation of the voltage) and this is simply considered normal.

And finally, there is a third group of devices - precise scientific/measurement equipment, old tube-based electronics, etc... that are as picky to the power voltage as it gets. If you own and operate one, you probably already know what to do in order to get it working right. Including, but not limited to using it only at night or employing some voltage stabilizer.

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All of those fluctuations are normal in regular residential electrical service. Any large loads, AC units, ovens, water heaters can/will cause a drop in your voltage. Power company regulators in their substations work to compensate larger fluctuations as do capacitor banks on overhead lines. These fluctuations normally will not harm appliances.

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When designing a circuit the rule of thumb is +/- 10% or document. So for example USB specifies 5v so when designing a circuit to run off USB make sure it works from 4.5v to 5.5v. applying the rule of thumb to 117v would give you 105.3v to 128.7v. in actual practice many devices are even much more tolerant. For example Enigizer mad a usb charger which accepts 100v to 240v per the label. Also many Japanese designed power supplies will work fine from 90v to 260v, and I even came across a power-supply rated for 90v to 480v.

For 117 +/- 10% don't give it a second thought. Outside of that range check the ratings of your equipment. First things to check are anything with a motor (fan, vacuum), transformer (furnace, doorbell, very old computer), or a heater (drier) as these are the most sensitive.

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  • This is a great point. It's pretty much a universal rule of thumb that electronics be able to tolerate +/- 10% in normal working operation (you have to pick SOME tolerance, infinite precision is impossible, 10 is a nice round number), and when it doesn't hold, the spec sheet will be very upfront about it. This is typically sensitive equipment that you won't see in a 'normal' house, save maybe hi-fi audio equipment. – DeusXMachina Jun 18 at 14:33

Many electronic devices are designed to compensate for fluctuations within the power grid. Even devices like microwaves and coffee pots have some sort of regulator that helps manage this fluctuation.

That's also the whole purpose of a power adapter, which converts your 110-120V AC outlet voltage to whatever specifications your device uses. Most of these adapters now accept 100-240V sources to be compliant across many different countries.

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    Not the "whole purpose". The main purpose is to be able to design items to use low voltage, which is how many things (e.g., most consumer electronics) have to work. The side benefit is that modern adapters can handle a wide input voltage range while providing a steady output voltage. But that's not the reason they exist - and if you look at older ones (e.g., perhaps from a 30 year old device) you will often find they can't handle a wide input range. – manassehkatz-Moving 2 Codidact Jun 17 at 18:56

Having worked for an electricity provider, I can tell you that your 3 volt variance is within the allowed tolerances. Your equipment should be fine; but, I would use caution as suggested, with the laser printer.

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While this is "normal" and as other have told you is not harmful to equipment, it could be a symptom of a serious electrical problem like a deteriorated neutral, undersized wiring relative to the circuit breakers/fuses in use, etc. It may make you feel better (and may catch early a problem that could turn into serious equipment damage or a fire later) to have the entire electrical system checked out by a professional or at least someone knowledgable.

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I'm in Maine, using Central Maine Power. Their website lists their voltage level commitment as 120V +/- 5%, or 114V to 126V. I think this is from a state mandate. I suspect you could find similar information on your power company's web site.

I sometimes watch the voltages reported by my UPSes, and find it isn't unusual for it to drop from 121V to 115V over a relatively short period. (As I write this, one UPS reports 117V and two others 116V.)

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