(The same question can apply to locations with 220/240V mains, if I am not mistaken.)

Frequently I see mixed ratings indicating that something is suitable for 110, 115, 118 or 120V (in the US). I've always referred to mains power as 120V but with the understanding that it varies because of:

  • Different means of generation (number of phases, etc.)
  • Line losses and imperfect conditions

When designing something, should one always test using the lowest expected voltage (110)? What reasons are there for the differences in mains voltage?

  • \$\begingroup\$ See this for US standards. The situation in Europe is even more complicated; for primer see this. \$\endgroup\$ Sep 15, 2017 at 19:48

3 Answers 3


In the US, the electric utilities are supposed to deliver power to residential customers at anywhere between 110 and 125 VAC RMS. The value 117 (or 117.5 or 118) is often seen on products, because that is the middle of the specified range.

If you're developing a product for general sale, it would be prudent to add a testing margin that's at least 5% or even 10% beyond the nominal range — perhaps 100 to 140 VAC RMS.

  • \$\begingroup\$ Same goes for the UK mains stuff - we now say 230V for most products because the range tends to be somewhere between 220V and 240V. \$\endgroup\$
    – Polynomial
    Feb 12, 2013 at 0:46
  • 6
    \$\begingroup\$ Actually in the UK, voltage was generally specified as 240V +/- 6% while mainland Europe used 220V, in both cases with some regional variations until the grids were tied up. As part of EU harmonisation, the compromise of 230V +/-10% was reached which covered both previous ranges in practical terms. \$\endgroup\$
    – user16324
    Feb 12, 2013 at 11:29
  • \$\begingroup\$ So the number usually varies because the test lab's voltage at the receptacle was 115V vs. 120V? Or is the choice of voltage listed on a label usually arbitrary? To me light bulbs are the most annoying because sometimes their brightness/power consumption and life are listed with different voltages. For example, a bulb might be labeled "100W 120V" but the very same bulb's package will claim it should last 2000 hrs at 115V. \$\endgroup\$
    – rob
    May 2, 2014 at 17:36
  • 2
    \$\begingroup\$ @rob: I'm really not sure what you're asking. The manufacturer is responsible for whatever the label says. This may or may not have anything to do with the actual tests done by a "test lab". Incandescent bulbs are particularly problematic because their lifetime is proportional the inverse of something like the 4th power of the voltage. Therefore, the lifetime test will be done at the lowest voltage they feel they can "get away with." \$\endgroup\$
    – Dave Tweed
    May 2, 2014 at 18:12

Adding to the answers here as to why the power company does play with the voltage: They will do their best to maintain your countries' accepted hertz value, normally at 50 or 60 cycles. They will sacrifice (brownout) voltage to insure the most important variable for equipment, hertz. Any change will make motors spin faster or slower, meaning timers and the like will not function properly. A voltage drop may be from 'imperfect conditions' but hertz variation is absolutely unacceptable.

  • 1
    \$\begingroup\$ This doesn't address the question. Although it's a useful side-note about power line frequencies, it misses the point of the question. \$\endgroup\$
    – JYelton
    Sep 20, 2014 at 18:38
  • \$\begingroup\$ Perhaps if you mention that for added context it would be useful. \$\endgroup\$
    – JYelton
    Sep 20, 2014 at 18:44
  • 4
    \$\begingroup\$ IMHO this is the definitive '[reason why there are differences in the mains voltage]'. \$\endgroup\$
    – Mazura
    Sep 20, 2014 at 19:15
  • 1
    \$\begingroup\$ @JYelton it's perfectly acceptable as an answer, and even stands alone. But even adding it in as complementary information is to be encouraged. Too many times we have people piling and repeating the same old info. \$\endgroup\$ Sep 20, 2014 at 20:48
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    \$\begingroup\$ Probably also worth mentioning here, the big, synchonous 'primary' mains generators are engineered to run @ 50 or 60Hz (depending on your country), and deviations from this frequency represent changes in their "shaft speed," as a symptom of overloading or overvoltage in those generators. Another reason why "hertz variation is absolutely unacceptable." \$\endgroup\$ Mar 8, 2016 at 5:03

Addressing just the reason why voltage varies, you are correct in thinking line losses play a role. No normal wire is a perfect conductor. Superconductors come very close, but for those of us working with normal electrical components, wires are just small valued resistors. As such, they experience a voltage drop when current flows through them, by Ohm's law. This is also why long-distance power transmission is done at high voltage, and stepped down as near to the customer as pratical.

You can observe this directly when a motor in your fridge, air conditioner, or clothes dryer turns on: the high starting current of the motor pulls the voltage in your house down, dimming any incandescent lights. Of course the running current is much less, so the lights appear to return to their original brightness, but if you were to measure carefully you would find they are a tad dimmer than they are without the motor running. With a large AC compressor even the current after starting is significant enough to dim the lights until it switches off.

Of course the electric utility attempts to mitigate this effect, but nothing is perfect.

  • 3
    \$\begingroup\$ FYI, if your lights get brighter under heavy loads, you have a faulty neutral connection, probably at the pole. Call the power company. \$\endgroup\$
    – Mazura
    Sep 20, 2014 at 18:31
  • \$\begingroup\$ Phil, this is very strange: the current in your house is not supposed to vary sensibly when your fridge is turned on, unless, perhaps, you use quite monster fridges. Seems like you have a big problem in your electrical installation. \$\endgroup\$
    – MikeTeX
    Mar 1, 2019 at 6:21

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