Our history determines our standards.

It’s one of those things I never actually recall stopping to think about as I flew through college and then into the working world, but when I picked up Tesla: Man Out of Time by Margaret Cheney, I found why our US grid is 60 Hz (AC at 110 V). It turns out it’s a combination of great genius and horrible compromise for the sake of business. As is so often the case.

Where did 60 Hz AC Come From?

The origin of 60 Hz AC, as many of you probably know, goes all the way back to Nikola Tesla, our favorite engineer. He first worked for, and then later was forced to compete viciously with, Thomas Edison. That competition is a whole story unto itself and one that has left me very cold toward Edison.

It’s funny how history leaves little things out, like how Edison hired kids to kidnap neighborhood cats and dogs so he could electrify them to show how dangerous Tesla’s AC distribution system was compared to his own DC system. Or how he electrified an elephant and recorded it, or invented the execution chair using AC and marketed the act as being “Westinghoused” because of George Westinghouse’s support of Tesla.

What does all this have to do with 60 Hz? Well, when Westinghouse wanted to build an electrical supply grid using the kinetic power of Niagara Falls circa 1888, he turned to Tesla, as Tesla had earlier demonstrated the benefits of AC and knew a few things about electric motors and power distribution. He had already done the analysis and already figured out that 60 Hz at 220 Vac was the most efficient means of doing so.

Thomas Edison

Edison--a genius, but a genius at any cost.


However, Westinghouse’s engineers had already committed to 133 Hz. In her book, Cheney said it best:

“When he so informed the engineers, he succeeded in rubbing them the wrong way and only after months of futile and costly experiments doing it their way, did they finally accept his word. Once they had done so, the motor worked exactly as it had been designed to. Sixty cycles has ever since been the standard for alternating current.”

Now, this paragraph is absolutely loaded with stereotypical engineer psychology: for instance, Tesla’s unerring precision and exactitude combined with a disassociation from the “feelings” of the other engineers. On the other side, the 133-Hz advocates were stubbornly typical in their disdain of this “outsider” (aka: “not invented here,” syndrome) and were determined to prove out their own work. 

Tesla's induction motor

Tesla's induction motor


Still, given the time, proving the efficiency of 60 Hz with respect to driving Tesla’s induction motor was critical, as motors were the heart of the industrial revolution, which was then in full throttle.

As to how we ended up with 110 Vac vs Tesla’s preference for 220 Vac, we can thank Edison, who already had an installed base of 110 Vdc, so there was a compromise to accommodate business. “A most un-American mental attitude,” said Tesla’s fellow Serb, Michael Pupin, who worked for Edison but had a hard time understanding why business would ignore the engineering experts.

Some things never change.

So, do you know why Europe is 50 Hz AC (15 to 20 percent less efficient)?


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  • SamCummins 2016-02-02

    I don’t know why Europe is 50 hz.  I don’t know why that would be less efficient.  The power factor would increase, but that may not correlate to efficiency. 

    • tonyr1084 2016-02-12

      I was hoping to learn more about why 60 Hz is most efficient.  So all I can offer is a guess:  I think it has to do with the rate of change in the magnetic flux.  Motors seem to work better on 60 Hz.  Maybe 70 Hz is too fast for the motor to convert the magnetic force into torque, same as 50 Hz may be too slow to take full advantage of it.

      Remember, this is just a guess.

  • Shae Abraham 2016-02-03

    Good story however I would like to see the more technical side as to why 60 Hz is the most efficient frequency.

  • jaykotecki 2016-02-05

    50 hz…it has to be a metric thing.

  • derfisherman 2016-02-12

    There were many reasons why 50Hz was selected for Europe, in fact, it has been selected for most of the world.

    60Hz is mainly the USA, some parts of south America, Canada and parts of Japan. In comparison, a “niche” standard, though it is slightly more efficient.

    Today, if designing an electrical system, we would select a far higher frequency, how high? No idea…..but aircraft and some shipping have used frequencies around 400 Hz for some years for reasons of overall lightness of the system.

    From the IOT:-

    (Induction motors turn at a speed proportional to frequency, so a high frequency power supply allows more power to be obtained for the same motor volume and mass. Transformers and motors for 400 Hz are much smaller and lighter than at 50 or 60 Hz, which is an advantage in aircraft and ships.)

    That the then prime movers were slow, most usually steam engines or water wheels in the late 19th Century, was also part of the reason, also lower than 50Hz frequencies had already demonstrated much too much flickering with lighting, was another….

    It was basically a consensus of many things business and practicality…..as was the 60Hz, later in the “day!”

    • tonyr1084 2016-02-12

      I usually don’t take guesses at things, but in this case, Aircraft AC IS 400 Hz for two reasons:  First, as has been stated transformers and motors are smaller and lighter while still producing plenty of power.  But the single best argument I’ve heard of is that with 400 cycle, instruments have less time to drift.  Navigation is more important than weight considerations, though weight IS a big factor.  Sacrificing efficiency for weight and navigational stability is preferred over efficiency, and the loss of efficiency is outweighed by safety and reliability.  Or so I hear.

    • tonyr1084 2016-02-12

      I usually don’t take guesses at things, but in this case, Aircraft AC IS 400 Hz for two reasons:  First, as has been stated transformers and motors are smaller and lighter while still producing plenty of power.  But the single best argument I’ve heard of is that with 400 cycle, instruments have less time to drift.  Navigation is more important than weight considerations, though weight IS a big factor.  Sacrificing efficiency for weight and navigational stability is preferred over efficiency, and the loss of efficiency is outweighed by safety and reliability.  Or so I hear.

    • tonyr1084 2016-02-12

      I usually don’t take guesses at things, but in this case, Aircraft AC IS 400 Hz for two reasons:  First, as has been stated transformers and motors are smaller and lighter while still producing plenty of power.  But the single best argument I’ve heard of is that with 400 cycle, instruments have less time to drift.  Navigation is more important than weight considerations, though weight IS a big factor.  Sacrificing efficiency for weight and navigational stability is preferred over efficiency, and the loss of efficiency is outweighed by safety and reliability.  Or so I hear.

  • billjratt 2016-02-12

    When you touch the input of your amplifier in the US, do you get a 60-cycle hum?
    We get 50-cycle hum in UK. I thought it was a NATURAL frequency and the one to use. although 60cps allows smaller, lighter components which is an advantage.

  • Xerox 2016-02-12

    Copy that! Or would you? The $64,000 question is “what would we do now with the benefit of hindsight?”.

  • Eric M. Jones 2016-02-12

    Here’s some considerations with AC frequencies that probably affected the decision:
    1) It had to be above the visual flicker frequency.
    2) It should ideally be divisible by many numbers BUT also a divisor of 360 (degrees).
    3) It should be low enough so that human contact will react strongly with it, and it turns out that frequencies near 60 Hz will cause the strongest “jolt” reaction. This is actually a good thing, since lower (or DC) and higher frequencies cause more tissue damage per delivered watt of power. Very high frequencies will fry you with no bodily response. I don’t have any Tesla writings to support this idea.
    4) Tesla was the early master of polyphase motors and generators. A 50 or 60 Hz device is relatively easy to build. A 133 Hz polyphase design would be a headache.

    • tonyr1084 2016-02-12

      Reply to your note #3:  I’ve seen the damage 28 volts AC at 400 cycle can do.  It’s far greater a burn than at 60 cycles.  But at the same time I once took a jolt from right wrist to left hand (through the heart) of 120 V 400 Hz.  Knocked me on my azz!  Left me dizzy but did no harm.  I’ve also take 120 v 60 Hz finger to finger (slippery plug) and it smarts.  However, 60 Hz is closer to the sinus rhythm of the heart, and therefore can be more dangerous to human life and sudden cardiac arrest.  I’ve also taken 65KV from a TV Flyback transformer to the chin. (I was just starting my young career and interest in all things electronic).  I don’t think anything has ever hit me as hard.  But you’d think with that high a voltage I should have been dead long ago.  The flyback transformer of a TV operates at very high frequencies, probably around 15KHz (guessing at that factoid).  Far more have lost their lives due to shocks across the heart at 60 Hz than anything else.  Probably because it’s most common, but I’ve never heard of anyone being shocked to death from the flyback transformer.

      Anyway, that’s my two nickels worth.

      • senormechanico 2016-02-12

        As a former TV technician and numbers aficianado (I remember every phone number and address I’ve ever had and I’m 70 years old), the horizontal sweep frequency which drives a TV flyback transformer was 15,750 Hz for monochrome shows and 15,734 Hz during color broadcasts.  It may now be the same all the time, but I haven’t serviced a TV in 25 years or so.
        I will never forget the burn (both hot and electrical) available to your finger when you remove the plate cap on a horizontal output tube when the set is running !  It just instantly bores a hole and leaves smoke in the air…

      • ilane 2016-02-12

        Gosh, those were the days! “Horizontal output tube” - talk about ancient English! I do remember that getting bitten by a many-miliamps-capable 150 or 180V DC supply hurt more, and “lasted” longer, than getting bitten by mains AC!

  • rivden 2016-02-12

    Great article and concise explanation for our 60Hz.  I really enjoyed the incorporation of the Engineering compromises to meet business needs and some of the other topics touched on.

  • edgolla 2016-02-12

    All very interesting.  Now, I would like to see a discussion of why the neutral is connected to earth ground in the U.S.  I am sure some would say for safety.  But, I content that most people that are killed by electric shock are killed by getting between Hot and earth ground and not between Hot and Neutral..  Ed

    • KenStewart 2016-02-12


      It makes no difference which side is connected to Earth ground.  The electrocution effect is the same regardless.  If the side we currently call Hot were connected to Earth Ground then the other side (neutral) would still present the same lethal voltage relative to ground.  Your question really is this: Why is either side grounded?  The reason is to anchor the electric grid relative to ground.  Imagine an electric system with two wires and neither was connected to ground.  The voltage between the wires might be guaranteed to be 110 VAC (for example) but the voltage relative to ground is not guaranteed to be anything.  It could float thousands of volts above or below ground potential.  Charged clouds passing overhead would change the potential of the electric system relative to ground even when no lightning strikes occur.  A direct lighting strike to an ungrounded electric system would make the electric grid float millions of volts from Earth ground.  The unpredictable voltage relative to ground makes this electric system much more dangerous.  Eventually some part of the electric system would arc to ground.  The low impedance of the arc would bring the electric grid back to ground potential for a time.  When the arc stops then the grid could again float relative to ground.  The location of the arc is also unpredictable.  It could be between a toaster and a sink, or a wire buried in wall whose outer surface is wet with rain water.  The unpredictable arc is both an electrocution hazard and a fire hazard.  The modern electric grid is more complex than the simple two wire system I’ve described here but the principals are the same.  Tying one leg of the electric grid to Earth ground guarantees that the other legs are at predictable voltages relative to the buildings they enter.  The buildings, of course, are grounded because they sit on the Earth.  A grounded electric system makes the problem of insulating the wires become predictable. 

      • nixxon 2016-02-12

        Hey! I believe we have that hazardous system here where I live in Norway. It is a IT (Isolated Terra) grid. It works all over the world - in Norway and in ... Albania… and… oh, I guess that’s it. (Older homes in Norway uses the IT system while newer homes use TN-C-S)

      • edgolla 2016-02-16

        Ken, Thanks for the extended reply.  It had a lot of good information.  What I was talking about is not whether the high voltage distribution system should be grounded.  I think it should for the very reasons you mentioned. But, I think it is foolish to ground one side or the middle (in the case of 240 V).  The output of the local transformer is isolated from all the effects that you mentioned.  Could you please give me your thoughts on not grounding only the lines feeding into the house from the local transformer.  Ed

  • stoneage 2016-02-12

    Here is a useful link at Wikipedia: https://en.wikipedia.org/wiki/Utility_frequency  With more than enough info about utility frequencies.

  • Easel Gord 2016-02-12

    For the last 60 years I have been , mistakenly apparently, thinking the the change from 50hz to 60hz was made because a very light flickering of a light bulb could become apparent to the human eye or mind at 50hz but not at 60hz .I distintley remember my father having to buy a new refrigerator (with a litle Ontario Hyro $) while I was in high school or 1952-1958.Old wives tales maybe?

  • uwezi 2016-02-12

    You asked the question why Europe would insist on a 15-20 less efficient mains frequency, when the US system is so superior… Since you don’t give any clue on where this number comes from I would like to point out that you cannot make such a general statement.

    Efficiency is not related to the number of cycles per second which you have in an AC system - on the contrary, inductive and capacitive losses for power transmission lines increase with increasing frequency. The lower mains voltage in the US system also contributes to a significantly lower efficiency, because of higher I^2 R losses in the wiring.

    Today the most efficient way to transport electricity is actually by the DC which Edison favored. No, Edison didn’t win the battle in the end, because the means of transforming DC up and down where not available at his time and thus he would not have been able to utilize this improved efficiency of today.

    The only half-way reliable reference to a higher efficiency for 60 Hz over 50 Hz which I could find relates to the long outdated arc-lights which used to illuminate cities more than 100 years ago.

  • ck722 2016-02-12

    Some air core fellow told me that 60Hz is the natural frequency of the universe since since 2*pi*60 = 377ohms which is the impedance of free space. —-

  • imaginewhen 2016-02-13

    Interesting to note
    When I was working in Canada the frequency was 25 Hz in 1957, I was there when they converted to 60 Hz within the next five years. The reason I bring up this point because the power station at Niagra Falls was where Telsa first installed his AC system.
    I often wonder why it was not 60 Hz, I wonder why it was 25 Hz you could actually see the lights flicker at 25 Hz in those days.
    The change over was an expensive operation all appliances with motors such as freezers had to be replaced, speed on washing machines and electric clothes dryers had to have pulleys changed, electric clocks had to have a motor replaced, I remember because I was there at that period of time.

  • Neodavid 2016-02-13

    At higher frequencies the capacitance of the lines would severely affect transmission over long distances. Even at the lower frequency of 60hz, large industrial motors heat significantly if leading capacitors to the plant don’t counteract the phase differential created by line resistance and capacitance in combination with the motors inductance.

  • mikekehrli 2016-02-13

    I’m surprised no one has mentioned resonant frequency.  I read the book that was referenced above, and one of the driving forces of 60Hz was seeking a happy resonant frequency with the power grid.  Remember, Tesla found the resonant frequency of the earth with his power station in Colorado.  He oscillated the ionosphere and found that it’s resonant frequency was around 5-6 Hz, if memory servers.  He also calculated the frequency that the power grid would best oscillate at when he came up with 60 Hz.

  • kevinrs 2016-02-14

    The frequency is partly a result of balancing the problems of inductance and capacitance on the power lines. DC(0 hz), you have the problem of inductance severely limiting transmission of large amounts of power over long distance. High frequency AC, capacitance would be the problem. So, decreasing frequency means more inductive resistance, but less capacitive, increasing frequency does the opposite. How efficient a particular frequency will be depends on the design of the grid, and the generators and motors used. Switching Europe to 60 hz on the existing lines would decrease efficiency, the same as switching the USA to 50.
    As far as the generators and motors, the same design has to turn faster at a higher frequency, so requires changes in turbines or gearing, for different frequencies. Higher rpm/frequency means higher watt output at the same weight, but also means more wear on the bearings, etc.

    • kevinrs 2016-02-14

      oh, also at the time, there were no solid state inverters, or dc-dc voltage converters, etc. any change to frequency or ac-dc to make a supply match what was required would have needed a motor built for the supply coupled to a generator to output the power needed.

      • edgolla 2016-02-16

        As far as I know there is no inductive loss associated with DC transmission.  The problem with long distance DC transmission is one of voltage conversion.  In the time frame when all of this took place, there was no practical way (as you mentioned in your second post) to convert large amounts of high voltage DC power (needed for transmission over long distances to avoid I2R losses. Not inductive losses) down to the low voltages needed in the homes and factories.

  • KIRON DAS 2016-02-14

    Thanks to Author.

  • SalceyForest 2016-02-19

    Interesting article, although like others I’d be interested to know the why’s and wherefore’s of 60Hz being the optimum frequency against 50Hz elsewhere.

    On a different note, I have a canal boat in the UK, where most of the electric is derived by a small (300W) generator. With the advent of wall-warts for everything, which are all rated to cover 100-240VAC it turns out I can run the 240V generator really slowly and everything works just fine. 180V is enough to make energy-saving bulbs work, so the gennie idles at 38Hz and all is well. It won’t produce full power of course, but I only need that for AC/DC!

  • Filbert11 2016-09-07

    I am with tonyr1084.  This article says “that” 60 Hz is better - but not “why” 60 Hz is better.  So - pretty much doesn’t even answer it’s own headline !!!  I guess the writer prefers drama and politics to engineering !!!

  • schmitt trigger 2016-12-09

    As part of my Master’s theses, I had a chapter where I investigated the reasons for all the world’s different voltages/frequencies.
    A quick summary:
    -The original generator frequencies evolved from the available prime movers rotational speeds.
    -Induction motors work better with lower frequencies. But for general purpose motors, 50 Hz is better. In Europe, where electric railways developed quite rapidly, there were even dedicated powerlines at 25 and 16 2/3 Hz..
    -On the other hand, for incandescent bulbs have lower flicker at 60 Hz.
    -For transformers, a major component of AC distribution, you require either more steel or copper at 50 Hz (because of the increased flux density at 50 Hz) as compared to 60 Hz. But the steel losses are higher at 60 Hz.
    Like everything else in engineering, there are compromises to be made.

    In the end, it all becomes a matter of nationalistic pride on which system was adopted.

  • PeterF 2017-07-31

    So some notes here refer to European railroads using 25Hz…. which was the standard used by the Pennsylvania Railroad.
    When they electrified tracks near NYC, they used 25 Hz, and eventually extended it to Washington, Harrisburg and New Haven…. All that was converted to 60Hz in the 1980’s (forcing the retirement of the venerable GG-1 locomotives) so the power could be commercially produced and purchase at lower overall cost. 
    It seems this conversation points to really HEAVY motors operating better at lower frequency.

  • Stan Cooper 2017-10-31

    There’s no appreciable efficiency difference between the two frequencies with appropriately designed equipment. 220V however is more efficient vs ohmic losses.

    Tesla chose 60Hz because it was an overtone of the so called Schumann resonance; the specific overtone being chosen as one with a useful frequency for application to rotating AC machinery. Precedence dictates it would more aptly be named the Tesla resonance.

  • Bren69 2018-11-19

    60 didn’t fit into the metric system when rewiring Europe after ww2