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Background of Worldwide AC Voltages and Frequencies

by Ron Kurtus (revised 3 February 2017)

The standard voltage and frequency of alternating current (AC) electricity used in homes varies from country to country throughout the world. Typically, either 120-volt AC or 240-volt AC is used. Also, most countries use 50Hz (50 Hertz or 50 cycles per second) as the AC frequency. Only a handful use 60Hz.

The standard in the United States is 120V and 60Hz AC electricity. However, due to fluctuations, the average measured voltage is 117 VAC.

(For a listing in various countries, see List of Worldwide AC Voltages and Frequencies.)

There is controversy over which frequency system is better. Also, the movement in many countries is toward using higher voltages.

You need to check your equipment specifications when using electrical devices in a country with a different voltage and frequency system than yours.

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion

How values were selected

The type of electricity delivered to homes and businesses was initially direct current (DC) but then changed to alternating current (AC) electricity. AC frequencies varied considerably, according to equipment used. For example, electrical generators at Niagara Falls put out 25Hz power.

Tesla starts AC

Early in the history of electricity, Thomas Edison's General Electric Company was distributing DC electricity at 110 volts in the United States.

Then Nikola Tesla devised a system of three phase AC electricity at 240 volts. Three-phase meant that three alternating currents 120° out of phase were combined in order to even out the great variations in voltage occurring in AC electricity. He had calculated that 60 cycles per second or 60Hz was the most effective frequency. Tesla later compromised to reduce the voltage to 120 volts for safety reasons.

(See the biography of Nikola Tesla for more information.)

With the backing of the Westinghouse Company, Tesla's AC system became the standard in the United States. Westinghouse chose 60 Hz because the arc light carbons that were popular at that time worked better at 60 Hz than at 50 Hz.

Europe goes to 50Hz and 230V

Meanwhile, the German company AEG started generating electricity and became a virtual monopoly in Europe. They decided to use 50Hz instead of 60Hz to better fit their metric standards, but they stayed with 120V.

Europe stayed at 120V AC until the 1950s, just after World War II. They then switched over to 230V for better efficiency in electrical transmission.

U.S. stays at 120V, 60Hz

The United States also considered converting to 220V for home use but felt it would be too costly, due to all the 120V electrical appliances people had. A compromise was made in the U.S. in that 240V would come into the house where it would be split to 120V to power most appliances. Certain household appliances such as the electric stove and electric clothes dryer would be powered at 240V.


For various reasons, Brazil and Japan have multiple standards.


In Brazil, most states use between 110V and 127V AC electricity. But many hotels use 220V. In the capital Brasilia and in the northeast of the country, they mainly use 220-240V. All operate at 60 Hz.


In Japan, they use the same voltage everywhere, but the frequency differs from region to region.

Eastern Japan, which includes Tokyo, uses 50Hz. In 1895, Japan purchased 50 Hz electrical generators for Tokyo from the German company AEG. This was the same as what was done in Europe. In 1896, the American company General Electric provided 60Hz generators to cities in western Japan, which includes Osaka and Kyoto.

It is unfortunate that they did not coordinate their efforts. Having different voltages and frequencies within the country not only must be confusing for the people but also can result in extra costs for appliances and adapters.


You can compare the different frequencies and voltages.


Both 50Hz and 60Hz have their advantages and disadvantages.


With 60Hz, the transformers can be smaller and less expensive than for a 50Hz transformer. Although the difference is small, it can add up in a system with many transformers. Using 60Hz results in less flicker on lamps, but that really is not a factor these days.

Hum and frequency noise are more readily audible with 60Hz and its harmonics than with 50Hz.


With 50Hz, electric power transmission over long lines favors it over 60Hz. The effects of the distributed capacitance and inductance of the line are also less at the lower frequency.

Although 50Hz transformers require more copper and iron, 60Hz transformers require more expensive plates to prevent eddy current losses.


Over the years, the trend has been towards higher voltages. Although lower voltages are safer, that is less of a concern these days with strict codes and regulations.

In the United States, 2-wire 120 volt service has been replaced with 3-wire 120/240 voltages or with three phase 120/208 voltages. In Europe many supplies were changed from 3 phase 4 wire at 127/220 volt to 3 phase 4 wire at 220/380 volt. In the UK most early supplies were 3 wire DC at 120 volts, but that was later changed to 240 volts.

When visiting another country

Bringing an electrical appliance from one country to another may require some special converters, transformers and adapters to allow the appliance or device to work properly.


Converters are typically used to decrease the AC voltage from 220V to the 110V level needed by the appliance.

They are only used for simple electrical products such as hair dryers, steam irons, shavers, or small fans. They are only used for short periods of time, can only be used for ungrounded appliances, and must be unplugged from the wall when not in use.

Converters cannot be used by electronic devices such as radios or computers. A transformer is used for those devices. The reason is that a converter simply cuts the AC sine wave in half, reducing the voltage. Electronic devices need the full sine wave to function properly.

Some converters will also change AC to DC. An example is converting 120V AC to 12V DC.


Transformers are used to increase or decrease the voltage and should be used with electronic devices such as radios, televisions, computers and other devices having electronics circuitry.

Transformers are more expensive than converters. They can also be used with electric appliances and may be operated continually for many days. A device like a hair dryer does not have any electronic circuitry. It simply has a heating element and electric fan, so it can use either a converter or transformer.

Dual voltage devices

Some devices have a built-in converter or transformer, such that they are called dual voltage devices. Most laptop battery chargers and AC adapters are dual voltage, so they can be used with only a plug adapter for the country you are visiting.

Plug Adapters

Outlet plugs are different in the various countries. Plug adapter must often be used when visiting a different country. These adapters do not convert electricity. Rather, they simply allow a dual voltage appliance, transformer or converter from one country to be plugged into the wall outlet of another country.

Frequency difference

Converters and transformers only change the voltage and not the frequency. The result is that a motor in a 50Hz appliance will operate slightly faster on 60Hz electricity. Likewise, a clock made for 60Hz will run slower in a country using the 50Hz frequency.

Most modern electronic equipment like computers, printers, DVD players and stereos are usually not affected by the frequency difference.


The voltage and frequency of AC electricity varies from country to country throughout the world. Most use 230V and 50Hz. About 20% of the countries use 110-120 V and 60Hz to power their homes. 240V and 60Hz are the most efficient values, but only a few countries use that combination. The United States uses 120V and 60Hz AC electricity.

Electricity is amazing

Resources and references

Ron Kurtus' Credentials


Utility frequency - Wikipedia

Mains electricity - Wikipedia

Guide to International Travel With Electrical Appliances - Good information from Lewis N. Clark

Elements of AC Electricity - Basic electronics tutorial site

DC and AC Electricity Resources

Physics Resources


Top-rated books on Electrical Power Systems

Top-rated books on Electrical Transformers

Questions and comments

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