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Electricity in the United States of America

Most people outside the United States of America think that the USA, and Canada and some other countries, use 120 V but it is not quite as simple as that.

Thomas Edison was a brilliant inventor but he was also a very astute businessman. Unlike Tesla, who was just as brilliant an inventor but who went bankrupt several times and died penniless. Tesla’s lasting memorial is the electric car and hi-tech electronics company founded in 2003 and named after him, but with which he had absolutely no connection.

Edison lost “The Battle of The Currents” because Westinghouse’s 240 V AC system allowed large amounts of power to be transmitted over long distances at 10 000 V (or higher) before being stepped down to 240 V into people’s homes whereas his own 100 V DC system required a power station on almost every street corner. But he was not going to give up his 100 V electric lighting system.

Incandescent lamps work just as well on AC as on DC. So he took the 240 V AC supply which had been forced upon him and used a centre tap on a transformer to convert it into two separate 120 V supplies.

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To get 120 V you connect between L1 and N or L2 and N.

Edison's 100 V DC lamps worked just as well on 120 V AC as before except that they were a little brighter and did not last as long until he started making 120 V lamps.

Today the 120 V sockets can be used for lighting and the sorts of things which people in 240 V countries plug into the ordinary sockets in their own homes: televisions, computers, washing machines, dishwashers, freezers, microwaves, power tools, etc. Of course the cables connecting the sockets to the main distribution board must be much thicker for the 120 V system because for the same power half the voltage means twice the current and so four times as much copper. Going from one country to another is simple, you just need a suitable adapter.

In North American homes 240 V is reserved for the sorts of high-powered devices which would not normally be plugged into an ordinary socket in a 240 V country, for example electric cookers and air conditioning systems, the sorts of things you do not normally take with you on holiday. Using a special 240 V connection for these is necessary because at 120 V the cables to carry the higher current would need to be very thick indeed.

Although almost all homes in North America receive a 240 V supply it is quite correct to call it a 120 V system because inside the home all the live wires are at 120 V above neutral - you cannot get a 240 V electric shock unless you touch both L1 and L2 at the same time. “The Battle of the Currents” was very acrimonious, and Edison made much of the fact that the 100 V used in his DC system was much safer than the thousands of volts used (he forgot to say in transmission) in the AC system: he even eloctrocuted dogs to prove it! When he lost the battle he continued to justify his use of 120 V for his lighting rather than the 240 V being supplied on grounds of safety, and this argument is still heard today. Electric shocks are discussed on another Page.

Homes are usually wired so that the amount of power used is balanced between L1 and L2, and this can sometimes cause difficulties for people who do not have a good understanding of the system when they are trying to trouble-shoot a problem where one socket is working and another one in the same room is not.

A personal request to my North America readers about this Page

The prime target readership for the maths and science Pages of my Web Site is teenagers studying for school-leaving exams and adults who stopped learning maths and science when they left school, although I know from my e-mails that my readership is not restricted to them.

I live in Kent, in Southern England, but about 30% of my readership live in the USA, and if this is where you live I would very much like to hear from you, to know how accurate and how helpful you think this Page is - you can e-mail me here. If you are a child or young person just tell me your age, if you are an adult a little more information about yourself would help me to pitch my reply at the right level.

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© Barry Gray November 2019