Voltage Level

I assume it went something like this:

Long, long ago when transmitting electricity was a new idea, people initially chose 100V or 200V because they are convenient numbers. But then those values had to have some official tolerance. So again, +/- 10% was chosen since it was also convenient and achievable.

As transmission lines grew in number and length, power generating companies tended to supply the highest allowable value to insure acceptable voltages down the line. 100 + 10% = 110V. 200 + 10% = 220V.

Eventually as designers created new AC electrical devices, they kept in mind both the highest allowable standard and what was actually measured on their power lines. Eventually "110" and "220" became the new nominal voltages in designer's minds. And eventually this unofficial standard became official.

You have to remember that long ago things were quite different than now. There were few official regulatiory agencies or scientific bodies overseeing many fields. Standards of all kinds just grew out of common experience and random chance.

Check here for your answer http://cr4.globalspec.com/thread/55947/Voltage-Multiples-of-11

Dear Sir,

Please refer to the book written by Mr. H.Cotton on Electrical Technology . There you will find a long formula on Electrical Power and that will give you why the KV is in multiple of 1.1 KV,11 KV,22 KV, 33 KV, 66KV, 132 KV, 220 KV .

Regards,

Ulhas Pradhan

Can you still buy/refer to "Electrical technology" by Cotton.

This was the book I used in my college in 1960.

It was a very good book.Happy to know somebody still recommends/remembers it.

Dear Mr. Srini,

I did my Engg. in 1965. Still I have the Book - ELECTRICAL TECHNOLOGY by H.COTTON and I refer in the event of any doubt. My Professor. Dr. T.R. NATESAN, has masterised/memorised this Book.

One of the BEST BOOKS.

SDR710

It is true only with respect to AC power systems. The first known man-made source of electricity is a cell, which is DC in nature. But, after realizing the disadvantages of the DC electric equipments, AC Electricity generating machines were invented. When these AC Machines were developed, the power of these machines was to be compared with the already available DC electricity, as it is human tendency to compare anything new with the existing ones. As in the case of steam engines.

When steam engines were invented, the power of the steam engines was compared with that of the horses, which were the power sources before the invention of the steam engine.

So, a value called RMS Value for AC Electricity was derived which compared the effectiveness of the AC Electricity with that of the DC Electricity. This value is the Effective Value of AC Electricity. As we were more interested in knowing the effect of AC electricity, all measuring instruments were and are designed to measure only the RMS value of AC electricity – may it be Voltage, Current, Power, etc.

But, for the designer sitting in the design lab, more than the effective value, the average value over a period on one sinusoidal cycle of AC Electricity was important. So, he designed an AC electric Generator, which would produce, on an average, a voltage over a period of one cycle, of say, 10kV (10 is a round figure, you know).

But, when this machine was built to the design and put to operation and when the output voltage was measured, it was found to be 11kV, as the meter was measuring NOT the average value but the effective or the RMS value. This relation existed for any voltage. So, a factor was arrived at – relating the RMS value and the average value, called Form Factor, which is the ratio of RMS value to the Average value, which for a sinusoidal wave form was about 1.1. Then, when the voltage was to be transformed, it was easy to have a whole number for the turns ratio of the transformer and hence all subsequent AC voltages became multiples of 11.