ANSWERS: 6

Nicola Tesla did pioneering work in the field of AC, and found that 60Hz was about optimal in terms of efficiency. However, even though 60Hz would be better, it didn't fit the neat paradigm of 1,2,5,10,20,50,100,200,500,etc used in metric conversions.

The answer Jeztyr gave has been distorted by the various people retelling it. Here's the original from IEEE: "Why does US use 60 cycles and Europe use 50 cycles? Many frequencies were used in the 19th Century for various applications, with the most prevalent being the 60 c/s supplied by Westinghousedesigned central stations for incandescent lamps. The development of a synchronous converter which operated best at 60 cycles encouraged convergence toward that standard. Around 1900, the introduction of the highspeed turbine led to settlement on two standards: 25 cycles for transmission and for large motors (this had been a compromise decision at Niagara Falls), and 60 cycles for general purpose systems. Meanwhile, in Germany, AEG  which used 50 cycles  had a virtual monopoly, and this standard spread to the rest of the continent. In Britain, differing frequencies proliferated, and Britain only settled on the 50 cycle standard after World War II." http://www.ieee.org/web/aboutus/history_center/bitsbytes/faqs.html So nothing at all to do with metric, and 60 being "better" really applies only to a certain synchronous converter which was partly developed with westinghouse in mind. The bottom line is it is a combination of historical accident and market forces: If AEG and Westinghouse were not such big players, there might now be a global standard which is different again.

how neutral grounding transformer's ratting is calculated?

Engineering calculations always involve the reciprocal of the frequency, the reciprocal of 60Hz produce repeating decimal places. During the early times, there were no calculators, most computations are done with pencil and paper. So, the brilliant engineers of Germany selected 50Hz which gives a reciprocal of 0.02 :) very neat. Why not use 100Hz instead?

The reason why we use a grid frequency of around 50Hz (60Hz in some countries) is because the pioneer of alternate current systems, Nikola Tesla, realized that it's a high enough frequency to be efficiently transformed by inductivelycoulpled circuits in a transformator, and yet low enough that there would not be major losses due to eddy currents in the transformator core and the wires. The interesting aside is, these frequencies (5060Hz) are also quite dangerous for the human body, as they will induce fibrillation of the heart rather easily. Much higher or much lower frequencies wouldn't. Bad luck.

In U.S the standards are 110V D.C and 60Hz frequency. The frequency is according to the design of the electrical machines.It's how many revolutions per second the rotor moves inside the magnetic field.
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