origin of voltages (why 120, 240, 440, 6900, 24900 Values)

In 1886, Nikola Tesla discovered a large number of electrical devices rated for operation at those voltages for sale on EBay. The price was low because nobody knew how to generate AC power properly in those days.

Tesla bought all of the devices and invented the necessary generating equipment. And that's what happened.

That's silly. They picked those voltages more than 100 years ago based on one of their back-to-the-future trips!

Roby wrote:

Look up ANSI standards C-37 and C-84. Those documents may give some insight into why these particular voltages were selected.

Keep in mind that historically 120V single-phase AC was much lower when commercial power generation started near the turn of the 20th century.

Selection of 110 v. was based on a complex set of requirements which included what would be optimum for a carbon filament lamp to operate at the maximum luminous level, give a reasonable long lamp life, overcome the voltage drop from the dc central station generator (in use back in the 1870's), provide a certain degree of safety, and provide design parameters for associated equipment such as fuses, switches, insulation thickness, etc.

Beachcomber

Because 121, 243, 449, 6932 and 24941 just seemed silly.

I can't give an authorative answer, but I have noticed that many of the voltages are often either multiples of 120V or 120V times sqrt(3) or

120V/sqrt(3) (~69V). Usually the multipliers are powers of 2, 3 and 10. Sometimes the "120V" figure is off somewhat. Examples of the first are 480V, 7200V, 345kV (multiple of 115, not 120), voltages like 277V, 69kV and 13,800V are multiples of 120V/sqrt(3). The sqrt(3) falls out of 3 phase power math. These voltage ratios probably comes from integer winding ratios in transformers.

Wasn't it 110 volts at one poin?

Michael Mor> I can't give an authorative answer, but I have noticed that many of

The beer drinker won as he could pee higher on the wall than the others, reaching 120cm. You don't hear of him as, after this, he was arrested for exposure and being a public nuisance.

Don Kelly @shawcross.ca remove the X to answer

It was all over the place between 100 and 130 in the earliest days.

In the era of Edison General Electric, when the same company made the whole system, including the lamps, lots of (to us) strange things occurred.

One issue was the matter of carbon filament lamps. The uniformity of these lamps was poor, but the lamp life then, as today, was a steep function of voltage. This posed a major problem in that lamps were not truly interchangeable. The earliest solution was to sort the lamps into voltage categories. However, because the lamps and distribution and energy were all supplied by the same entity, it was not a problem to run one neighborhood or city at 110 to use up the 110 volt lamps, and another area at 125 to use up the 125 volt lamps. This practice declined with the advent of better lamps and was pretty much obsolete by World War One.

Check out the IEEE historical section. The added voltage was built to compensate for voltage drop. Edison determined that the optimum voltage for his carbon filament lamps was about 100 volts. However, based on the physics of the Edison DC Central Station, the longest practical run at that voltage would be just a few city blocks and the drop averaged 10 volts or so at the maximum distance. Being DC, there was no way to boost the voltage. Thus, 110 volts became optimum at the generator/central station.

Beachcomber

As systems progressed, utility companys raised the voltage to 115,

120, and eventually a maximum of 125 volts as it became economically more efficient to transmit at the higher voltage. By this time, tungsten lamps had replaced the carbon filaments