挺有意思的 电压等级变化

raisinbran, eventhough you concluded,let me supplement as this was a topic intriguing me.My search concluded as below:

Commercial Electric distribution was first done by Edison and his Manhattan DC lighting system was opened in September,1882.Edison took 100V as the voltage for his incandescent lamps.( Why 100 V?Round number,safe electrical voltgae to handle,but high enough to save copper in conductors)But it soon became clear  that at the fag end of street the lamps were not burning well due to voltgae drop in line.Edison had to raise the generator voltage to 110 V ( there by distribution voltage became 110V!)to take care of the voltgae drop in line.To save copper, Edison raised the voltage to 220 V by adopting three wire Dc system .

When in 1886, first AC power distribution was installed at Great Barrington,Massachusets, Westinghouse and Stanley adopted 500 V as generator voltage 3 KV as transmission  voltage and finally 100 V as distribution voltage.When first 3 phase Ac line was put up by Siemens to connect Lauffen with Frankfurt,generator voltage was 55V( half of 110V) and transmission was at 15Kv to be raised to 25 kV later.

According to clause 3.2.6 of Red book,(IEEE 141-1993),later distribution and transmission voltages  were developed as multiples of 110 
ie utilisation voltages of 110,220,440,550 V and distribution voltages of 2.2,4.4,6.6,13.2 Kv and transmission voltgaes of 22,33,44,66,110,220Kv 

In US to keep the utilisation voltage same at full load, supply voltage was kept little more high ie 115 V.Then a new series occurred ie utilisation voltages of 115,230,460,575 V and distribution voltages of 2.3,4.6,6.9,13.8 and transmission voltages of 23,34.5,46,69,115,138,230 Kv etc,

As a result of continued problems with the operation of voltage sensitive lighting equipment and voltgae insensitive motors on the same system and the development of the 208Y/120V network system, supply voltages were raised again to multiples of 120 V 
120,208Y/120,240,480,600V as utilisation voltages and 2.4,4.16,2.4,4.8,12,12.47Y/7.2 Kv as distribution voltages.