The voltage at which the US operates the national grid varies from 240KV to 640KV. There are a few smaller voltages used but the are not common. Notice the voltage is at KILO-volts. This is the voltage which comes out of the power plants and on to the grid. Except for a very few exceptions it is always within a few ticks of 60 HZ. It is not possible to connect to the grid at any other frequency other than 60 cycles. The voltage has to do with transformers and the standards which were developed in the early part of the 20th century. AC power is much more efficient when it is transmitted at higher voltages and then stepped down to house voltage of 240 volts. This power is single phase and can be split into two circuits at 120 volts each. This is the way it is done and it is not going to change on this continent. In the second half of the last century all US power is at 60 hz. Gonk has already explained most of the reasons. Outlaw saw fit a while back to actually make a few 2150s which ran at the higher voltage but they likely need 60 hz power. That means you either run some converter circuitry or you don't buy one.