I’m not an electrician, although I’ve done my share of DIY home wiring over the years. Ever since I started driving electric cars, which utilize 240-Volt charging stations, I’ve been curious why REcolorado (Denver’s MLS) refers to 110-Volt and 220-Volt service instead of 120-Volt and 240-Volt service. And they’re not alone in doing so.
Recently I asked someone at Xcel Energy to explain this dichotomy. What I was told was that 120 volts became the standard as a result of the Rural Electrification Act in 1930. As for how you get 240-volt service, it’s created when two 120-volt lines of opposite phase are combined.
Nevertheless, most Americans continue to refer to electricity as being 110 or 220 volts, even though those voltages no longer exist and haven’t for nearly a century in the United States.
Using a voltmeter, I verified that my own home’s outlets are running 120 volts and 240 volts.
I've wondered that too. There's a good history at https://www.quora.com/Is-the-power-system-in-the-US-technically-110-115-or-120VAC-How-about-220-or-is-it-240-or-235VAC :)
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