A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

How come the U.S. uses 120 volt electricity, not 240 like the rest of the world?

November 20, 1992

Dear Cecil:

How come the U.S. is practically the only country in the world where household electricity is 110 volts instead of 220 volts?

Cecil replies:

The penalty of leadership, champ. While inventors in many countries contributed to electric power technology, the U.S. was way out front in putting that technology to practical use. In the early days, lower voltages were the most practical for electric lights — higher voltages burned out the bulbs. So the hundreds of power plants built in the U.S. prior to 1900 adopted 110 volts (or 115 or 120 volts) as their de facto standard.

Trouble was, power transmission at higher voltages was more efficient — you didn't have to use so much copper in the wires. By the time most European countries got around to making big time investments in electricity, the engineers had figured out how to make 220-volt bulbs that wouldn't burn out so fast. So, starting in Germany around the turn of the century, they adopted the 220-volt (or 230- or 240-volt) standard. But the U.S. stayed with 110 volts (today it's officially 120 volts) because we had such a big installed base of 110-volt equipment.

But don't worry that we're stuck with a technological dinosaur. Fact is, homes with standard 3-wire electrical service in most parts of the country get 240 volts. The three wires that come in from the street are 120 volts positive, zero volts (neutral), and 120 volts negative. (I know, this is alternating current, not DC, so we can't really say "120 volts positive," but don't bother me with details.)

Take the neutral and either of the other wires (the usual practice) and you've got 120 volts. But tap into your plus-120 and minus-120 and you'll get a 240-volt jolt, handy for energy-hungry appliances like air conditioners or electric stoves and clothes dryers. The telltale sign in the fusebox is a special double-width circuit breaker that straddles the plus-120 and minus-120 bus bars. Not the most vital fact in the world, but at least next time you're poking around in there when the lights blow you'll have some idea what you're looking at.

Static

Dear Cecil:

Are you going to explain to your readers that, with the three-phase wiring prevalent in the world, the two lines are only 120 degrees apart in phase, and not of opposite polarity as you stated? And that therefore tapping across them provides only 208 volts, not 240? Or do you figure nobody will miss the other 32 volts AC? And that explaining three-phase polarity isn't worth the space, justifying your fudge?

Cecil replies:

Sarcasm plays better when you have at least a general idea what you're talking about, Robert. Three-phase power is used primarily in commercial applications, not homes. When I was an electrician's apprentice, I remember we installed it in a garment factory for use with portable electric cloth cutters. The electrical service in most U.S. homes is 240 volts single phase with a center tap, giving you the 120 volts needed for most household uses. To be fair, New York, in this as so many things, is an exception. There 208 volts is the standard high-end voltage.

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