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Straight Dope Classic from Cecil's storehouse of human knowledge20-Nov-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? --
Mark, Berkeley, California
Dear Mark:
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? --Robert
Goodman, Bronx, New York
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.
--CECIL ADAMS
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