208/240V, was: 25 Hz Power (L |
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Danny Burstein (dannyb@panix.com) Sun, 12 Mar 2006 01:14:11 UTC
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In <telecom25.98.17@telecom-digest.org> wollman@csail.mit.edu (Garrett Wollman) writes: [ misc snip ]
> The standard residential supply in North America is 240 V single-phase
Just to expand on this a bit as I've run into it far too many times
As the above poster mentioned, "240V" is usually a "real" (more or
However, in multi-apartment dwellings in NYC and in a fair amount of
(That 120 degree number is coincidental and has little to do with the
Anyway ... in the three phase situation, a circuit that's using one
HOWEVER, if you grab two of the hot legs for the circuit, you're NOT
120V * (square root of 3) = 208V
(For the physical layout reason why this is often done see below ...)
Now in most, but certainly NOT all cases, appliances designed for the
Motors, depending on design, may either work slower (and/or less hard)
This is a common problem with air conditioners.
Heating devices, such as a furnace, will usually work ok _but_ will
Where you see serious issues is in things like a broiling element, or
The difference between 208V and 240 V is about 15 percent. So you'd
But ... in reality, since the voltage is down by 15 percent, the
That adds up ...
Similarly, a broiler element or a toaster wire frame will _not_ get as
(I'm leaving out the complicating factors of different reistances at
So again, something designed for 240V will probably work on 208V, but
Techish design issue:
The practical design that gets you two legs of three phase in many NYC
The transformer is tapped by three wires (plus neutral) that are 120
These then pass through each apartment, with the first one getting
All three legs continue to the roof for the three phase motor used by
Sidenote: I once saw an Isotec power strip that said, in big letters,
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