TELECOM Digest OnLine - Sorted: Re: 208/240V, was: 25 Hz Powe

Re: 208/240V, was: 25 Hz Powe

William Warren (
Mon, 13 Mar 2006 00:08:39 -0500

Danny Burstein wrote:


> As the above poster mentioned, "240V" is usually a "real" (more or
> less...) 240V based on tapping two 120V legs against each other. If
> they're (that is, both wires) coming off opposite sides of the
> transformer, you get a simple addition (120 + 120 = 240) [a].

> [a] I'd personally consider that design
> to be two-phase, since the legs
> are 180 degrees apart, but the
> rest of the world disagrees with
> me and calls it single phase.


I think the confusion comes from the use of ground as a reference: a
wye-connected three-phase circuit has three distinct phase angles **
with respect to ground **. In a center-tapped secondary delivering
240/120 to a home, there is only one phase angle with respect to
ground. The polarity is different, of course, but the absolute value
of the angle is the same.

The disparity might also be an accomodaction to history: some
electrical systems used to deliver two phases of power separated by 90
degrees (and thus requiring four wires). See and for

If I had to guess, I'd say that electrical engineers decided to call
our usual 240/120 CT arrangement "Single phase" to distinguigh it from
the four-wire two phase systems, and possibly because engineers think
in terms of differences: two wires only have one angular difference.

If there is a "Two phase" system in current use (pun intended), I
think it would be a delta-connected service with one leg grounded. I
don't know what an electrician would call it, but "Delta breakers" are
commonly used for this arrangement, i.e., there are panels with
two-pole breakers that control two legs of a delta feed, with the
third being grounded. Come to think of it, maybe this is the
"standard" for delta: any electricians care to comment?


William Warren
(Filter noise from my address for direct replies)

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