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

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

Robert Bonomi (
Tue, 14 Mar 2006 03:40:30 -0000

In article <>, Danny Burstein
<> wrote:

> "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.

"Two-phase" described 2 hot-wires that were 90(!!!) degrees out-of-phase.

It is demonstrably single-phase, as there is only one secondary coil.
It just happens to have a 'center tap', which is at 'relative' zero potential.

The rest of Danny's dissertation is spot on. :)

> [TELECOM Digest Editor's Note: I know of a janitor in an apartment
> building who always used 25 watt _240 volt_ light bulbs in the
> exit signs in his building (even though the fixures were the more
> standard and customary 120 volts.) He said those 240 volt bulbs
> (which were in difficult to reach places) _never_ had to be changed;
> they would on burning for several years. He did not like the idea of
> getting out a step ladder to climb up and change a bulb in an exit
> sign if he could avoid it. PAT]

Years? *SNORT* make that decades or *centuries*. expected lifetime
of a light-bulb is approximately a 15th-order inverse relationship.
Halve the voltage, and lifetime will be roughly 2**15th (32,768) times

The 'lumens' (actually lumen-hours) of light output, per dollar of
electricity supplied will be far, *FAR* inferior to the standard 120v
bulb. There are standard "rough service" bulbs (lifetime for which is
at a rated 130v) that are "close" to as dollar-efficient in energy
usage as the 120v bulbs, but with a greatly improved lifespan. They
don't cost much more than _quality_ 'standard 120v' bulbs, assuming
you can find somebody that carries them.

[TELECOM Digest Editor's Note: Precisely! But a 25-watt bulb at
240 volts in a 120 volt fixture still presents a reasonably decent
glow to point people in the direction they should walk to reach a
fire escape, for example, which is I think how he was doing it. PAT]

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