TELECOM Digest OnLine - Sorted: 208/240V, was: 25 Hz Power (L

208/240V, was: 25 Hz Power (L

Danny Burstein (
Sun, 12 Mar 2006 01:14:11 UTC

In <> (Garrett
Wollman) writes: [ misc snip ]

> The standard residential supply in North America is 240 V single-phase
> center-tap, with 120 V nominal on each leg, at 60 Hz. However, some
> residences, particularly in New York City, have 208 V three-phase wye
> with 120 V nominal on each leg, still at 60 Hz.

Just to expand on this a bit as I've run into it far too many times
... and it may answer some wierd problems otehr people see here and

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.

However, in multi-apartment dwellings in NYC and in a fair amount of
other places, including some locations and places here and there
... what you've got is a "three phase" feed from the transformer; that
is, three separate wires each 120 degrees apart.

(That 120 degree number is coincidental and has little to do with the
120 Volt figure).

Anyway ... in the three phase situation, a circuit that's using one
hot wire and a neutral gets you that same 120V as in the single phase

HOWEVER, if you grab two of the hot legs for the circuit, you're NOT
getting 240V but instead, because it's 120 degree out of phase, you
get (again, that's separate from the 120V... ) you get:

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
nominal 240V will work ok at the lower voltage. But there will often
be some diminishment.

Motors, depending on design, may either work slower (and/or less hard)
-or- may pull higher amperage to do the same work (and will be
hotter ... burning out sooner).

This is a common problem with air conditioners.

Heating devices, such as a furnace, will usually work ok _but_ will
take longer to heat up. Similarly, something like an oven will take
longer to come to temperature (but should be ok once it stabilizes).

Where you see serious issues is in things like a broiling element, or
for that matter when you try boiling water ... Let me explain:

The difference between 208V and 240 V is about 15 percent. So you'd
think it would take 15 percent longer to get your kettle going.

But ... in reality, since the voltage is down by 15 percent, the
current is also down by 15 percent. Meaning the wattage (power) is
down by roughly 25 percent.

That adds up ...

Similarly, a broiler element or a toaster wire frame will _not_ get as
hot, so won't be toasting (crisping) the food as well.

(I'm leaving out the complicating factors of different reistances at
different temperatures. Feel free to work on them at your leisure)

So again, something designed for 240V will probably work on 208V, but
check it out first.

Techish design issue:

The practical design that gets you two legs of three phase in many NYC
structures (and other places):

The transformer is tapped by three wires (plus neutral) that are 120
degrees apart.

These then pass through each apartment, with the first one getting
legs "a", "b", and the neutral (and a safety ground). The next
apartment gets "b" and "c". The third gets "a" and "c". Rinse, lather,

All three legs continue to the roof for the three phase motor used by
the elevator and, perhaps, some central air condtioning equipment,

Sidenote: I once saw an Isotec power strip that said, in big letters,
that it only worked on 208V and should NOT be used on 240V. I've never
been able to track that one down again...

Knowledge may be power, but communications is the key
[to foil spammers, my address has been double rot-13 encoded]

[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]

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