Yep----that's all single phase is, assuming you want 240 or 208 volts. If
not, all you need is one leg to neutral, but it has to be the proper leg if
you have delta service. The high leg to neutral in that case would be 208
Volts, and is usually the center leg, or B phase, on the panel. Be sure
to run a ground. If none of this makes sense, get someone to help that
better understands electricity.
Okay , I have to disagree with you . Delta configuration , all three legs
should be at specified voltage in relation TO EACH OTHER . Whether or not
any leg shows a potential to ground depends on how the service is grounded -
the service at the cabinet shop I work at appears to have one leg at ground
potential . I have no idea what code says . Wye connected , same in relation
leg/ leg , but quite often the center connection is grounded , giving half
of rated voltage between any leg and ground .
Of course I could be full of shit , it's been a long time and some of the
details are a bit fuzzy .
No. In a center-grounded Y, line-to-line is sqrt(3) or 1.732 x line
to neutral. 208 line-to-line 3phase is 120 line to neutral. 480 line
to line 3phase is 277 line to neutral, used in some lighting
In a delta with one leg grounded, line to line and line to neutral
will be the same for the other two legs.
I was wondering how it worked out to sqrt(3) seeing it should be based on
trig. But it does. The answer using trig is 2 * cos(30deg) which is the
same as the sqrt(3). With trig, you just form a triangle representing the
voltage vectors. So you have two legs for 120 V at 120 deg out of phase
and the vector from tip to tip is the 208 leg. It's a triangle with one
120 deg angle and two 30 degree angles. The lengths of the sides are 120,
120, and 207.846....
Isn't there also a configuration where they put a center tap on one of the
delta windings and use that as the ground? I think I saw that once on some
book or web site where they had 220 V 3 phase and the center tap gave you
110 V to two of the 3 phases. Ground to the other phase would be some
other strange value (190 I think). I've never actually worked with 3 phase
circuits so I don't know if this is actually used or not.
Ah I see now, 240 V 3 phase delta with one winding center tapped for the
ground gives you 120 V to two of the phases and 208 to the third phase.
The 190 number is what you get if you do the math using 110V/220V instead
of 120V/240V for the numbers.
The key word here is *delta* (or Wye wiring).
A phase and C phase to neutral are 120 volts. B phase to neutral is 208
volts. I'm not disputing phase to phase voltage, which, in the case of
delta, should be 240 volts. I suggested that if the motor required 120
volts that one must then select the proper phase for achieving the desired
voltage. If I'm wrong, then the wiring in my shop, and past shops I've
owned with three phase delta service, have a serious problem, because that's
the voltage, measured to ground with a respectable meter. It's also why
there used to be blank spaces in the panel in my previous installations.
The current shop has more than one panel, so all single phase service comes
from a single phase panel.
I don't usually mind looking ignorant , as you say I learn lots when better
minds step in . My downfall is the math . I useta know a couple of formulae
for specific items , but never could "get" algebra . I still know a few
formulae for things , just not these things ...
And a day I didn't learn something is a day wasted . I learned a *bunch*
Thanks for the education , I appreciate it .
On 26 Feb 2007 00:58:36 GMT, with neither quill nor qualm,
firstname.lastname@example.org (Curt Welch) quickly quoth:
Read any good books lately, Curt? <bseg>
=========================================================CAUTION: Do NOT look directly into laser with remaining eyeball!
I read the new Miller 2007 catalog this week. That's a good book. :) I
recently read most of "Welding Technology Fundamentals" by Bowditch
Bowditch and Bowditch for the two welding classes I'm taking. That's also
a good book.
I'm currently reading "Consciousness Explained" by Daniel Dennett. It's
good, but a bit slow - I haven't picked it up in about a month now but it's
on the top of the stuff I'm reading. I found Darwin's Dangerous Idea by
Dennett to be a far more interesting read. I find his views refreshing.
I'm also trying to get though "Spikes, Exploring the neural code". It's
mostly math which makes it fairly slow going. Not too long ago I read
Dawkins' "The Selfish Gene". That's a classic. I've got Minsky's new book
"The Emotion Machine" sitting in the "to be read" queue. I'm not sure if
I'm going to find that very interesting or not. His SOM is a classic I
read not too long ago. I've got about 20 books and a large assortment of
technical papers on AI related stuff sitting to be read. Not to mention a
nice backlog of Popular Science, and New Scientist, Robotics, and
Scientific American, and other such magazines waiting to be read.
But sadly, nothing on political issues like global warming in the stack. :)
On Sun, 25 Feb 2007 21:11:17 GMT, "Harold and Susan Vordos"
Interesting! Took me a minute to figure this one out though it's
probably quite common as you say. (I'm not an electrician)
Start with a Y-connected xfmr producing 240 line-to-line 3phase. The
center of the Y is merely that--center. We haven't defined a neutral
Now connect a voltage (xfmr winding) of 69 volts to the center that is
in phase with B. Make the other end of that neutral. Now lines A and
C are 120V WRT neutral and are opposite in phase, while B to neutral
is 208 volts, in quadrature with A and C -- leading one and lagging
the other. Result: A and C are 120 line to neutral and 240 line to
line (like single phase power) while AB, AC and CA comprise 240V
threephase. I can sure see how that would make a lot of sense in a
shop. The center of Y-connected threephase loads will be 69 volts
above neutral, which doesn't matter a bit as long as they're not
connected to anything.
Or do what they probably really do and just use a delta transformer and
center tap one of the delta windings for the ground. That gives you your 69
volts to what would be the center of the Y if it existed.
How does a 3 phase panel work? I've never seen one. It will have 4 wires
coming into the box right, and if it's 3 phase delta, then you end up with
240/240/240 across each phase and 120/120/208 from phase to neutral right
(for 240 delta service)?
Is it like a home panel with the phases alternating so that every 3rd
breaker is the same phase? So to hook up a 3 phase feed, you need a
breaker with 3 gangs feeding it? And for 120 circuits for lights etc you
just use breakers in the A and C phase slots? And for 240 you can use any
two side by side breakers (assuming it's a pure 240 circuit you are
creating and not a 240/120?) Or do you just always use A-C phase pairs for
220 V circuits so the ground is always 120V to both?
Is this type of 240 V 3 phase service common in commercial settings because
it gives you the standard 120V circuits at the same time?
I guess 208 Wye gives you 120 on all phases to ground. Is that a common
service as well?
I've always specified and used Square D products. They are amongst a few
that are still using copper buss bars, and are of reasonably high quality.
Well respected in the industry, although not cheap. What I'll describe is
proper for Square D. I have no experience with other panels.
As you allude, the panel accommodates a breaker with three poles. There are
three buss bars that run vertically in the panel, with the center bar the
high leg when wired delta. That is the second position on the breaker
panel, so if you use the three phase panel for single phase, that position
may not get used. For sure it won't for 120 volt service, but there's
nothing preventing you from using it for 240 volt single phase service.
The panel is wired with the three phases, a neutral, and a ground. Five
wires total. Not all three phase service has the neutral. If there are no
120 volt loads, it's possible that a panel wouldn't be able to provide such
service considering it's no longer legal to use the ground as a conductor,
although it would work fine.
Or do you just always use A-C phase pairs for
You need not use them in pairs. Each slot has its own breaker, single pole.
You'd have a blank space in the panel where the high leg would be if it
wasn't used for either 240 volts or three phase service.
All depends on the load needs and panel size. Because 1/3 of a three phase
delta panel is wasted when the panel is used for 120 V service, you may find
that a second panel, strictly single phase, is needed. That's what I've done
with my shop. Everything is fed from the same transformer source, but it
separates at the pole and goes through two different meters. That, in my
case, was a good thing, because I'm on a demand meter that changes the price
of power when I exceed 50 kw demand. 50 kw is the size of my induction
furnace, so I'm damned glad to not have everything running through the
Indeed, and is far more common than delta. There are some issues with
delta that present a fire hazard, according to PUD (our provider). They
really push the star (wye) service, which is 208 volts, but that's not in
the best interest of almost anyone that runs motors. While they'll usually
run on 208, they draw more amperage and run somewhat hotter. In some
instances, devices don't perform as intended when they're designed around
240 volts. A friend had this problem when he moved his saw sharpening
service from a delta wired shop to a wye wired shop.
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.