Re: AC Mains panel.... What on earth do I have here???

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "David Sharpe by way of Terry Fritz <teslalist-at-qwest-dot-net>" <sccr4us-at-erols-dot-com>

Paul

What distribution system you are describing is a called in the trade a
"high-leg delta" configuration.  Imagine 3 standard 240V (120x120)
transformers wired in a delta configuration.  Tie any one of the
transformer CT's to building neutral (you now have 120 + 120
across that one transformer to neutral).  If you measure the voltage
to the opposing unused hot feeder it will be to the ratio of approximately
115+115+190V.

The high leg feeder should have some kind of distinguishing phasing
tape, as a warning that there is no reference to neutral (except through
two transformer windings).  I've personally seen several machine shops
in our locale wired this way.  It allows total utilization of kVA capacity
on pole, versus wiring 120-208 wye (and tossing effectively 50% of
your capacity away since you would only be using half of the transformers
LV windings).  FWIW.

Regards
Dave Sharpe, TCBOR
Chesterfield, VA. USA


Tesla list wrote:

 > Original poster: "Paul Kidwell by way of Terry Fritz 
<teslalist-at-qwest-dot-net>" <tmb-at-ieee-dot-org>
 >
 > Hi Everybody,
 >
 > I'm part owner of an engineering firm. I was looking at our
 > AC service to see what I had available and I came across
 > something that had me a little confused. Our office outlets
 > all measure real close to 120 VAC but we have three phase wiring
 > coming into the back of out building. I'm no great expert
 > when it comes to polyphasic power distribution, but seeing a
 > nice 120 VAC at my outlet, 240 at some of out heavy
 > equipment, and three phase power coming into the back of our
 > building something didn't seem quite right. (from what I've
 > read, if I had 120 VAC at the normal outlets, I should have
 > been seeing something other than 240) So I took a bit of a
 > walk out back to see what was coming in.
 >
 > Here's what I found...
 >
 > On the outside of our building is a pole with a trio of 100
 > KVA pigs that we share with one other building.
 >
 > There are four large feedthroughs on the sides of our building,
 > (one neutral plus one for each phase)
 >
 > On the inside is a metering cabinet that has a set of three
 > current transformers that are connected to our meter (in a
 > small meter box below this cabinet) and the three distribution
 > lugs that run off to the breaker and fused disconnect
 > panels mounted near by. At the bottom of the cabinet is a fourth
 > lug for neutral.
 >
 > Almost everything (office and most of our other equipment)
 > is connected through one large breaker panel to neutral and
 > the two outside three phase lugs.
 >
 > One other breaker panel handles most of our heavy three
 > phase equipment and has connections to all four lugs.
 >
 > Our wave solder machine (no longer functional :/ ) has it's
 > own separate 100 amp fused disconnect also connected to all
 > four lugs.
 >
 > I used a meter and checked the lugs in the metering panel,
 > (that's as close to where power came into the building as I
 > could get without a ladder)
 >
 > I had 240 VAC between any two of the three phase lugs. (closer
 > to 245, actually)
 >
 > I next checked the voltages between the neutral lug and each
 > of the three phase lugs...
 >
 > Going from neutral to either of the outside three phase lugs
 > gave me 120 VAC
 >
 > (this is where things got interesting)
 >
 > Going from neutral to the center lug gave me... 214 VAC???
 >
 > Hmmmm....
 >
 > Ok, if I ignore the center one of what I'm calling the three phase
 > lugs, then everything looks exactly like what I have in my house.
 > (120 VAC between neutral and either feed lug and 240 VAC between the
 > feed lugs )
 >
 > I don't really want to drag a scope back there, but at home the two
 > feed circuits are in phase but opposite polarity.
 >
 > Is it the same situation here???
 >
 > And how does the center lug relate to the other two so I still get
 > 240 VAC from it to the other two???
 >
 > On closer inspection I found a penciled in not across the top of
 > the wave solder machine disconnect where someone wrote 120-208-120
 > so I guess I'm seeing what I should be.
 >
 > Here's another thought. Some time ago I acquired a triple stack
 > variac (3 x 580 volt -at- 30A) My original plan was to parallel the
 > three so I'd have a maximum of 90A. Using such an arrangement at home
 > would result in having 240 VAC on your output lines when you
 > have full voltage output on the variac, but setting the variac at
 > zero would wind up with 120 VAC with respect to neutral on both
 > lines. (I've been told that this isn't a great way to do it)
 >
 > Seeing I have this wondrous center lug here, could I use the outer
 > two lugs to run my lights, contactors, RSG phase control, ect.; and
 > the center lug to feed to power the triple stack? (that way, zero set
 > on the variac would mean zero output to the pig.)
 >
 > Thanks,
 >
 > Paul