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

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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