[BC] Electrical Code

[Milton R. Holladay Jr.]

As most, if not all, 3phase generators these days of a size that a station
would use only provide 120/208 wye, those with stations having them are set
up for wye.
I encountered one open delta corner grounded service  provided  by "Nuke"
Power in NC; 120V was xfmr derived.
Installation of a generator required change to a wye which they derived from
the two phases present with three xfmrs in, I presume, a Scott T
arrangement.
M
----- Original Message ----- 
From: <PeterH5322 at aol.com>
To: <broadcast at radiolists.net>
Sent: Friday, October 21, 2005 11:55 PM
Subject: Re: Re: [BC] Electrical Code


> > If the three phase is Delta connected (Not common anymore) then
> > the voltage is 240 volts leg to leg and one of the leg to leg
connections
> > is
> > normally center tapped to form the neutral and 120-0-120 for single
phase
> > load.  Either end of the center tapped transformer winding to neutral is
> > 120
> > volts and is usable the third leg (often reffered as the "wild leg") can
> > not
> > be used to neutral as it varys wildly depending on the load and current
> > unbalance.
>
> 120/240 three-phase (208 volts from the B phase to the neutral) is
> probably much more common than 120/208 three-phase.
>
> Certainly more new high-powered installations are 120/208 Wye or 277/480
> Wye, with 120/240 single-phase being derived locally, using a
> customer-owned "dry type" transformer.
>
> This is especially true where the customer specifically requested either
> of those distribution voltages, and he agreed to provide his own 120/240.
>
> But lower-powered commercial and industrial installations are still
> 120/240 Delta three-phase, for the most part, especially under about 300
> KVA.
>
> Open Delta is almost always 120/240 single-phase with a small "kicker"
> transformer for the third phase.
>
>
> It is really "grounded B phase" delta which is now not so common, and, in
> fact, is no longer approved for new installations.
>
> (The NEC refers to this in an oblique way by stating that "delta
> breakers" are not allowed.
>
> (Yet, Square D, and others, still provide such "delta breakers", for use
> as replacements in existing installations.
>
> (One advantage of a "grounded B phase" three-phase system is you can
> control and distribute three-phase power while utilizing a common 120/240
> single-phase panelboard.
>
> (You still distribute the three-phase using four wires, A, B, C and
> protective ground, but there are no breakers in any B phase, as the B
> phase is at ground potential, and, in fact, it is tied to the protective
> ground at the service panel, thereby giving this system its name ...
> "grounded B phase", AKA, "corner grounded" three-phase Delta).