[BC] Electrical Code

[PeterH5322@aol.com]

> 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).