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#67141 07/08/06 11:27 PM
Joined: Oct 2000
Posts: 2,722
Broom Pusher and
Member
Circuitman:

Quote

scott35, i have a transformer connection manual that shows the hi leg of a 120/240 volt delta on "C" phase always. but you say code says should be "B".wonder why the difference? BTW it's a GE transformer connection manual.

As viewing the 4 Wire Delta Transformer Network (3 interconnected Single Phase Transformers),
the Lines (or "Phases") encountered would be:
*** "A" ***,
*** "N" ***,
*** "B" ***,
*** "C" ***

On the Transformer which uses the Center Tapped Secondary Winding's Terminal Bushing, the first Line Conductor (on the left side) would be Line *** "A" ***.

Next to it, and in the center of the three bushings, would be the Grounded Neutral Conductor - or Line *** "N" ***.

The last Line Conductor (on the right side) would be Line *** "B" ***.

Line *** "C" *** is the from the Paralleled Termiantions, which tie the farthest _LEFT_ side Secondary Bushing's Terminal on the _LEFT_ side Transformer, to the farthest _RIGHT_ side Secondary Bushing's Terminal on the _RIGHT_ side Transformer.

When the Service Entrance Conductors are ran into Service Equipment, the "Rotation" follows the "Right-Hand Test-Bypass-Block As Power Leg" configuration. The Conductor on the "Right-Hand Side" is the one tagged Orange (the High Leg).
The "Rotation" stays this way, until the Feeders reach "The Customer's Side" (normally, the "LOAD" side of the kWh Meter).

This is the point where "NESC" and "NEC" begin and end (in a matter of speaking!).

The NESC "Ends" on the LOAD side of the kWh Meter, or at the LINE side of the Service Disconnect.
Where the NESC "Ends", the NEC "Begins".

NEC wants the Orange Colored Conductor (the High Leg), to be in the *** "B" *** Position, on the Bus Kit of 3 Phase Panelboards, Switch Gear, and Disconnect Switches.

If the Grounded Neutral Conductor (Identified Conductor, or simply "The White Conductor") is "Used" in any 3 Phase Panelboards, Switch Gear, or Disconnect Switches "Downstream" of the Service Equipment, then the Line *** "B" *** "High Leg" shall be tagged Orange.


Quote

one other ? why is the hi leg 208?

This one has been covered a few times before, but it never hurts to ask when needed!

The "High Leg" on a 240/120 VAC, 3 Phase 4 Wire Delta system is approximately 208 VAC to Ground (to the Grounded Neutral, or more correctly, the Center Tap of the 1 Phase 3 Wire "Section" of the configuration).
It may be 190 VAC to Ground, it also may be 220 VAC to Ground - all depends on the loaded conditions of the Transformer Network.

Typically, the "High Leg" will be 1.732 x the L-N Voltage of the 1 Phase 3 Wire "Section" - which equates to roughly 208 VAC.

Simplest explanation of why this equals out like it does:

Phase Shifts and a Series Adding Winding configuration.

Detailed explanation:

Too much stuff to type out at this time! [Linked Image]

Best if you perform a search in this forum area, along with the Electrical Theory section, using a topic like "4 Wire Delta", or "Transformers".

Good luck.

Scott35


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
#67142 07/09/06 11:47 PM
Joined: May 2004
Posts: 116
S
Member
All -

Once again thanks to everyone that has provided information and insight.

I know I can always get great answers and learn things here.

Steve


Thanks
Steve
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