3W 3PH corner grd delta

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wieand

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I have seen a three phase corner grounded delta service which is both three phase three wire 120 volt and simultaneously a three phase three wire 240 volt service. the way it is done is three pole mounted tubs are wired delta and the b-phase is grounded. This makes the 240 delta service with the corner grounded
Also two of the tubs have the center tap brought out and connected as phase conductors along with the grounded b-phase. This panel is then a 120 delta service although only used as 120 single phase load center. Is this a code violation?
 

wirenut1980

Senior Member
Location
Plainfield, IN
I agree with Larry. A corner grounded delta service would not have any wires coming from the centertaps of the transformers (X-2 bushing). If there is a conductor coming from the centertap of one of the transformers, then you would have a "center-tap" grounded delta service and would have 4 service wires (assuming one conductor per phase).

Edit to add, a wye secondary also has conductors connected to the X-2 bushings of all three transformers and those are connected together...
 
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jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
It is not physically possible to have both a 240V L-G delta and a 120V L-G wye connection from the same transformer.

The OP seems to be describing a 240V corner-grounded closed delta transformer with two transformers center-tapped at 120V. Let's call these center taps as a' and c'. We can connect use these two transformer connections as a 120V corner-grounded open-delta transformer.

The resultant 3-phase voltages are:
System 1
Vab = 240V, Vbc = 240V, Vca = 240V; Vag = 240V, Vbg = 0, Vcg = 240v

System 2
Va'b = 120V, Vbc' = 120v, Vc'a' = 120V; Va'g = 120V, Vbg = 0, Vc'g = 120V

While I have never seen a system like this, I do not see a voltage problem as long as there are no connections from a or c to a' or c'.
 

Smart $

Esteemed Member
Location
Ohio
While I have never seen a system like this, I do not see a voltage problem as long as there are no connections from a or c to a' or c'.
I foresee a problem with someone trying to run a MWBC out of the single phase load center. Phases are only 60? apart, instead of 120? like a 120/208 single phase panel. The shared grounded conductor's currrent on a balanced, wired-as MWBC will exceed that of either Line conductor.
 

jim dungar

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Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
I foresee a problem with someone trying to run a MWBC out of the single phase load center. Phases are only 60? apart, instead of 120? like a 120/208 single phase panel. The shared grounded conductor's currrent on a balanced, wired-as MWBC will exceed that of either Line conductor.
I never said this was a desirable installation. I can foresee many potential wiring problems but that is what happens when people work on systems they do not understand.

For a 3-wire 120V circuit, the shared line conductor can never carry any more than the vectorial difference of the other lines. It is a standard open-delta connection that happens to have only 120V per phase.
 
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