3 Phase 240 V Panel?

[gar]

210510-1725 EDT

Look at this paragraph from the first post:

"The panel appears to be a 3 phase panel, it has three lines entering the terminals at the top of breaker. When measuring voltage across each phase to ground (A,B,C) voltage is 125V through each phasehowever when measuring voltage across phases A to C reads 250V, B to C reads 250V, but phase A to B gives a reading of 1V; practically nothing."

Each phase wire (A, B, C) to neutral or EGC is 125V. That alone looks like a center tapped single phase source. Note 2*125 = 250. So it is not a wye source.

If this was a two transformer open delta you could not have two center tapped secondary coils with the center taps connected together. You can have one center tapped secondary connected to a neutral and/or EGC.

Now suppose the secondary BC has an open circuit in its path, then B is unlikely to be close in voltage to A. Putting a reasonable load on B to neutral would cause a major shift in voltage at B.

Next suppose the secondary BC is a conductive path. and that the primary has an open circuit, then a loaded B to neutral would pull down B toward neutral because the impedance of the secondary of a transformer with its primary open will not look like a low impedance.

Next suppose B to C is a short circuit, then the voltage A to B would be 250 V.

Everything points to a three phase panel being used as a single phase panel. Put a scope with channel 1 from neutral to line A, and sync on this signal. Use channel 2 to observe various voltages. Line C to neutral will be 180 degrees from A to neutral. I believe B will be in phase with A.

.l

 

[gar]

210511-2031 EDT

cgardener1:

Tonight on my main panel (240 V center tapped) measurements at the panel with an approximate 12 A load change on one phase (Phase A) were:

Phase A ---- 123.1 V drops to 122.3 or 122.4, a change of 0.8 or 0.7 V. This is the loaded phase.
Phase B ---- 123.3 V rises to 123.7 or 123.8, a change of 0.4 or 0.5 V. This is the phase with no load change on it, and with the load change on the other phase.

A typical home might see a greater change than at my home, but not highly different.

Thus, I suggest you find a 1500 W space heater to use as a test load. Then using the neutral bus bar as your reference measurement point see how voltages change with different loading points, and measuring points, but all voltage measurements made relative to the neutral bar. Report back.

.

 

[LarryFine]

Phase A ---- 123.1 V drops to 122.3 or 122.4, a change of 0.8 or 0.7 V.
Phase B ---- 123.3 V rises to 123.7 or 123.8, a change of 0.4 or 0.5 V.

Thus, a change in voltage drop on neutral of 0.3 V., or almost half, which is logical.

 

[gar]

210511-2340 EDT

LarryFine:

That is correct and something I discussed in the past.

If the measurements were directly at the transformer terminals, then the result would be different. But at my main panel I see substantial wire impedance compared to the transformer impedance. I think I posted somewhere here the results of making measurements directly at a transformer's terminals. This was done on a small transformer, about 200 VA, but that does not change basic electrical theory. When measuring directly at the transformer terminals I see the effects of transformer primary impedance. In my home experiment the wiring voltage drops over compensate the transformer primary impedance voltage drop.

.