Phase voltage of a 3 phase 208 system is 208\1.73 = 120v what about 240 3phase?

[jim dungar]

Most services that began as 1-ph had line-to-neutral primaries, which is necessary to obtain an open-Delta service with only two primary lines. Line-to-line primaries would require all three phases even with an open Delta, minimizing the advantages.

A 3-phase service requires three conductors, it does not matter if it is 3 hots or 2 hots + 1 neutral. The deciding factor is the primary distribution system

 

[LarryFine]

A 3-phase service requires three conductors, it does not matter if it is 3 hots or 2 hots + 1 neutral. The deciding factor is the primary distribution system

Yes, I'm well aware. When I say 'two lines', I'm not including the system neutral, which I've never seen omitted.

I've heard of SWER systems, but I can't imagine any utility would place two transformers on such a system.

 

[jim dungar]

I've heard of SWER systems, but I can't imagine any utility would place two transformers on such a system.

SWER stands for Single Wire Earth Return. An SWER circuit does not have enough conductors to provide 3 phase.

The advantages of an open-delta typically come from the number and sizes of the indivual transfromers not the number of conductors.

I can't remember ever seeing a utility distribution system with only 2 hots + 1 neutral. Most of the utility poles I have seen were either 3 hots + 1 neutral or 1 hot and 1 neutral.

 

[LarryFine]

SWER stands for Single Wire Earth Return.

Again, I knew that. (I did, honest!)

An SWER circuit does not have enough conductors to provide 3 phase.

As I mentioned never being done.

The advantages of an open-delta typically come from the number and sizes of the indivual transfromers not the number of conductors.

The vast majority of open-Delta services I've seen had a smaller high-leg conductor, and the 3ph loads were a small portion of the entire load.

I can't remember ever seeing a utility distribution system with only 2 hots + 1 neutral. Most of the utility poles I have seen were either 3 hots + 1 neutral or 1 hot and 1 neutral.

There are plenty of them around here. Keep in mind that Richmond is a relatively old city. There are plenty of two-line (+ neutral) primary systems that supply high-leg services around here.

Of course, they're fed from 3ph systems, and often have all three phases on the pole, but there are only two transformers, each single-ended, and the high-leg transformer is obviously smaller.

In fact, a friend of mine grew up in a house that had a high leg brought to the house for just the AC compressor, served via a separate 3p disconnect, and I've wired several new services to existing high-leg systems.

In those cases, I installed 200a or 225a 1ph panels and separate 100a or 125a 3ph panels, all with reduced high-leg conductors. I'll see if I can grab some pics of transformers and services this week.

 

[richxtlc]

If you look at Rick C's third diagram on the right it sure looks like the center tapped portion is the same as a 120/240 volt house service.

It looks like the same but as I stated the input is single phase for the house service with two hot legs 180 degrees apart with a center tap on the secondary winding. The center tapped delta is a phase-to-phase centertapped winding with the phases 120 degrees apart. Although they have the same voltage characteristics the phase angle relationship is different.

 

[LarryFine]

It looks like the same but as I stated the input is single phase for the house service with two hot legs 180 degrees apart with a center tap on the secondary winding. The center tapped delta is a phase-to-phase centertapped winding with the phases 120 degrees apart. Although they have the same voltage characteristics the phase angle relationship is different.

Nope. The transformer with the center-tapped secondary, whether part of a Delta or not, is fed from a single primary, has a single core, and is a single winding, just like home.

In fact, a Delta can be made up with three center-tapped-secondary transformers, as long as only one tap is bonded. The Delta neutral is not symmetrical to the primary system.

 

[jim dungar]

There are plenty of two-line (+ neutral) primary systems that supply high-leg services around here.

Of course, they're fed from 3ph systems, and often have all three phases on the pole, but there are only two transformers, each single-ended, and the high-leg transformer is obviously smaller.

Oh, it appears you are calling the 3W open-wye drop from the pole to the transformers or a spur circuit run for a few hundred feet a 'system'.

My point is that, across the country the predominant transformer design for 240/120V 3P 4W is an open delta because of the cost saving in transformers not because the system is fed with an open-wye (or vee).

 

[LarryFine]

Oh, it appears you are calling the 3W open-wye drop from the pole to the transformers or a spur circuit run for a few hundred feet a 'system'.

Only because we, as the electrician, only care about "our side" of things. What would be the correct term for a primary "branch circuit?"

My point is that, across the country the predominant transformer design for 240/120V 3P 4W is an open delta because of the cost saving in transformers not because the system is fed with an open-wye (or vee).

Invariably (and redundantly - sorry), in my experience, the open-Delta services I have seen were those with mostly 120/240v loads and a smaller amount of 3-ph.

Any new POCO transformer banks are 208Y/120v 3ph. After all, who would specify a high-leg service for an all-new installation.

(Yes, I realize the Delta vs. open-Delta is a separate discussion from Delta vs Wye.)

 

[richxtlc]

It looks like the same but as I stated the input is single phase for the house service with two hot legs 180 degrees apart with a center tap on the secondary winding. The center tapped delta is a phase-to-phase centertapped winding with the phases 120 degrees apart. Although they have the same voltage characteristics the phase angle relationship is different.

Ok, I guess I have to clarify my answer, a normal overhead service that supplies a residence has a single phase to ground primary and a secondary that has two hot legs and a center tapped neutral from the secondary winding. The phase angle difference for these two hot legs are 180 degrees apart. I won't restate the above.

 

[LarryFine]

Ok, I guess I have to clarify my answer, a normal overhead service that supplies a residence has a single phase to ground primary and a secondary that has two hot legs and a center tapped neutral from the secondary winding. The phase angle difference for these two hot legs are 180 degrees apart.

Okay. In a high-leg Delta, how would the thansformer with the center tap be any different?

Each of the primaries of a typical 3ph Y-D bank is also fed line-to-neutral. If each secondary had a center tap (and many do*; they're just not connected), the two lines would appear the same: what you call 180 degrees apart.

Added: * In 240v Deltas, that is. In 208Y/120v banks, the secondaries are reconnected in parallel inside the can to put out 120v, using only the left and center bushings.

As a matter of fact, we had a rather lengthy thread (we won't repeat) on the semantic argument of whether the lines of a center-tapped secondary were actually 180 degrees out-of-phase, or merely of opposing polarity.

 

[rattus]

Okay. In a high-leg Delta, how would the thansformer with the center tap be any different?

Each of the primaries of a typical 3ph Y-D bank is also fed line-to-neutral. If each secondary had a center tap (and many do*; they're just not connected), the two lines would appear the same: what you call 180 degrees apart.

As a matter of fact, we had a rather lengthy thread (we won't repeat) on the semantic argument of whether the lines of a center-tapped secondary were actually 180 degrees out-of-phase, or merely of opposing polarity.

Yes indeed, an inverted sine wave is 180 degrees out of phase from the non-inverted wave. Same as six and a half dozen.

Also, the 240V secondaries in residential systems are not connected in a delta because they share a common neutral. That is the difference between residential single phase and the high-leg delta. Everyone knows that already though.

 

[LarryFine]

Also, the 240V secondaries in residential systems are not connected in a delta because they share a common neutral. That is the difference between residential single phase and the high-leg delta.

A residential service comes from a single secondary with a center tap. It is two 120v windings connected in series, but it's a single winding on a single core, with the tap bonded to ground and used as the neutral.

A high-leg Delta service has three of the same 240v secondaries joined in a triangular fashion, with the three juntions providing the line terminals, and one winding with a center tap bonded and used as the neutral.

What you're describing as three windings that share a common neutral is a Wye, not a Delta. The Wye has three 120v secondaries, with one end off all three secondaries joined, bonded, and used as the neutral.

The difference between a Delta and a high-leg Delta is the latter has a secondary with a grounded center tap. A Delta can be non-grounded, have one phase (or corner) grounded, or have a neutral derived.

The only reason a high-leg Delta even has a high leg is because of the conductor we opt to ground. However, that center-tapped 240v secondary is exactly the same as a single-phase 120/240v secondary.

 

[rattus]

What you're describing as three windings that share a common neutral is a Wye, not a Delta. The Wye has three 120v secondaries, with one end off all three secondaries joined, bonded, and used as the neutral.

No Larry. I am describing three secondaries from three transformers on three separate poles. The primaries are typically connected in a wye, and the three center-tapped 240V secondaries share only a common neutral/ground wire. Each primary is connected to a different phase. Some of your neighbors receive a single phase service +/- 120 degrees out from your own.

My point is that these secondaries cannot be connected in a delta as they are in a high leg service.

 

[LarryFine]

No Larry. I am describing three secondaries from three transformers on three separate poles.

That's not a 3-ph transformer bank; that's three individual 1-ph transformers feeding separate services, and not germain.

The primaries are typically connected in a wye, and the three center-tapped 240V secondaries share only a common neutral/ground wire.

Again, you're describing separate 1-ph secondaries. The three neutrals are common because they're all grounded, of course.

Each primary is connected to a different phase. Some of your neighbors receive a single phase service +/- 120 degrees out from your own.

No disagreement there, but that has nothing to do with this sub-discussion, which is comparing the center-tapped secondary of a high-leg Delta to that of a 1-ph service.

My point is that these secondaries cannot be connected in a delta as they are in a high leg service.

Well, one of them can, which leads to the point (okay, my point):

If you put all three of those transformers with the 120/240v secondaries onto a single pole, connect them in a Delta, and only ground ONE center tap, you have a high-leg Delta.

And, if you compare that one, center-tapped-and-grounded secondary to the secondary of one of your individual 120/240v 1-ph transformers, you will see absolutely no difference.

There's 240v between the two ends, and 120v between each end and the neutral. As Mr. Trotter said in My Cousin Vinny, "I-dentical!"