High Leg Voltage

[kwired]

Open delta’s are generally used where the three phase load is small, and the majority of the load is single phase. The high leg is seldom used for single phase loads. This works well with utilities because the single phase load transformer is larger than the second transformer the high leg comes from. With a full delta, it overloads the one winding that the neutral taps from, that’s why the manufacturer limits the neutral to phase loads.

Open delta is occasionally used around here for rather fixed loads where the major load is a single three phase motor or something similar.

On occasion to supply three phase to a remote site where the primary distribution does not have all three phase conductors available. The POCO generally will want to use all three phase conductors if the needed kVA is over a certain level though. Do have a few existing installs that are higher capacity still out there but they wouldn't approve them today without all three phases if they were a new application for service.

 

[ggunn]

I meant they [closed delta high leg services] are rare in this area.

Around here as well, at least for the ones I have seen. In fact, I myself have never seen one,

 

[kwired]

Around here as well, at least for the ones I have seen. In fact, I myself have never seen one,

They are common around here. Particularly on farms, but you will find them in cities/villages particularly serving older existing facilities. Most new facilities tend to get a wye supply if they are three phase. On the farm we still see new 120/240 full delta services all the time, the primary load is usually three phase motors and a rather limited amount of 120 volt load in comparison. Why go with 208 when the motor is rated for 240? Plus there often is some single phase motors in the mix, most those ideally would rather see 240 instead of 208.

 

[Hv&Lv]

We still build a 2 pot open delta bank and 3 pot closed delta banks 120/240

Heck, we have one place with a two pot 120/208

 

[tortuga]

Heck, we have one place with a two pot 120/208

Open wye ? that's interesting.
I think the best of both worlds would be to eventually convert hi-leg delta's to 220Y127, as there is not really 120V incandescent lighting anymore, and with VFD's preferring a wye system unless I am forgetting something else that can't run on 127 in commercial? it would be a win win.
Then phase out 208 in favor of 220 both the utility and customer instantly benefit from reduced voltage drop.

 

[hillbilly1]

And an open delta can easily be designed to have a very large amount of 120V while it is more difficult with closed deltas, especially in padmount construction.

Yes, that’s why the utilities have a larger transformer for the single phase loads, but that’s not done with pad mount or dry type transformers because the windings are the same size do all three phases. That’s why dry types are limited on unbalanced loads.

 

[Hv&Lv]

Yes, that’s why the utilities have a larger transformer for the single phase loads, but that’s not done with pad mount or dry type transformers because the windings are the same size do all three phases. That’s why dry types are limited on unbalanced loads.

I have built an open delta with single phase padmount transformers.

 

[hillbilly1]

Open wye ? that's interesting.
I think the best of both worlds would be to eventually convert hi-leg delta's to 220Y127, as there is not really 120V incandescent lighting anymore, and with VFD's preferring a wye system unless I am forgetting something else that can't run on 127 in commercial? it would be a win win.
Then phase out 208 in favor of 220 both the utility and customer instantly benefit from reduced voltage drop.

Lighting is only part of the loads where open deltas are used. POS, office equipment and other loads are 120 volt, and not adaptable like modern lighting.

 

[hillbilly1]

I have built an open delta with single phase padmount transformers.

Yes, but that’s rare. Dry type are a different animal with windings built together. You were using single phase padmounts looped together, not a single pad mount. It could be done the same way with dry type, but usually three phases is already in the building, so a closed delta would be used.

 

[jim dungar]

That’s why dry types are limited on unbalanced loads.

Not always.
I know Schneider Electric offered one with no limit on its loading.

The issue is loading when it is the common three-legged core construction.

 

[hillbilly1]

Not always.
I know Schneider Electric offered one with no limit on its loading.

The issue is loading when it is the common three-legged core construction.

Exactly, didn’t know Schneider made a special one.

 

[jim dungar]

Exactly, didn’t know Schneider made a special one.

I think it was brand labeled, so there must be others.

 

[Hv&Lv]

Open wye ? that's interesting.
I think the best of both worlds would be to eventually convert hi-leg delta's to 220Y127, as there is not really 120V incandescent lighting anymore, and with VFD's preferring a wye system unless I am forgetting something else that can't run on 127 in commercial? it would be a win win.
Then phase out 208 in favor of 220 both the utility and customer instantly benefit from reduced voltage drop.

Here’s a quick “back of the napkin” drawing.

 

[Birken Vogt]

Heck, we have one place with a two pot 120/208

Explain please. How is it constructed and what does it supply and why?

 

[LarryFine]

He just did. Look at each of the four 120v secondaries individually, from winding to line.

 

[kwired]

I have built an open delta with single phase padmount transformers.

There are some of those around here as well. Might be less likely to see new ones but there are existing ones still operating.

 

[Jraef]

Plus you would need a non slash rated single pole breaker.

AND a panel that will allow you to install one on the B phase...

This is an under rated aspect of the entre question rendering it, for the most part, moot. For 3 phase panels that are rated 240/120V or 208/120V, ALL of the single pole breakers are "slash rated", meaning they can ONLY be used where the L to G potential is 120V maximum. You CANNOT use slash rated breakers on a 208V to ground potential circuit. The only "straight 240V" rated breakers are 2 pole or 3 pole (and the 2 pole are more expensive than the 120/240V rated 2 pole breakers).

So if you were to WANT to use the B phase to N as a 208V single phase circuit, the only options would be:

• Use a 2 (or 3) pole straight rated breaker and only use one pole, (which is pointless, because you might as well use 240V and NOT use the neutral). This would ONLY make sense if you were somehow stuck with a motor that was rated for only 208V single phase. I have never seen one.
• Buy a 480/240V 3 phase panel and use the entire thing at 240V, which may be confusing to someone in the future. In some brands, you can mix breakers of different voltage ratings, but in reality, you are always buying a 480V rated panel, so it will be bigger and more expensive than an equivalent 240/120V one.

So to be able to use that B phase to neutral, it will cost you MORE than what you might think in one way or another AND be risky for some future person who doesn't understand the ramifications and connects a 120V load to it.

 

[electrofelon]

AND a panel that will allow you to install one on the B phase...

This is an under rated aspect of the entre question rendering it, for the most part, moot. For 3 phase panels that are rated 240/120V or 208/120V, ALL of the single pole breakers are "slash rated", meaning they can ONLY be used where the L to G potential is 120V maximum. You CANNOT use slash rated breakers on a 208V to ground potential circuit. The only "straight 240V" rated breakers are 2 pole or 3 pole (and the 2 pole are more expensive than the 120/240V rated 2 pole breakers).

So if you were to WANT to use the B phase to N as a 208V single phase circuit, the only options would be:

• Use a 2 (or 3) pole straight rated breaker and only use one pole, (which is pointless, because you might as well use 240V and NOT use the neutral). This would ONLY make sense if you were somehow stuck with a motor that was rated for only 208V single phase. I have never seen one.
• Buy a 480/240V 3 phase panel and use the entire thing at 240V, which may be confusing to someone in the future. In some brands, you can mix breakers of different voltage ratings, but in reality, you are always buying a 480V rated panel, so it will be bigger and more expensive than an equivalent 240/120V one.

So to be able to use that B phase to neutral, it will cost you MORE than what you might think in one way or another AND be risky for some future person who doesn't understand the ramifications and connects a 120V load to it.

Just a slight correction: the standard bussing provision for Siemens panel boards (regardless if spec'd at 240 or 480) is for a type BL or BQD. The BQD is a 277/480 rated breaker so it could be used for this application.

 

[Flicker Index]

Every facility needs 120v outlets.
High leg service can be present for mostly three phase, as well as mostly single phase users.

Legacy manufacturing facilities with mostly 3 phase motors often had high leg delta while farm houses with mostly domestic, but had a three phase motor or two that required three phase often got open delta.

For open delta, you really shouldn't put any single phase load on the second transformer.

 

[tortuga]

For open delta, you really shouldn't put any single phase load on the second transformer.

240V line to line loads go on the 2nd transformer (stinger) all the time (A-B load), what I try to avoid is putting Line - line 240 across the open jaw (B-C).
And if possible identify the open phase for the next guy.