Delta tranformer

[Riograndeelectric]

I have been an Electrician for 25 years. I hate to sound stupid But I do not understand the reason for a 3 phase Delta corner grounded system with a wild leg. I understand how they work and how they are wired. can some one explain the reason for having a delta and what they were used for .

I just love it when a DIY thinks they are a Sparky and wire a single pahse 120 circuit /outlets or lights to the wild leg and then fry everything connected to to it. I saw carpenter hard wire his tablesaw to the wild leg and then turn it on. it burned up the Motor. I think if he had thought about it for a minute maybe the fact the 95% of B phases had no breaker.

 

[Dnkldorf]

Rio, you are talking about 2 different systems here.

3 phase corner grounded and a 3 phase center tap.

The 3 phase corner ground is just that, one of the phase conductors is grounded, thus providing a path for fault current.

The 3 phase center tap, is three phase delta, with one of the phases center tapped for a nuetral. These are normally used for 3 phase equipment with a very small amount of 120v loads available for lighting and such. 2 of the phases will measure 120 to nuetral, and the third (high leg) would measure 208v to nuetral.
Between each ungrounded conductor, they should measure 240V.

 

[drg]

I always wondered why this high leg delta is out there,

would it be because you can get 120v for lighting and receptacles, 240v for single phase appliances and 3 phase 240 for motors that require it ?

The 208 high leg is just not used in any single phase applications and only there because of the 240v transformer winding configuration in 3 phase.

I never installed one but have worked with one in a old public school just doing a lighting system addition a few years back, this is the best sense I can make of this and only my opinion with no references to back this up , just a working guess but would like to know just like you.

 

[Riograndeelectric]

Rio, you are talking about 2 different systems here.

3 phase corner grounded and a 3 phase center tap.

The 3 phase corner ground is just that, one of the phase conductors is grounded, thus providing a path for fault current.

The 3 phase center tap, is three phase delta, with one of the phases center tapped for a nuetral. These are normally used for 3 phase equipment with a very small amount of 120v loads available for lighting and such. 2 of the phases will measure 120 to nuetral, and the third (high leg) would measure 208v to nuetral.
Between each ungrounded conductor, they should measure 240V.

I meant to say center taped mid point grounded with wild leg. what is pupose behind having the wild leg? Thanks

 

[roger]

I meant to say center taped mid point grounded with wild leg. what is purpose behind having the wild leg? Thanks

There is no purpose for it but it can't be helped due to the way the delta is generated. It is an unusable point of a delta as referenced to the grounded conductor.

Roger

 

[Smart $]

I meant to say center taped mid point grounded with wild leg. what is pupose behind having the wild leg? Thanks

The wild leg, or high leg as it is sometimes called, is nothing more than a "side effect". Typically, a 240Δ/120, 3?, 4W system is used only in industrial situations where at least 95% of the load on the system is 3? or 240V 1?.

Unless the number of 120V circuits is quite low (like four max, IMO), anyone installing such a system should consider adding a single phase subpanel...

 

[LarryFine]

I meant to say center taped mid point grounded with wild leg. what is pupose behind having the wild leg? Thanks

It's not done for the purpose of having a high leg; the high leg is simply the result of having a center tap on the opposing secondary.

The high-leg Delta system is generally used where the majority of the load is 240/120v, with some 3-phase loads added on. You could almost say that this system is the result of modifying a 240/120v 1-ph system.

I have a friend who used to live in a house that had a single #10 run alongside the 3-wire service drop, and ran through a spearate 3-pole disconnect solely to supply the central-AC compresor.

3-phase Y systems are relatively newer, and used where the majority of the load is 120v. They have a higher source impedance than Delta when supplying 208v 1- and 3-phase loads due to two secondaries in series supplying line-to-line loads.

Conversely, Delta systems have a lower source impedance than Y systems, especially for line-to-line loads, since each secondary basically supplies the load attached across it. Imagine three individual 1-phase sources connected in a triangle.

Grounding a Delta system is not necessary for 3-phase loads, but limits line-to-ground voltages during fault conditions, especially primary-to-secondary faults. When a center tap is grounded, the high leg is the result. Corner-grounding is the next best thing.

 

[drg]

To discourage carpenters and scare DIY's?

 

[roger]

Typically, a 240Δ/120, 3?, 4W system is used only in industrial situations where at least 95% of the load on the system is 3? or 240V 1?.

Do you have a reference to back that statement of 95%?

I know of numerous 240Δ/120, 3?, 4W services that supply a facility where there are only a small number of A/C units supplied by the three phase and the larger portion of the service load is single phase.

This is the reason that the "Kicker" or "Lighter" winding is larger than the other winding(s)


Roger

 

[Smart $]

Do you have a reference to back that statement of 95%?

I know of numerous 240Δ/120, 3?, 4W services that supply a facility where there are only a small number of A/C units supplied by the three phase and the larger portion of the service load is single phase.

This is the reason that the "Kicker" or "Lighter" winding is larger than the other winding(s)


Roger

Something I read somewhere, but it was in reference to equal windings and /or poco supplied (also equal windings). Note I did say 240 single phase is included in the 95%, and if load balancing is done as it should be, the load would be "lightened" across the center-tapped winding for the half-voltage loads to balance the overall.

Perhaps it would have been better stated as the half-voltages do not imbalance the L-L loads by any more than 5%, calculation-wise, as a recommendation not a requirement.

 

[Riograndeelectric]

Thanks Guys.

 

[roger]

Something I read somewhere, but it was in reference to equal windings and /or poco supplied (also equal windings).

You should drive around and look at some POCO pole mounted delta services.

Perhaps it would have been better stated as the half-voltages do not imbalance the L-L loads by any more than 5%, calculation-wise, as a recommendation not a requirement.

Where do you find this recommendation? And why would it be necessary?

Roger

 

[Dnkldorf]

I know of numerous 240Δ/120, 3?, 4W services that supply a facility where there are only a small number of A/C units supplied by the three phase and the larger portion of the service load is single phase.


Roger

Roger, I seem to recall that when you do this, the transformer must be derated.
Memory thinks something to the effect of 57%...due to the fact the transformer would be unbalnced and produces more heat..

Is this correct?

 

[roger]

Dnk, an open Delta would be 58% of the closed Delta in the capacity it could serve.

There is no question that a 4 wire Delta must be sized in consideration of the of the unbalanced loads the single phase 240/120v loads will/may cause. So, if we were installing a single dry type transformer the overall KVA of the transformer would need to be increased, and if we were installing a "bank" of transformers, the center tapped winding would be sized accordingly.

Smart, I have never heard of this 95% figure.

Roger

 

[al hildenbrand]

In my experience, the 120/240 V 4 wire delta was used for occupancies with some 3? load. What I describe below is a not-uncommon older installation around the 7 county metro area I'm in.

The classic setting is a corner neighborhood grocery store set in a residential neighborhood. This "grocery" is the old Mom & Pop business that was possible in the '30s to the '60s (and before). Todays convenience stores and large-corporate-grocery-chains have long since competed Mom & Pop out of business, but many of the buildings remain along with their electrical configurations.

The PoCo transformer bank supplyed a number of 120/240 1? single family dwellings originally. Then as the neighborhood developed, the corner grocery was built. The PoCo simply added another can on the pole to give the grocery it's 3?, and, maybe, if there was enough load in the neigborhood along with the new 1? load in the grocery, increased the size of the original transformer.

The grocery has a couple 3? machines in it, and a lot of 120 and 240 V load, with some 120/240 V multiwire load.

The PoCo transformer supplying the 120/240 V load, will be sized larger than the transformer(s) connected to the high leg, sometimes tenfold.

The PoCo is not motivated to alter this transformer bank into any other configuration and will support it until the owners of the various properties served alter their properties.

The PoCo, at least in my area, will not install brand new 120/240 V 3? transformer banks for new buildings. . .120/240 V 3? is no longer "supplied".

*****

One of the "economies" of the 4 wire delta, especially in the early days of building the distribution grid out into the non-electrified areas of the country, was the PoCo line crew only had to bring two lines (two phases) to a two transformer bank. The line crew would connect the primary of the two transformers in an "Open Wye" and the secondaries of the two transformers in an "Open Delta", and, viola!, the customer(s) had 240 V 3? and 120/240 V 1?.

The PoCo minimized the line crew labor by not installing the third line, and also saved another transformer for installation somewhere else.

In those first days, the first connected load was low. As load was added, the distribution lines could be upgraded and the transformer sizes increased as required. It was more important to get the customer energized, in the beginning.

 

[infinity]

One of the "economies" of the 4 wire delta, especially in the early days of building the distribution grid out into the non-electrified areas of the country, was the PoCo line crew only had to bring two lines (two phases) to a two transformer bank. The line crew would connect the primary of the two transformers in an "Open Wye" and the secondaries of the two transformers in an "Open Delta", and, viola!, the customer(s) had 240 V 3? and 120/240 V 1?

What is an open wye?

 

[al hildenbrand]

Yup. Open wye.

This is just over at Snelling and Selby in St. Paul.

There are two PoCo primary conductors. . .that's it. Only 2 of 3 phases. The neutral below the transformers (connected to a ground under the pole) serves double duty as the PoCo primary neutral.

The secondary side of this transformer bank is a 120 / 240 V 3? 4 wire open delta.

 

[LarryFine]

What is an open wye?

 

[drg]

Thanks for the information everyone , it's good to know things like this and have a good understaning of things we encounter while in the field.

All to often while in the field trying to learn this trade we get a answer that is wrong , misleading, and a lot of times just stupid but the person giving the wrong information is so convincing of their views it makes your head spin.

Being here at Mike holts forum one of the greatest things you will learn is how to persue a definitive answer to a question that is a must if you really want to progress in this field, also just as important you will learn to avoid a windbag that will fill your head with a lot of useless and possibly dangerous information .


Thanks again to the members who share their knowledge in this field and take the time to help others who really have that desire to learn .

 

[sparksaflying]

is there any way at all to get 120 volts from all 3 phases on a delta xfer