Open Delta Load Calculations 4

[knole2]

I have encountered an open delta 3-phase 240V service in PA and have been confused by so many different explanations. I understand the B (high or stinger leg) is at 208V to ground, but are the phase to phase voltages still 240V and this is how they get the 3-phase loads from this system?

 

[stevenal]

Yes.

 

[knole2]

Just another question, can I use a single pole breaker to serve a 208V, 1-phase load as long as it is on the B (high leg)?

 

[waross]

Single phase loads are 120V, 120/240V or 240V. A maximum of 120V to ground.
Three phase loads are 240V.
These are the voltages of a four wire delta system.It doesn't matter if the delta is open or closed.
Single phase loads should not generally be connected to the wild phase.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rhatcher]

waross, it may be that the OP is talking about a 240V split phase delta (high leg, wild leg, or red leg delta), as shown on the last page of this link. This makes sense of his reference to a 208V 'high or stinger leg' on a 240V, 3 phase transformer.

http://ametekprogrammable.com/support/FAQs/DF_AC_Distribution.pdf

In this case, I think that a single phase 208V/240V breaker would be ok since it interrupts the power side of the circuit with the other side being the neutral as in any 'normal' single phase circuit. Of course, I may be wrong based on code or standard practice in his (or your) area.

For knole2, I am not sure why you posted the same question twice in such a short time. This creates confusion and cross-posting between the two original posts and is not recommended. Also, as you see above, your terminology may not be correct. Check the link and determine what you really are working with and let us know since there is a big difference between an open delta and a split phase delta.

 

[waross]

Hi rhatcher;
I think that we agree. What I know as a four wire delta is the same as you call a split phase delta.
A normal 240 Volt single phase circuit is 120 Volts to ground from either leg. Using a wild leg for single phase exposes electricians to 208 Volts to ground when they expect only 120 Volts to ground.
There may be code issues with using the wild leg for single phase loads. It is not usually done. Either a single phase panel is used for single phase loads and a three phase panel used for three phase loads or a three phase panel is used and every third breaker space is left empty unless a two pole or three pole breaker is used.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rhatcher]

waross, I do agree with your post on the other thread that said that it is not a good idea to place a single phase load on the open leg of an open delta transformer. This idea is not based on code since I do not know the code for this. The idea is based on the concept that the voltage may not be stable when the single phase load(s) change, especially if there are no three phase loads attached.

I also think that I understand your concern about having a single phase breaker that is not marked for voltage and that has 208V instead of 120V. You are concerned that an electrician may connect a 120V load to it. Is this correct?

If so, I say 'shame on the electrician'. In my experience I have seen many 277V panels (480V transformer with multiple 1 pole breakers connected line to neutral) used for industrial lighting. Some are marked 480V/277V on the front and some are not. For some facilities such as mine, the marking is a circuit designator that indicates the source voltage if you know how to read it. In my facility, panel designators beginning with an 'E' means 480V delta, panel designators with a 'P' means 240V delta, and panel designators with an 'L' means 480V/277Y. In any case, if an electrician connects any load to any panel without first checking the source voltage then he has made a mistake and noone can be blamed but him.

If your concern is that the electrician may touch the output from the panel and, as you said, be 'exposed' to 208V instead if 120V, then he has again made a mistake. In either case, 120V or 208V, he is liable to be seriously injured or killed.

Finally, there may be code issues against using the wild leg on a split phase, four wire transformer for single phase loads. As I admitted before, I do not know if that is the case. Other than code, I don't see a problem with it from a power distribution standpoint.

 

[waross]

Hi rhatcher;
Good points.
I have seen similar situations. And I have seen some others also.
But there are a few things that set this question apart from your plant.
Not just he use of an open delta but the possible confusion about split phase delta and open delta when this may be both lead me to some assumptions dangerous as that may be.
Open delta often implies a small plant that may not have an electrical staff.
The electrician may be sent in by a contractor and not be familiar with wild legs. I have worked in large geographic areas where the utilities did not allow 240 Volt full delta services. In some areas an open delta four wire service would be allowed to add a three phase motor load to a single phase service. The utility would discourage single phase loads on he wild leg. This was mostly in rural areas for irrigation pumps where three primary phases were not present.
I have met and worked with a great many good electricians who have never seen a "Wild leg" panel. I would not say shame if they were sent unknowing to a wild leg panel and assumed that it was 120/20 rater than 120:240/208 which they had never seen.
I respect your position. This is mine.
Respectfully
Bill


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rhatcher]

Waross, you've had some good points throughout this thread, that is why I gave you a star last night (for this thread and another). And, you have two more good points from your last post.

First, like many engineers, I am very familiar with transformer theory including open delta and split phase delta. But, I've never actually seen a panel fed by a split phase delta transformer. I agree; If a person is not very careful when they do encounter one, they could be in for a nasty surprise.

Next, there is a lot of room for confusion when the wrong terminology is used. Is it an open delta or a split phase (high leg) delta? The OP called it an open delta with a high leg reading 208V to 'ground.' This is obviously erroneous.

You have made two good points here that led me to look into this a little more. I went back and looked at the reference link I provided again this morning and yes, the connection diagram and the terminology for this tranformer can be confusing if you are not careful.

At first glance at the connection diagram in the link, it may appear that the 'C' leg is grounded but a closer look shows that it is the high leg that is grounded. I wondered; "Am I seeing this right, the high leg is grounded? Then why do they call it the high leg?"

So, I did a little more research and found another source that calls the center tap a neutral (makes sense if it is grounded) and calls the opposite line lead ('C' phase on the reference I provided) the 'high leg.' Of course, this source made no reference to the center tapped 'neutral' being grounded.

Then there is the the terminology 'high leg', 'wild leg', and 'red leg' that are sometimes applied to the split phase transformer. Where does that come from? The voltage from the 'high leg' isnt the highest voltage that can be derived from the transformer. There is nothing 'wild' about the voltage of the 'wild leg' since it is fixed relative to the other transformer voltages. And finally, to make matters worse, the NEC (Article 110.15 of the 2005 Code - I am not a code expert) states that the 'high leg' should be identified in a panel, preferably by coloring it orange, not red (red leg??). All of this can be very misleading and confusing.

This is why I say that the electrician is responsible for knowing what he is connecting to and cannot assume anything. I've seen cases of an electrician smoking something because he assumed that he knew what the source voltage was. And, unfortunately, I've seen electricians shocked because they assumed that the neutral conductor was not hot or because they assumed that the power was off after throwing the (wrong) breaker.

That is why I believe that the electrian should always check all voltages line-line, line-neutral, line-ground, and neutral-ground before doing anything. That is my practice and is the practice that I insist that my workers use. After all, an assuming electrician is on his way to being a dead electrician.

So, I do respect your points and I have given you another star for your last post. I definitely agree with you that there is a lot of room for confusion. Is it an open delta or a split phase delta. For the split phase delta, is the center tap the neutral or the 'high leg.' Is there an intentional ground connection or is it a floating ground. And, of course, this is all made even worse if you start mixing the terminology like the OP did. Again, thanks for your comments and keep up the good work. You've definitely enlightened me.

 

[waross]

Hello again.

After all, an assuming electrician is on his way to being a dead electrician.

That's worth a star.
Some jurisdictions will only supply a wye service but there are still some legacy systems.
Even where delta systems are not used, exceptions may be made for rural areas where three primary phases are not presently installed.
Four wire or split phase delta systems are used to feed single phase loads from a delta transformer bank. One transformer is used as a classic 120:240V single phase transformer. By code the neutral or center tap of the transformer must be grounded. This gives phase to ground voltages of 120V, 120V, and 208V. This is the "High" leg.
Why red leg for an orange conductor? This connection was in use generations before orange colored building wire was readily available. Red colored wire has been available for much longer.
One or two transformers may be added for three phase delta service. With a four wire delta system it doesn't matter whether it is open or closed delta.
I have seen just about every combination of four wire delta that has been used.
For open delta:
The basic system is a conventional 120:240V single phase service where a second transformer is added to supply a small three phase load. This transformer is often called a "Teaser" transformer.
In this configuration you may see a large (50 KVA or 100 KVA) transformer supplying a residential neighborhood with a teaser transformer added to supply a small three phase customer. possibly a small repair shop with one or two three phase motors.
There are services where the three phase load is larger and both transformers are the same size.
Then there are closed delta four wire systems.
A four wire system is an economical way to provide an acceptable ground point to a delta transformer bank.
An industrial installation with large three phase loads combined with a lot of single phase lads such as lighting may choose to use a four wire delta system.
Note; Open delta may be any mix of transformer sizes that do the job.
Closed delta single phase loads are fed 50% from the transformer dedicated to single phase and 50% by the other two transformers acting together (assuming equal impedances). Unequal size transformers in a full delta bank usually means wasted capacity.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rhatcher]

waross,

Thanks for the explanation on use of single phase transformers in various combinations to derive open delta and split phase transformer banks. I am familiar with these ideas but seldom see them since I work in an industrial/marine environment with 99% wye or delta three phase transformers. A little refresher is always good and it is even better when it is a detailed explanation from someone obviously familiar with them.

It is a shame but I think the OP is not around anymore. If he was, I would recommend a detailed reading of your last post along with studying the diagrams in the link I provided (or elsewhere) to make sure he knows what he is looking at before he gets into it. He should probably also talk to a licensed electrician in his area that is familiar with the local code requirements.

By the way, I think somewhere along the line there has been a change in the 'star' system. It would appear that an individual can only give one star to another for any given thread since I have already given you two during this thread but only one shows. Also, it would appear that once a star is given to an individual that it is shown for all of his posts in a given thread. I guess that means I just have to owe you one from before and one more for the great explanation in your last post. Thanks again.

 

[waross]

Don't worry about stars. I value comments such as yours from a respected friend much more than stars.
Stars are fun but not that indicative of excellence. It is easier to get stars for simple answers to newbies simple questions than it is to get them from the real experts here. A personal thank you is much more gratifying than a star.
Thank you for the kind words.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Compositepro]

I'll give Waross another star. I live in a rural area and my building is split-phase delta, so this is a very relevant discussion. I think the only licensed electricians in this county were grandfathered-in when it became required.

 

[waross]

Thank you Compositepro;
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter