Single phase transformer fed by delta high leg

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Schewe

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Hello all, I'm new to the forum and have what may or may not be a silly question. At work we have a delta high leg service, A and C are 120v phase to ground, and B is around 200v to ground. There is a machine at work that utilizes a 220v/110v transformer, this being fed by 2 legs of said service. Does it matter if the high leg feeds on side of the primary winding, or should they both be the 120v legs. Can this cause a funky issue with the neutral on the secondary?
 
You should double check things however.

The transformer itself doesn't care about which phases you use.

The OCPD _might_ care, in that OCPD might only be rated for 240V L-L 120V L-N . In your supply panel you must use a full rated 240V breaker, not a slash rated breaker.

But the biggest question is making sure you understand your service. Depending upon the POCO transformer design, the high leg might be totally balanced with everything else, or might only be intended for 3 phase loads.

-Jon
 
winnie said:
You should double check things however.

The transformer itself doesn't care about which phases you use.

The OCPD _might_ care, in that OCPD might only be rated for 240V L-L 120V L-N . In your supply panel you must use a full rated 240V breaker, not a slash rated breaker.

But the biggest question is making sure you understand your service. Depending upon the POCO transformer design, the high leg might be totally balanced with everything else, or might only be intended for 3 phase loads.

-Jon
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This^^^

The high leg is a stable Voltage and the transformer as installed only cares about its turns ratio to give you the 120V, so it will do so stably. If you have OCPD in series with that transformer primary or secondary it will only care about the current through it and that should also function dependably.

There are two problems.

If you have 120V already from the regular 120 V secondary windings, why are you adding another transformer to get more 120 V connected in a way that could be non standard and unique across the entire US. The problem is not that it will not work, it probably will effortlessly. The problem is that you could be the only one in the entire US connected this way. You want to know why this implementation was chosen before proceeding.

If you want to say this connection was made to balance the load on the transformer, I would say you are an idiot. And probably should not be wiring anything without standard oversight, someone who knows what they're doing.

A standard red leg transformer is for regular 3 phase loads with usually only a small 120 single phase secondary load. if it's all 120 single phase load at the red leg secondary, the textbook says you can only utilize 57% of the kVA rating of the transformer. Never heard of defeating the textbook on this by loading high leg to neutral with another transformer. That is until you came along.

Back to square 1, what do you want to accomplish. If you want all 120V for single phase loads you should have chosen a Y secondary transformer, or a suitable single phase transformer loaded on the available primary supply Voltage.

The problem you should look at is the original square 1, what do you want to accomplish, and also take a good look at the guy who solved this problem with your unique and non standard solution. He is either brilliant and gifted or accidentally and occasionally lucky.
 
If you have 120V already from the regular 120 V secondary windings, why are you adding another transformer to get more 120 V connected in a way that could be non standard and unique across the entire US. The problem is not that it will not work, it probably will effortlessly. The problem is that you could be the only one in the entire US connected this way

The OP stated the transformer was part of a machine. I have found a lot of machinery that is supplied for multi-voltage installations and has a control transformer with taps accordingly. It certainly is not a unique.situation.
 
@augie47 makes a good point. This is probably some sort of control transformer, providing power for the low voltage bits of a larger piece of equipment.

But the point about checking the service still stands. There are several different 'flavors' of high leg service, and where you place a single phase load is determined by the particular type.

As I recall,
1) balanced high leg delta, where all three legs are pretty much the same, and it would be desirable to balance single phase loads across the legs.
2) unbalanced high leg delta, where the 'base' leg is built with a larger transformer, and is intended to serve all L-N and single phase loads. You only use the high leg for your 3 phase loads.
3) high leg open delta, which is even more unbalanced, and where you can have degraded voltage regulation if you place a single phase load across the 'open jaw'.

If this machine is a large 3 phase load with a small single phase control transformer, then it pretty much doesn't matter which legs the transformer is connected to.

If this machine is a large single phase load with a small control transformer, then think more carefully. Hopefully this isn't a case of 'every third slot in the panel is empty, so we want to connect there'.

Jon
 
I can also see using a transformer to feed a remote load, say in an out building, where a second grounded conductor would not be a problem.

The 57% loading has to do with the overall transformer bank rating between a closed delta and an open delta. Say you have (3) 33.5kVA single phase transformers in a closed delta, which would be rated 100kVA. If one transformer was removed you would then have an open delta of 57% of 100kVA or 57kVA. But you could start with just two transformers in an open delta at 86% of the two transformers (2 x 33.5 x 86% = 57kVA) and get to the same effective rating. But if the provided value is already based on an open delta no adjustments are needed.
Numbers have been rounded for convenience.
 
Seems a bit convoluted. We Brits keep it simple. Single phase is just single phase 230V for domestic, Industrial is 400V three phase. That's all.
 
Besoeker3 said:
Seems a bit convoluted. We Brits keep it simple. Single phase is just single phase 230V for domestic, Industrial is 400V three phase. That's all.
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It appears convoluted because you are not familiar with it.
The most common issues we are discussing is how the voltages are created by transformers not what the voltages actually are.

Why don't you contribute your transformation details. Is everything a a three phase transformer on a common core or do you have single phase transformers combined into banks?
 
jim dungar said:
It appears convoluted because you are not familiar with it.
The most common issues we are discussing is how the voltages are created by transformers not what the voltages actually are.

Why don't you contribute your transformation details. Is everything a a three phase transformer on a common core or do you have single phase transformers combined into banks?
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It IS convoluted because it is. And yes, I do use single phase and three phase units.
 
Besoeker3 said:
That's the point - it COULD be simpler for you.
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It is not complicated for qualified/trained personnel. Our voltage systems have been in available for more than 100 years, although this particular one is rarely used with new installations which means many people have not seen it.
It is typically untrained or inexperienced individuals that make it complicated. But once they get their questions answered things go smoothly.
 
jim dungar said:
It is not complicated for qualified/trained personnel. Our voltage systems have been in available for more than 100 years, although this particular one is rarely used with new installations which means many people have not seen it.
It is typically untrained or inexperienced individuals that make it complicated. But once they get their questions answered things go smoothly.
Click to expand...
But it can so much simpler. The USA is about 300 million with a variety of different voltages/systems. My way of contradiction Europe, also around 300 million, simply has uniformity.
 
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