delta-wye

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ishium 80439

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Denver foothills via the Horn of Africa
I had a customer ask me a question that I'm not sure I know the answer to. He is a commercial real estate agent and is looking at this building that is fed with 3PH 120/240V delta/wye. He has a potential client that wants to know if the existing service will accomodate refrigeration compressors (I didn't ask how many or what they were for). My response vacilated between "it depends on the actual equipment" and "if it is listed at 208V nominal than it should eat 240V 3PH".

I have very little (read next to zero) experience with this configuration. Can anyone advise me on the feasibility of newer epuipment running at this voltage?

As a side note why does it seem that most industrial areas built between the 50's and 70's (at least that's what it seems like from my limited foray) were fed with this configuration? Was/is there any advantage over a straight wye?

Thanks in advance for your input.
 
You will most likely get far more sophistictaed and better answers than mine, but to get the ball rolling:
I think your terminology might be suspect, in all likelyhood you are speaking of a 120/240 3 phase delta situation. In this area such installs were common-pace where there was limited 3 phase load (compared to single phase). It was a less expensive manner for the utility.
Although equipment is often interchanged, it is important to note NEC 110.4 states "the voltage rating of the equipment shall not be less than the nominal voltage...." so to operate 208v rated (only) equipment on a 240 system would not be Code compliant (might work, but not compliant)
 
Ishium, you have to remember that electrical distribution has been around and evolving for a while now. If you subtract the high leg and its transformer(s), you will be left with a plain and simple 120/240v 1ph service, exactly.

In fact, most of the older high-leg Delta services started out as 1ph services, and the high leg was a relatively simple POCO upgrade to add more motor HP while keeping the existing lighting and 1ph loads intact.

The newer high-leg services installed in an existing area were done because to suit the existing distribution equipment. You will rarely find a new high-leg Delta service installed these days; it's all Wye now.

A friend's parents lived in a house that had a #10 wire run alongside the typical open three-conductor service drop. That wire had only one load: the original central-AC compresor unit. It was a high-leg from an open Delta.
 
Basically

120/240V 3 phase = delta

2 legs @ 120V to ground, one @ *208V to ground*, 240V between any two.




120/208V 3phase = wye

all 3 legs @ 120V to ground and 208V between any two.


I have not personally seen refrigeration equipment that was designed spcifically for either 240 or 208.


*I have been told that the calculations say 208 volts on the high leg but I always seem to read in the high 190's in my area.
 
This is where I get a bit lost. The POCO told me that the configuration was delta/wye. I have heard of this before but never did more work on these types of systems beyond making a visual inspection of the equipment. I'm not exacltly sure what the hybrid is or what the advantages might be, but I can tell you that I have been told by both the POCO area planner on this site and a lineman at another that there is a difference between this and a straight delta system.

So to clarify my main question (as opposed to seeking some general education) if unit has a name plate that has a rating for "208/240" (as many motors do) or some such I don't see any problem with this equipment being installed on this system as long as there is no neut for some goofy reason that I can't think of. Would you agree?
 
ishium 80439 said:
This is where I get a bit lost. The POCO told me that the configuration was delta/wye. I have heard of this before but never did more work on these types of systems beyond making a visual inspection of the equipment. I'm not exacltly sure what the hybrid is or what the advantages might be, but I can tell you that I have been told by both the POCO area planner on this site and a lineman at another that there is a difference between this and a straight delta system.

So to clarify my main question (as opposed to seeking some general education) if unit has a name plate that has a rating for "208/240" (as many motors do) or some such I don't see any problem with this equipment being installed on this system as long as there is no neut for some goofy reason that I can't think of. Would you agree?
Click to expand...

If it's marked 208/240 no problem.
As far as goofy is involvd, it does not necessarly need a neutral to operate.
If the equipment does need a neutal, be sure the (neutral) load is not fed from the "high" leg (208)
 
ishium 80439 said:
The POCO told me that the configuration was delta/wye.
Click to expand...
I've never heard of that as a source. Either there are two conductors of equal voltage to neutral, or there are three. It can't be both.
 
I agree, you can not have a delta/wye system from a single transformer bank on the secondary side; it has to be either a delta or a wye. The lineman is correct in that you probably do have a delta/wye (primary/secondary) configuration, a wye/delta (primary/secondary) configuration, or an open wye/open delta (primary/secondary) configuration.

Measure the voltages and look at the number of transformers to see what is actually there. :)
 
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