High leg

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laketime said:
Why have a high leg? What was the original intention of the design?
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It is my understanding that the high leg was used a lot in rural areas so farms could could have three phase but not effect the homes were most electric ranges, water heaters, pumps were only rated for 240v and would not run on 208v. I'm not sure if it was designed this way or just a convenient side effect.
 
maloo said:
It is my understanding that the high leg was used a lot in rural areas so farms could could have three phase but not effect the homes were most electric ranges, water heaters, pumps were only rated for 240v and would not run on 208v. I'm not sure if it was designed this way or just a convenient side effect.
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I'm pretty sure the 208-N connection has never been legal to USE in that coonection. But the 240 A-B, B-C, and C-B connection is just fine for the items listed.... The 208 is a byproduct of one and a 1/2 transformer windings - making it difficult to provid OCP for, and unstable due varying loads.


It is also my understanding that the split phase was only as a conveinence to provide for 120 lighting and general outlets in many industrial situations where only the delta would be provided otherwise. I assume it may also be the case for rural farming areas too???

While 208 WYE is very easy to reconnect for 208/120, and very friendly to 120 loads it is not the greatest for industrial motors - and a delta would be preffered for 3 phase loads. And you can still get a very 120 frindly edison circuit out of it too. The BEST of BOTH worlds... Back when the demand for 120 was much less in those areas.

And breaking up a delta transmission lines into single split-phased transformers is easier, making more remote single phased areas easier in an shared Residential/Industrial area.
 
Why the Open Delta?

Why the Open Delta?

The diagram in the previous post tells the HOW this (240/120V + 240V, 3-phase) is done. How you get 120/240V like a regular service plus the 3 phased 240V.

The WHY is that is that if you had a typical 240/120 center-grounded transformer service and you needed a SMALL capacity for 3-phase, the utility could put in a SMALL second can, center-tapped to provide the other two legs of a 240V 3 phase service. (New meter then, too.)

Most NEW installations just provide 3 phase, 120/208 from the "get go". Another topic then is how often you encounter loads that MIGHT fail due to the voltage difference between 208V and 240V.
 
EEChemE said:
The WHY is that is that if you had a typical 240/120 center-grounded transformer service and you needed a SMALL capacity for 3-phase, the utility could put in a SMALL second can, center-tapped to provide the other two legs of a 240V 3 phase service.
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Actually, the second transformer does not need a center tap for grounding/neutral use; the original one has that.

What you described is the origin of the high-leg, open Delta service: a modification of existing 1-phase services
 
LarryFine said:
Actually, the second transformer does not need a center tap for grounding/neutral use; the original one has that.

What you described is the origin of the high-leg, open Delta service: a modification of existing 1-phase services
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I agree with Larry.

Roger
 
roger said:
I agree with Larry.

Roger
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mivey said:
And...deep breath & hold...close eyes...clinch fist...I do too. :grin:
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goofy.jpg


"Well, gawrsh!"​
 
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