Open delta service

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jpfarren

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I'm working on a building that has a 120/240, 3 phase, 4 wire (counting the center tapped neutral), open delta service consisting of a 10 kva transformer and a 15 kva transformer with a center tap. Phase C is designated as the high leg (208 volts to ground).

There are a couple of 3-pole circuit breakers supplying larger loads, lots of 1-pole circuit breakers on phase A and B only and a few 2-pole circuit breakers between phase A and B. No single pole circuit breakers on phase C because it's the high leg with 208 volts to ground.

My question: is it ok to put 240 volt, 2-pole, loads on phase C-A and/or phase C-B or can they only be on A-B?

Thanks,

Jon
 
Re: Open delta service

I don't see why you can't go from A-B, B-C, or A-C to get your 240.
You mentioned a couple of LARGE loads are three phase. The size of the three phase loads on the reduced size transformer may be the reason they only have their 240 loads on A-B.
If this works....go to this post for loading calculations.
http://www.mikeholt.com/codeforum/ultimatebb.php?ubb=get_topic;f=11;t=006060#000000
If that didn't work....
Type in OPEN DELTA in the search box, about twelve pop up, one of them has diagrams showing LOADING calculations.
 
Re: Open delta service

I would say that it is preferable but not mandatory to take the 240V directly off one of the windings because phase shift is introduced when you connect even a resistive load to the open phase. This phase shift in effect lowers the VA rating of the two transformers. We are still beating this sick horse in another thread.
 
Re: Open delta service

On your 2 pole c.b.'s "slash" breakers are a violation, make sure the rating is 240VAC not 120/240VAC

frank
 
Re: Open delta service

Good call mpd. I'm supprised that the high leg is on the "C" phase. It sould on the "B" phase & properly identifed as required in Art. 215.8/230.56 &408.3(E).
 
Re: Open delta service

The electric utility requires the location of the High ? to be on the R/H side of a meter fitting, that may have been carried on to the inside wiring by mistake. It should have been changed but it would be expensive now. :D
 
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