240V Delta High Leg Used for Single Phase 240v

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Jason Spray said:
Can you utilize the high leg of a 240V 3P 4W Delta system as one of the 2 hot legs for a 240v single phase piece of equipment?
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Yes.
It's only "high" with respect to it's reference to ground.

But because of that, you have to be a little careful of what the 240V single phase load is. If your connection requires 2 hots and a neutral, it's not a good idea to use the high leg as one of the hots. That's only because inside of the equipment they may be tapping one hot to neutral for 120V controls and if that takes place from that high leg, the controls fry.

Case in point is a clothes dryer. Those are typically used in residential where the issue of a high leg doesn't exist, but I had a T-Shirt company buy old used resi-grade clothes dryers (from thrift stores to save money) and they fried 4 of them before calling anyone. :slaphead:
 
Jason Spray said:
Can you utilize the high leg of a 240V 3P 4W Delta system as one of the 2 hot legs for a 240v single phase piece of equipment?
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For a straight 240, yes. I would not even consider it for 120/240 equipment. One down side is that you will have to use straight 240 rated breakers.
 
kylelee said:
Usually there is no neutral in 240 3P delta system right ?


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There is no neutral in a regular delta, but a high leg delta has a center tap on one transformer that creates a neutral for making 120v available for some loads.

You will get 240V line to line, 2P and 3P loads.

120V line to neutral off the tapped transformer ends, A and C, but 208V from point B to neutral.





high-leg-delta5.jpg
 
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It helps to remember that the high-leg delta started out as a modification to existing single-phase services. A small conductor was added to the service by feeding it through the meter and to what was called a delta breaker, which supplied the one new 3-phase load, such as air conditioning.

Two 120-volt lines were picked up from the panel buses as usual, and the third line fed into the 3-pole breaker with a fourth breaker body section (picture a shunt-trip breaker with one body section with no switch), and back out of the 3-pole breaker along with the other two lines.

delta7.jpg delta3.jpg

In fact, a friend's father's house had an electrically-similar set up for the central A/C. There was no delta breaker, but the fourth wire (red #10, if I remember correctly) dropped from the pole with the three service conductors and fed an outside 3-pole disconnect that went directly to the compressor unit without entering the house.
 
MAC702 said:
Not only can you, but most of the time, you should include the high leg in your 240V 2-pole loads. This helps balance the panel in most cases.
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Not necessarily. In many high leg services the high leg has much less capacity than the other two and the POCO does not want you to balance the load over all three phases. We sometimes encounter this when we are designing a PV system; many times the POCO wants us to interconnect only on the 120V to N lines as if it were a split phase service.
 
ggunn said:
Not necessarily. In many high leg services the high leg has much less capacity than the other two and the POCO does not want you to balance the load over all three phases. We sometimes encounter this when we are designing a PV system; many times the POCO wants us to interconnect only on the 120V to N lines as if it were a split phase service.
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That makes sense especially with open high leg deltas. Often the second tranny added is smaller with limited capacity compared to the center tapped one.
 
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