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Author Topic: Open-Leg Delta  (Read 2895 times)

Jonathan Johnson

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Open-Leg Delta
« on: August 21, 2023, 01:36:40 PM »

Found this in the wild the other day -- see attached photos. It's a transformer bank supplying 240V three-phase (delta) service using only two transformers from only two primary phases. The larger transformer is center-tapped to provide 120/240V split single phase service. I don't know the nature of the three-phase load in the building, but I suspect it's for the booster heater for the commercial dishwasher.

Sorry for my poor drawing skills and the hideous green paper, but I've also included a diagram.

Note that while all three primary phases are available on the pole, only two are fed to the transformers. The second lead of each transformer primary is bonded to neutral/ground. The "high leg" is 208V to neutral/ground.

There are other possible ways to configure an open delta connection. For example, L2 could be bonded to neutral/ground, and the one transformer wouldn't be center tapped.

I'm posting this as a matter of curiosity; it's something that you could run into, and maybe this will help you understand "what is going on?" when you see this unusual configuration.
« Last Edit: August 21, 2023, 01:49:20 PM by Jonathan Johnson »
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Brian Jojade

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Re: Open-Leg Delta
« Reply #1 on: August 21, 2023, 02:15:56 PM »

Uncommon? Yes. Unusual, not really.

Wild leg delta is used when there is need for both single phase AND 3 phase connections in the same system.  Common when you have 240v devices alongside 3 phase devices.

If you look at a breaker panel and see a blank every third spot, or you don't seen any single pole breakers every 3rd slot, that's a pretty good indication that you're dealing with this type of system.

The high leg is typically indicated by orange, but code lets orange be used for non-high legs if there isn't one. (stupid)

Anyway, I've certainly run into situations where idiots have seen open breaker slots and decided to use that as an extra circuit, assuming that it would be a 120v connection.  They can use it for a lot of devices that have auto ranging power supplies, so it could go years before it's noticed that something is wrong.

ALWAYS measure voltages before turning stuff on.  ALWAYS.
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Frank Koenig

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Re: Open-Leg Delta
« Reply #2 on: August 21, 2023, 03:02:08 PM »

Theyíre common as dirt around the San Francisco Bay Area which is mostly served by Pacific Gas and Electric (there are a few local utilities that buy power from PG&E such as the City of Palo Alto). You see them on most light industrial buildings. Easy to recognize as thereís one big pot and one little pot on the pole, as in your picture.

--Frank
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Frank Koenig

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Re: Open-Leg Delta
« Reply #3 on: August 21, 2023, 03:15:10 PM »

Just noticed that in your circuit diagram youíve got one phase of the input grounded. Iím pretty sure this is incorrect. The input to the two transformers uses two of three symmetrical phases and the POCO rotates which two of the three they use from transformer-pair  to transformer-pair so as to balance the load across the phases. There is no hard reason one of the phases couldnít be grounded but generally isnít at the distribution level. (All three insulators are the same size.)

--Frank
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Jonathan Johnson

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Re: Open-Leg Delta
« Reply #4 on: August 21, 2023, 05:32:16 PM »

Just noticed that in your circuit diagram youíve got one phase of the input grounded. Iím pretty sure this is incorrect. The input to the two transformers uses two of three symmetrical phases and the POCO rotates which two of the three they use from transformer-pair  to transformer-pair so as to balance the load across the phases. There is no hard reason one of the phases couldnít be grounded but generally isnít at the distribution level. (All three insulators are the same size.)

--Frank

Well, that was an assumption on my part. But for the transformers, there appeared to be only one insulator on the top, so I made the assumption that the other lead of the primary was grounded.
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Frank Koenig

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Re: Open-Leg Delta
« Reply #5 on: August 21, 2023, 05:57:05 PM »

Well, that was an assumption on my part. But for the transformers, there appeared to be only one insulator on the top, so I made the assumption that the other lead of the primary was grounded.

Could be. Different POCOs have their different ways. For example, when traveling I've often seen unblalanced (one side grounded) single-phase distribution lines. There is none like that around here. --Frank
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Brian Adams

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Re: Open-Leg Delta
« Reply #6 on: August 23, 2023, 10:51:41 AM »

My fairly small shop has a 200A 3 phase Delta service. I don't really use the wild leg, but I guess it's nice that it's there if I need it. A few years ago I asked about changing the transformer to 208V and I was told it would cost around $10-12k, which just isn't worth the cost.
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Tim McCulloch

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Re: Open-Leg Delta
« Reply #7 on: August 23, 2023, 02:34:19 PM »

High leg Delta exists mostly in places that need a single leg of 240V single phase.  Things like air conditioner compressors, small-ish motor loads in fixed tools, etc. that don't use 3 phase.  It's a really flexible service but too few electricians or end users understand it.
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Stephen Swaffer

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Re: Open-Leg Delta
« Reply #8 on: August 25, 2023, 12:51:03 PM »

There is at least one building in my area that the installer installed two 3 phase panels adjacent to each other on a high leg service.  One is the normal with a high leg and a few 3 phase loads.  The other is wired  Phase A, Phase A, Phase B with all 120 volt loads.  Very creative to say the least.  But that is also why code requires phase color coding to be documented-not that it always happened and especially not before code inspectors existed around here.
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Adam Kane

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Re: Open-Leg Delta
« Reply #9 on: August 25, 2023, 03:00:00 PM »

We used to provide audio and lighting at an event that the previous provider was no longer working since he "blew up about half of his equipment the year before." When I heard the story, I immediately knew what kind of electrical service was available.

We showed up the first time and there was a panel in a small utility building behind the stage. Every third spot was open in the panel, but no markings otherwise indicating a wild leg. Working as an electrician alongside my audio career paid off again. I tied in and went on with my day. I can picture the other guy flipping on the breakers in his distro not knowing what was about to happen.

Before we left, I got my Sharpie and made a few notes on the panel cover so the next guy might have a chance.

That event doesn't happen anymore, the venue is no longer, and the small building with that service was town down about a year ago.

During my time as an electrician, I saw a handful of buildings with these services, but not often. Mostly in situations where there was only a handful of small 3-phase loads relative to single-phase. Think a single large air handler on a building full of office suites. Last I knew, code prohibited using that wild leg for anything other than one phase of a 3-phase load. Meaning you can't use a single breaker in the panel for a 208-volt single phase load like a well pump, heater, motor, etc. While it would technically work, code didn't (doesn't?) allow it. A double pole breaker across the two 120/240-volt phases was required.
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Re: Open-Leg Delta
¬ę Reply #9 on: August 25, 2023, 03:00:00 PM ¬Ľ


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