Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums > AC Power and Grounding

Open-Leg Delta

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Frank Koenig:

--- Quote from: Jonathan Johnson on August 21, 2023, 05:32:16 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.

--- End quote ---

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

Brian Adams:
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.

Tim McCulloch:
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.

Stephen Swaffer:
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.

Adam Kane:
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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