cctalk Digest, Vol 88, Issue 2

Jonathan Chapman lists at
Tue Jan 4 17:14:56 CST 2022

> High-leg delta is independent of open- or closed-delta.

That's correct.

> Open delta uses two single-phase transformers primaries connected to
> phases AB and BC.

A to C is also valid, presumably it's rotated if there's a lot of open delta in an area (again, why?) to balance phases.

> On high-leg, one of the secondaries is center-tapped and split
> single-phase is fed from the center tap and either end of the secondary.

Right, and you get a mostly unusable "high leg" w.r.t. neutral, usually 208V though I don't know what it ends up actually being in open delta with poor/uneven loading. The 120/240 power available is also supposed to be derated, IIRC it's only supposed to be 5-10% of total service load.

High-leg delta exists so you can have 120/240 lighting and appliance loads in a building that consumes mostly 3-phase, like a machine shop with an office. In most areas you aren't allowed to have more than one type of service to a building (not sure if that's true for double-fed sites, never seen one with two kinds though). I've heard the Power Company usually doesn't want to install high-leg delta anymore for a variety of reasons: the load limit, people not understanding they need to skip a breaker, 120/208Y having become the usual form of smaller service three phase, etc.

> Said configuration can be a 3-winding full delta configuration or the
> open-delta as detailed above.

Right, there's one or two of those services in town here for a couple of small commercial buildings. Four wire high leg open delta off the pole, two transformers on the pole.


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