"What is the 3 phase 208v wild leg usually used for in commercial panels?"

You are more confused than you realize. Let's assume you do understand the 240/120v 1ph system: a single transformer secondary winding with a grounded center tap. The two hots are condicered 180 deg. opposed to each other.

You could do the same thing with two separate 120v-secondary transformers: join one end of each (in phase!) and the junction becomes the center tap, which we ground. That gives us two points that each provide 120v to ground, and 240v between them.

A Y-system is basically the same thing, except we have three 120v-to-ground secondaries, all joined at one end, the grounded neutral. The hots are only 208 between one another, because of the 120-deg. phase angle.

A center-tap-grounded Delta system is actually one transformer connected exactly as a single-phase 240/120v transformer, plus two more, also 240v hot-to-hot, connected as an equilateral triangle; the Delta.

The phases are normally identified as A, B, and C, with A and C as the two ends of the center-tapped secondary, and the third hot as B, aka the "wild leg". A and C are each 120v to ground, but B is 208 to ground, so not useable for line-to-neutral loads.

An "open Delta" system omits one of the two extra transformers. It still works, but there is a bit less "stability" in keeping the voltage steady with varying loads; in other words, a greater source impedance.

The Y-system is mostly used where there is mostly line-to-neutral loads. The Delta is best for high-current 3-phase loads, and the open Delta (generally on the way out of use) is where there is a combination of large 240/120v 1-ph loads, with a bit of 3-phase loads.

The corner-grounded Delta is just what it says; a triangular connection with one phase grounded. It's used where only line-to-line loads are used, and no secondary has the center tap connected to anything.

Larry Fine
Fine Electric Co.
fineelectricco.com