As to your L-L connections using the "Wild Phase" of the 4 wire Delta ["A" to "B", or "B" to "C"], it should be acceptable with the NEC, since this is like any other type of L-L single phase connection and load.
As long as you use 2 pole units with a 1 pole rating of 240 VAC, there is really no problem. Please note that the SINGLE POLE RATING needs to be 240 VAC, not just simply the complete unit, or 2 pole rating. This is to assure that the device can withstand a L-G fault from the "Wild Phase" and not fail from exceeding the 1 pole fault rating.
This type of connection will not imbalance the system unless a majority of L-L loads were connected only between "B" and a certain Ungrounded Conductor - such as "A", and there was little or no L-G [L-N] and / or 3 phase L-L-L loads.
A uniform balance of loads, regardless of which "Phase" is used, will be the best method of connecting L-L loads on the 4 wire Delta.
As far as an L-G connection using the "Wild Phase" ["B"], it would work just fine to run a 208 volt 1 phase load, but the Grounded Neutral [Common Grounded Conductor] would not be able to "Balance" the current as it would between the Ungrounded Conductors on that coil ["A" and "C"]. This would cause the complete load current to flow from the "B Phase", through that "Phase's" Conductor, through the load, through the Grounded Neutral Conductor, through the Center Tapped point on the "Lower Winding", then through the "Lower Winding's" coil to either position "A" or position "C" where it will return to the coil of it's origin.
Once again, the OCPD used will need a 1 pole rating of 240 VAC, and since this will be a 1 pole device, it needs to have a rating of at least 10KAIC @ 240 VAC for the single pole breaker. Fuses will easilly conform to this rating, since by default they are listed at the L-L system voltage for the AIC rating [unless incorrectly used - such as if 240 VAC fuses are used on a 480 VAC system. even if the system was a 480Y/277 3 phase 4 wire Wye, the L-G voltage of 277 VAC is higher than 240 VAC].
For these reasons, I believe the NEC will not allow any loads connected L-G on the "Wild Phase" - be it either "B" as normal outside of metering sections, or "C" within metering sections.
I seem to remember a while back, the NEC allowed Welders to be connected L-G using the "Wild Phase" of a 4 wire delta. Not sure which NEC version this was, or which Article it was found in - just remember my Father talking about it being allowed at one time.
One of the members whom is more fluent with the NEC and older versions can verify this
[hint-hint to Don, Bill, Steve, Virgil, Scott W!!!].
Hope this is helpful and makes sense. Feel free to post any questions or comments you might have regarding this techno-babble.
Scott SET
