High Leg Delta

[kwired]

It is useless for a 208 volt load,

It will work, just normally not a very good idea to do so, unless maybe is a very limited load like for 208 volt controls or something. It won't balance things like using all three lines to neutral on a wye system does. Assuming B phase is the high leg, on a full delta that B to N loading flows through both A and C halves of the delta. On an open delta system all that current flows through which ever side of the delta is the closed side, in addition to any other current already flowing on that side.

 

[Carultch]

It is useless for a 208 volt load,

In theory, it could work. In practice, you're facing the limitations of slash-rated breakers, if you attempt. Most single pole breakers in this voltage class, are slash-rated at 120/240V, requiring the nominal voltage-to-ground to not exceed 120V, and the voltage across poles to not exceed 240V nominal. Even if the load can be powered by 208V, the single pole breaker's limitations govern.

You'd need a straight-rated breaker that's rated for 240V, without 120V being part of the rating, to be able to do this. One way, is to use a 2 or 3 pole breaker that is straight-rated, and not use the remaining pole(s). Another way, is to use a 480V panelboard, whose breakers would at minimum, be rated for 277V nominal to ground.

There's very little practical need to do this, since most loads that can work with 208V, are also built to handle 240V. Either by configuring jumpers on a device like a motor, or with a device that automatically adjusts, like a DC power supply.

 

[electrofelon]

I think there is excessive drama about this. last year i ran a load off the 208 high leg. It was a single phase machine that needed 220 nominal. Tons of drives, i looked at the data plates and 208 was a better fit than 240+. Also its expecting line to neutral, so i didnt want to risk giving it two hots. Just used a 3 pole breaker. Its fine.