I'm looking to install a ductless mini-split system to cool some computer equipment in a building where the main A/C is turned off when unoccupied. It's a 9000 BTU heat pump, electrically rated at 9 amps RLA at 208/230V (1 phase), minimum circuit ampacity 15 amps. The electrical panel most convenient to the mini-split is a Square-D 3-phase 120/240V delta panel which has no open 2-pole 240V spaces. However, there are several single-pole 208V 'wild leg' spaces open. What should I look out for if wiring this mini-split to a single pole 208V breaker/circuit?
Primary issue: there are no 240V straight rated single pole breakers in the QO line
Your first problem is that you can't just use a single pole breaker for this job, as all single pole QO(B) breakers are slash rated 120/240V, which means they are only suitable for 120V-to-ground circuits, not the 208V-to-ground you find on a high leg. This can be overcome, but only by "pairing" the high leg in question with a non-high-leg circuit of the same size and using a QO215H (2-pole, 240V straight rated) or QO315 (3-pole) for the breaker. (If you have a NQ or NOxx panelboard instead of a QO loadcenter, you may need to order the QOB versions of the aforementioned breakers.)
Secondary issue -- the local disconnect
Since you have a single hot and a neutral, instead of two hots, you'll have to choose and wire the local disconnecting means more carefully than normal. In particular, the disconnect will need to be 240V straight rated; this is something you'll have to check actual labels to figure out, as information online is often inconsistent on these devices. You'll also need to use a wirenut to splice the neutral through the disconnect, as the neutral bars on pullouts can't be isolated from the case.
Of course, if you want to play it safe, you can use a 240V rated general duty, non-fusible safety switch with the neutral isolated and a grounding kit fitted.