This is my panel.

https://www.amazon.com/Siemens-Circu.../dp/B005GLDS8O

I am concerned about capacity of the panel. I have a dedicated 12/2 at the panel that I am thinking of using for a 230V/15A air compressor. The compressor is 115V/15A but can be rewired for 230V. If I can add this do I use a 15 or 20A double pole breaker? The reason why I ask this is because the compressor has a surge current on start up that might exceed 15A.
I would also want to use the remaining slot for another 20A circuit. Would I be within capacity of the panel?

 

You just ran this 12/2 yourself, or you just "found" it? At any rate, if it's 12 gauge for the entire run, then put it on a 20A double pole breaker.

By the way, at least some of those single pole breakers can be swapped out for cheaters like you have in the other slots, giving you additional circuits.

 

I ran it, dedicated to the panel. So at 240V is it pulling less amps?
As far as additional circuits isn't there a limit to how many there can be on the panel?

 

Sure, a given panel might not accept those cheaters in all slots. But I see you've already got one in the second from the bottom row, so I assume the ones above it are good to go. You should make sure your service supports the total load, but you can't just eyeball things or count up breakers and say that it "looks full". Plus in real life you're not going to use many of these power toys at the same time. The 200A you have now is pretty beefy.

As for your other question, I didn't realize you were asking about whether you should wire the compressor for 240V or not. My knowledge of compressors is limited to my small 21 gal 120V one in the garage, so I don't know if they behave differently at 240V. I'm interested to know if they fill the tank faster, or just use the same wattage at lower current draw.

 

Quite a busy panel. You may be at or maxed on duplex breakers. Quantity and where they can be used in the panel is posted right on the door. It's a wiring diagram. You can post it if you don't understand it.

Running a heavy draw item, like your compressor, won't use any less power on 240v but connecting it that way will balance the load to your panel increasing motor starting efficiency and reducing lights dimming on startup.

 

Did you use gray UF-B cable or the yellow NM-B cable? Yes, when running on 240 volts the amperage drops in half, but you have that current draw on 2 poles instead of just 1 pole.