Man I've been putting this off for too long...

I need to wire a compressor to panel, it's a mid-sized upright (45gal), I need to figure what wire size for a
run of either 12-15' or 20-25' depending on final location. This will be a temporary, so no romex.

Based on the motor specs, what size breaker and wire, and what type of wire? Wire will be running across
the floor of a small room to get through the wall to the compressor, though I could run in on the surface of
two short walls.

Motor:

208-230V
15 amp
SF 1
L.R. amps 93
Output 2.98 KW
Emerson definite purpose compressor motor.

What do i need here?

12 ga should work but I’d do 10ga. I like overkill when it comes to sizing wires and also like more circuits than needed. Lots of house/building fires start from overloaded wires.

20 amp breaker.

I wouldn’t leave it on the floor. Too much chance of damage. Code wise, that would be a big no no. But, even if you don’t care about codes I still recommend against it.

Silly question, 15 amps per side or 15 amps total?

It just says 15 amps on the motor. The label on the compressor says 3.5 amps running, but it's
older and that may inaccurate from the days when they lied about HP. (around 2006)

Any problem with stapling romex right to the wall then? I could hang EMT as well and pull, looking for the easy button here.

Temporary? I’m sure that would be fine. Technically Romex needs to be protected from damage, typically being inside walls, but it’s run like that all the time without issue. I have exposed Romex in the rafters of my shop and it’s fine.

You could run armored cable too. Your run isn’t long so wouldn’t be expensive.  I’d probably run 12 ga armored cable if it’s only a temp install. If it was permanent I’d probably run 10 ga just so I could have a bigger unit at a later time if I wanted to. Will it have a plug or hard wired? If hard wired then it should have a disconnect near the compressor. If a plug then the plug acts as the disconnect.

12 ga will be fine but as was said I'd go with 10 to handle any start-up surges.

Hard wired, I'll p/u a disconnect when I get the wire.

You could even do a motor rated DPDT switch.

12g is fine.

Personally I would bump it to 10g.

If the panel is in sight of the equipment then you likely don’t need an additional disconnect.

Update: running the compressor or 20A breaker and ~20' of 12/2 Romex temporarily; everything is running just fine.

Changed plan to moving unit into storeroom, which is really wasted space, and piping air out to garage area to save
a tiny bit of floorspace there as well. I'll update the wiring to a more permanent situation at that time.