installing a power switch between Nema 6-15 device and 6-15 outlet

The type, the shape of the switch is not important as long as you respect few things:
- to be closed in a case or in wall, somehow that nobody will touch the wires when they have electrical energy in them. If it is a box then make sure is isolated and the wires or the screws of the switch do not touch any metal part in case the box is metallic.
- You have to know the voltage and the current of the air conditioner.
-The switch must be minimum the same current and voltage as the air conditioner. That is the most important part. Higher rated current is even better.
- If the switch is double, then is even better, meaning to interrupt the hot and the neutral.
- Make sure the wires between switch and air conditioner or its socket have the proper size. For that you need to know maximum current of the air conditioner.

@viki2000,

There is no neutral in the 240V.  Two hot legs.  Not sure why anyone would switch a neutral wire...it's a dangerous practice from people installing lights on the cheap.  The neutral provides a path back to your panel, and shouldn't be broken.  Otherwise, stray current will seek an opportunistic (uncontrolled) path to ground.

For motors, you can't look at the running rating.  The inrush could be 2x the normal load.  The circuit breaker and any disconnect ("switch") has to accomodate the much larger startup load, or risk meltdown or welding the contacts in the "on" position.

That's what I thought, the switch only closes/opens one hot. And the motor control switch and box type switch seem not the one I want to go further to it.
I found this, 6-15 power strip, it costs $120!!!! but that's the only one I could find. Will this work?
http://internationalconfig.com/icc6.asp?item=56514

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The other option I can do is, replacing the power plug and outlet to some standard types as you mentioned in another post so that I can have more choice of power strips and switches I can install. What standard type plug and outlet can replace this 6-15?

I forgot that is USA. In Europe there is 230V and many times 240V for hot and neutral. The 2 phase give 380-400V.
I forgot that in USA is 110-120V between phase and neutral and when we speak about 240V then are 2 phases.
We should not worry about interrupting the neutral to the load as long as we have the ground wire.
Anyway, in case of 2 hots, then is clear we have to interrupt both and a double switch is needed.
For industrial applications are all kind of switches and circuit breakers, different shapes and sizes.
To recommend one we need to know the current, voltage, power of the air conditioner, The inrush current will be good to be known, but  think we ask too much.
The Square D catalog is a good one, but are many other brands as Telemecanique, Siemens, ABB, Klockner Moeller, Omron...and more.

aircontioner is 240v  ac, 15amp

Double poles refer to 2 hots, what does it mean single phase?
And why does it have to be motorized switch? Just curious

Good find viki2000.  Much cleaner than the external box or disconnect box.

The 6-15R should have a 1-1/2hp rating.  So a switch with 2hp rating should work.

Here is the dimension, wiring and datasheet info, bottom of the page:
http://www.leviton.com/OA_HTML/ProductDetail.jsp?partnumber=3032-2W§ion=41455&minisite=10251
http://www.leviton.com/OA_HTML/ProductDetail.jsp?partnumber=MS302-DS§ion=40132&minisite=10251

What do you mean by "motorized", where did you see that?
"Motorized" general meaning is that there is a motor as actuator, but here is not the case.
If you look at the title of the product from Amazon where is mentioned " Single Phase AC Motor Starter, Suitable as Motor Disconnect" then:
- the word Motor there means is suitable to start an electrical motor as the one in your air conditioner, as the one from the pump and the fan inside the air conditioner units.
- Single Phase means here that the motor uses 2 wires, no matter if is phase-neutral for 120VAC or phase-phase for 240VAC. The switch will interrupt the both wires.
Other types are the common 3 phase systems or multiple phase systems, special designs.

Thanks. So I drew wiring diagram that I think. I haven't opened the outlet box yet, so don't know what wires available, but imagine something like this. So I don't wire neutral, only two hots and ground from source to switch, then to outlet. Can you take a look and let me know?

If you look in the "Instruction sheet" from next page at bottom:
http://www.leviton.com/OA_HTML/ProductDetail.jsp?partnumber=MS302-DS§ion=40132&minisite=10251

then there is a pdf file and on the 1st page we see next wiring diagram:

Thank you very much. I guess I have to open it to see how it's wired actually.
This helped me a lot. I always wanted to work on electrical wires in my house and work place, this will be my first wiring project. :)
THANKS!

Sorry, I had to choose only one best answers, was switching around answers of you both  to decide. This really doesn't mean any, both answers were great.

Wiring diagram:

Do you think we are finished here?
Is not about the points, it is about your life and your house.
Hold on a moment, if this is your first time working with electricity home then you have to respect some rules.
I'll come back in a moment.

OK. thanks.
For wires, I guess I choose 30amp 10 guage to be safe.

Romex 10 guage
http://www.homedepot.com/p/Southwire-10-3-NM-B-W-G-By-the-Foot-63948499/204724935

1) Shut off the electricity in your house or at least the branch with the the electricity where you work, the circuit breaker which powers the wires to your future switch for air conditioner.
NOW, make sure you lock or at least put a big sign on a big paper to the power panel where the circuit breaker or the fuses are turned off. Somebody may come and without any bad intention, will power up the line where you work and maybe will not be able anymore next time to ask questions here and you will not finish your project. Very important. I know enough people with experience that died or were very close to die due to such small mistake. So lock or big warning, if it is somebody else in the house.
2) The wire size.
At 15A you need minimum 10AWG, better 8AWG, because you do not want your wires hot, especially during the summer time.
Have a look at some tables below;
https://www.eol.ucar.edu/rtf/facilities/isff/LOCAL_access_only/Wire_Size.htm
http://www.keywolf.com/American_Wire_Gauge.php
3) The circuit breaker.
The circuit breaker has to be dimensioned in such way that will permit to your air conditioner to work in its normal operational state and also to avoid tripping off  when you start up the air conditioner due to inrush current.
The circuit breaker does not protect your air conditioner. The air conditioner has its own fuses or circuit breaker or thermal/magnetic overload protection.
The circuit breaker from your house, situated in power distribution panel, protects the wires, the cable between the power distribution panel where is circuit breaker is installed and the end of the cable, in your case the future switch.
If a short-circuit appears along the cable, the circuit breaker trips off avoiding the cable to be melted due to high currents.
The same if the short circuit will appear in your switch or the socket.
You have to make sure that in case of short-circuit the switch is not melted and the circuit breaker can protect it.
In other words, the thermal overload of the circuit  breaker should not be higher than 30A.
Remember 2 things: AWG8 goes up to 24A and AWG7 to 30A and the switch is max. 30A.
I do not know what wires and cable do you have installed, but you should check that.
I would use AWG8, then circuit breaker of 25A.
But if you already have installed AWG10 for 15A then you choose circuit breaker 16A or 20A max.
The 16A is at the limit of your air conditioner rated current and may trip off, but 20A will be  a bit too much for the AWG10 wires.
You have to avoid tripping the circuit breaker and running the wires hot.
These values in Amperes refer to RMS value of the current, the normal rated current.
When we speak about inrush current, which is a pulse of hundreds of microsecond to milliseconds, then we have to take in consideration the instantaneous tripping current, sometimes, depending by circuit breaker type may be the magnetic characteristic of the circuit breaker.
For normal home usage there is curve B or C or D, most common being B type:
http://en.wikipedia.org/wiki/Circuit_breaker
The B type has 3-5 x max. rated current of the circuit breaker. In other words, for your case if you have AWG10 wires and B16 circuit breaker, meaning 16A rated RMS and curve type B, then can deal with the inrush currents 48 up to 80A.
My first try would be with AWG10 and B16.
In case is tripping off right away due to inrush current then I would go with C16 and if are still problems then I would try with D16.
In case is tripping off after a while, the circuit breaker running too hot, then I would increase to 20A and I would watch the wires, cable to see if they are hot or not. In case they are warm to hot, then you must change them to a bigger size as AWG8 or AWG7.
If this is a new installation and you can put any wires sizes and any circuit breaker you want, so you are not dependent by any wires in the walls, then I would go from beginning with AWG9 and B20 circuit breaker or even a bit over dimensioned as AWG8 and B25.

THANK YOU VERY  MUCH

The switch specified will take up to 8AWG, but you'll have to crimp on ring terminals, which is not as easy and requires more parts.  The side screws or back stabs only hold up to 10AWG size.

Wiring straight to motor, there are fewer wires.  For what you're suggesting, you'll need to ground both boxes (switch & receptacle).  Use pigtails and a wire nut to connect everything to the incoming ground wire.  This switch may not have a ground screw.  If it does, make a pigtail into the wire nut.

So ground is needed for receptacle, got it.