http://www.leviton.com/sections/techsupp/nema.htm

This shows all of the standard receptacles and plugs.

They are designed so that, with two exceptions, the plugs will only fit into the corresponding rectacle of the same voltage and current rating.

The excepts are that 15 amp/120v plugs can be also plugged into 20 amp/120v receptacles. And likewise 15amp/240v plugs can 20amp/240v receptacles.

In general the rectpacle is matched to the voltage and current rating of the circuit.

Note the different series of 240 volt receptacles.

The 6-xx is for 240 volts ONLY and does not have a neutral.

The 10-xx series has a neutral and is for 240/120 volt loads. However, it does not have a GROUND. It is used for older dryer and stove installations.

New dryer and stove circuits will have 4 wires (2 hots, neutral, and ground) and will use the 14-xx series.

A 240v window AC would use the 6-xx series.

Actually after I did a lil thinking and looked at the different receptacles I think it's all about the amps. I noticed the smaller 230v plugs are 15amp and 20amp and I CAN use smaller wire gauge. The 30amp larger receptacles get the larger gauge wire.