I am replacing a wall unit AC for my parents that was running on a 220 line, 18,000 BTU standard three prong plug. I have purchased a new unit that is 25,200 BTU 220 Volt 20 Amp Dedicated Circuit (Perpendicular Plug). I wasn ot aware of the plug diffrence. Do I need to return this or can I somehow alter their outlet receptical to accept this "Perpendicular Plug". I have been looking all over and there is no information anywhere! I was trying to do something nice and I am so confused and frustrated! Please help!

You say the old unit had a "standard three prong plug". You don't mean a "standard" 120v plug do you?

What did the prongs look like on the old plug/receptacle?

A tandem plug...

a tandem plug...

I'm sorry but that tells me nothing.
Describe the holes in the receptacle, or better yet post a pic.

I'm sorry...I'm really bad at this. I appreciate your help but I returned it for one that had a matching cord to receptacle. It was just easier! Thank you anyway - you've been very kind.

A plug chart is here. http://www.stayonline.com/reference-nema-straight-blade.aspx. What your probably looking at is a NENA 6-20 configuration.

Note that one of the female sockets loots like a "T". This allows 15 A and 20 A 240 plugs to be inserted.

12 AWG wire should have been run for 20 Amps.

The blade configurations don't allow a 120 V stuff to be plugged into a 240 V outlet. The major difference between a 20 A 120 V and a 20 A 240 V is which side of the outlet the blade is horizontal.

Usually 240 V receptacles have a single outlet. They are not usually duplex receptacle in the US.

Now, does this help answer your question?

Since, we are not exactly sure what it is.

You also DO NOT want to use a Tandem breaker. I.e. Circuit Breakers - Tandem Breakers (http://electrical.about.com/od/panelsdistribution/a/tandembreakers.htm)

You want a 2-pole single handle or two interlocked handles.