Sounds like 220v 2-phase.  Two hots and one neutral/ground.  If its
marked 20A, I believe the connector is probably an L6-20 twist-lok.
Not difficult to wire up in most US homes.
  My computer room is running from two APC Matrix 5000 5kva UPSs,
which have 220v inputs and 110/220v outputs.  I ran two 30A 220V
(L6-30 twist-lok) circuits from the two outer poles on my breaker
panel to power the UPSs, and everything (be it 110v like the regular
Alphas/Suns/SGIs and such or 220v for the Crays) runs from the UPSs.
  Having the equipment run from dedicated 220v 2-pole breakers is
very, very nice.  Definitely worth the trouble.
       -Dave McGuire
On October 31, Merle K. Peirce wrote:
  I just checked ours - it's 3 prong.
 On Tue, 31 Oct 2000, Dave McGuire wrote:
  On October 31, THETechnoid(a)home.com wrote:
  Last weekend I aquired a system/36 model 5360.
Have not powered it as
 yet.  The plug does not fit either of my dryer sockets but is marked
 250vac, 20amp.  Is this a three-phase machine?  If so I think I am in
 trouble..... ;-) 
   Does the plug have three prongs or four?
             -Dave McGuire
  
 M. K. Peirce
 Rhode Island Computer Museum, Inc.
 215 Shady Lea Road,
 North Kingstown, RI 02852
 "Casta est qui nemo rogavit."
               - Ovid