Upgrading Shore Power to 50A 220V

[nopeda]

I've been on 30A 120V power at the dock for years and would now like to upgrade to 50A 220V, or what they refer to as 50A 250V at the marina office. I haven't put a meter on it yet but so far suspect it's closer to 220 than 250. The socket on the pedestal is like the one pictured here:

Layout 1

They say it's 3-pole 4-wire. I'm guessing X and Y are the two hot legs and W is the neutral. Between X or Y and W would be 120V+/- and between X and Y is the 220/250V. Right? How does the ground tie into it though? With only 3 poles do we run a separate wire from the boat to the pedestal, and if so how do we tie in?

Also can anyone suggest the cheapest place to get a plug like that? So far the best price I've found is $68.18 here:

Cooper Wiring Devices 63CR65 Plug CR 50A 125/250V 3P4W H/L Armored

Thank you for any help!
David

[awpptdt]

That may be the best price you will find as they are real prowd of them. I converted my 30amp to a 220 50 amp. Why, I added a second a/c unit and that pushed the load over the limit. So I installed a master breaker that disconnects the two hots and the neutral. The plug actually has four wires, the two hots, the neutral and the ground. The ground connects thru the metal housing where as the other three use the plugs. I also installed two sub panels one for each of the two 120 legs. There are no 220 devices on my boat so I split the 120 loads between the two 120 sub panels. Good luck, J.T.Duncan