Welcome to ECN; another 'Jersey Guy'!!
OK, as Redsy said, you have to provide an "ESI" form to PSE&G for requesting electrical service.
That said, some of your info above don't 'register' right with me.
"I'm dealing with a 3phase, ungrounded, system. The owner has a private submeter and wants to install a PSE&G meter. He says his meter shows about 800 to 2000 kwh per month. He currently has a 200A, 3phase service, but PSE tells him that with those numbers he's giving them, he will need a 2500A Service! Can someone explaing their method of determing service required? Thanks, Walter"
Where is the electric service coming from now? A 'private submeter' has to be coming from a utility source, unless there is a private source of energy.
Perhaps, based on the address/block-lot you gave PSE&G, there is an existing PSE&G service and meter?? Maybe your client wants to 'add a meter' to an existing service??
Based on "3phase, ungrounded, system.", what is the voltage? 230 volt, 3 phase, 3 wire? 460 volt, 3 phase, 3 wire? Sounds like an old 'factory district'.
PS office is in Clifton for your area; marketing is in Secaucus last I heard.