[Guy]

As usual , Richard has put down bread crumbs for you too follow. He is probably spot on (as usual). The coil on the side of the contactor that is connecting you to street power needs an input to tell it to energize when street power is available. Keep looking for the one wire that is different on your new contactor, from what you see on your old contactor. It will probable be 12 0r 14 gauge.  If you are totally confused, an industrial electrician will solve your problem in a flash. Not a good term to use.