You can buy from most Generator Companys a (looks like a small box car) with a trasfer switch,generator & main all in it.The way we used to do it at Casinos (they Do NOT want to have dn time period).Buy one of theses self contained units run a new feed from Secondary side of Transformer & then from Unit to Switchgear. Then (most time there was room for extra conduits) , We'd even pull wire & be ready to terminate ,We would shut dn building pull off existing terms both ends put on new ones . Pull out existing wires when after gear was engerized.(with 70-E now that could be a problem).Question you say you have a 3 phase 4 vwire 120/240 that must be a Delta thus a high leg right. Another note you can have a transfer switch ahead of a main IF its a Service rated transfer switch.
