A stock 240V ATS wired up for 120V would likely detect incoming power as either a failed phase or improper voltage (depending on how you wired it) and fail to operate properly. Would be a trivial thing for the manufacturer to correct with software- and who knows, maybe they did? It might be as easy as flipping a DIP switch on the controller board. A lot of transfer switches are good at a variety of voltages, with the "acceptable" voltages easily adjustible through a wide range, so a higher voltage single-phase ATS might be adjustable down to 120V.
If you can't tell from online documentation of the commercially availible switches whether it will operate at 120V, you'd have to contact the manufacturer(s). Usually, when ordering an ATS or generator, I'll simply give the manufacturer's sales rep my specs and let them figure out what parts I need. Half the time, they come up with other parts & options I'd never have thought of, or point out flaws in my plan, like "oh, by the way, those 200kW generators you asked for are the budget model and don't parallel. Want us to quote you one that does?"
Last edited by SteveFehr; 12/03/07 09:01 AM.