Re: ATS question.
If the contacts are rated for 480 volts at 100 amps, there is no problem using the unit for a lesser load, assuming that physical size is not a problem, and you can trip the switch properly.
If the switch is automatic, what is the voltage rating on the relay coil (activating mechanism)? If it happens to be a 120 volt coil, then you are "golden." If the coil requires 480 volts to automatically switch from one source to the other, then 120 volts may not be enough to activate it, or it may hang partially activated.
If the coil is marked 480 volts, and you knew someone with a Variac that could dial up or down AC voltages, you could find the actual activate and drop-out voltages. Or just test it with a lead cord, and if it trips check for resistance across the contacts. If 120 volts will not trip the switch, it still might be possible to use it with a small (as in low current) step-up transformer to drive the coil when the alternate power is present.
In any event, you want to be sure that the contacts trip firmly, and are not just lightly touching while being held back by the return spring due to low coil voltage. The loose contacts would be a high resistance point, and a very heavy current draw would result in heat, possible burned contacts, and in the worst case a fire.
Someone said "Making good decisions comes from experience, experience comes from bad decisions." I say there are three kinds of people: those who learn from their mistakes, those who learn from the mistakes of others, and those who never learn.