Can a 200 amp automatic transfer switch be used with a 100 amp service?

Answer for USA, Canada and countries running a 60 Hz supply service.

No. The ampacity of the transfer switch is rated at 100 amps. What this means is that the manufacturer of the switch states that the mains, breakers installed are all rated for a maximum of 100 amps. What can be done, is to install a 100 amp breaker in the 200 amp service and use the output of the breaker to feed into the line side of the automatic transfer switch. The load side of the ATS will be a sub panel that have your preferred loads that you want to stay on during a power failure.

This said, it is a complicated job in as much as you have to know wire sizes, ampacity of wires and proper electrical workmanship. Such a project should be left to a licensed electrical contractor to take out the proper permits and call for proper inspections. By taking this route it will leave you confident that if any mishaps happen to the installation down the road your insurance company will be behind you 100%.

As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.

Before you do any work yourself,

on electrical circuits, equipment or appliances,

always use a test meter to ensure the circuit is, in fact, de-energized.

IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB

SAFELY AND COMPETENTLY

REFER THIS WORK TO QUALIFIED PROFESSIONALS.