A generator that powers a house needs a way to connect that keeps utility power and generator power completely separate. That is what a transfer switch does, and it is not an optional accessory.
The reason is straightforward and serious. If a generator is connected to house wiring while the utility connection remains intact, the generator’s output travels backwards through the service and into the utility transformer, where it is stepped up to distribution voltage. Line workers repairing what they believe to be a dead line encounter it. This is the origin of the practice being illegal essentially everywhere.
There is a second reason that matters to the homeowner directly. If utility power returns while the generator is running and connected, the two sources are out of phase and the generator is generally destroyed instantly.
A transfer switch makes the two connections mechanically exclusive. It is physically impossible for both to be closed at once.
There are three common arrangements.
A manual transfer switch with a set of selected circuits. A small panel is installed beside the main, a handful of important circuits are moved into it, and a switch selects utility or generator for those circuits. This is the most common approach for portable generators and the cheapest.
An interlock kit on the main panel. A sliding metal plate prevents the main breaker and a generator inlet breaker from being on simultaneously. This lets any circuit be powered, with the user managing load manually by switching off what the generator cannot carry. It requires an interlock listed for that specific panel.
An automatic transfer switch, usually paired with a permanently installed standby generator. It detects loss of utility, starts the generator, transfers load, and reverses the process when utility returns.
Sizing matters more than people expect, particularly with motor loads. A well pump or an air conditioner draws several times its running current at startup, and a generator that comfortably handles the running load may stall on the start.
Inlet placement, cord rating, and generator grounding all form part of a correct installation.
A useful reference: found this on Dotnetcloud
