Backfeeding with a Generator is Dangerous.
Backfeeding a house, an apartment, or business is a dangerous practice.
People who backfeed their houses connect the generator to an outlet with a heavy power cord. This allows electrical power to enter the main panel through the outlet’s branch circuit breaker. The panel distributes the power to the rest of the house through the other branch circuit breakers. It also feeds power out through the main breaker to the utility company’s transformer, which then converts it back to thousand of volts and attempts to energize all neighborhood utility lines.
Energizing the utility lines in this fashion is dangerous and illegal. Line electrical workers, tree trimmers and damage assessment focusing to restore power to the neighborhoods may unexpectedly encounter high voltage on the utility lines and suffer a fatal shock.
If the main breaker is turned off, it alleviates the hazard, but that requires remembering to shut the main breaker off, and relies solely on the user-customer to do so. There is no guarantee that someone else will not turn the main breaker back on and suddenly energize the neighborhood wires.
There is no safeguard in this arrangement.
Please make sure your main breaker is open before using your generator.