Technical details and the social contract mandate that your generator is never connected to the main power grid. The generator should be wired to an enclosed AC transfer switch. This switch will connect either the generator or the main grid to your home, but never both.

Some detail: If the generator is wired to the main grid it can prevent restoration of main grid power. While an AC transfer switch will perform the task, many jurisdictions mandate additional safety precautions (which can be quite expensive).