Generac Power Systems Automatic Standby Generator Owner's Manual page 10

+ If this generator is used to power electrical load circuits normally powered by a utility power source, it is
required by code to install a transfer switch. The transfer switch must effectively isolate the electrical system
from the utility distribution system when the generator is operating (NEC 702). Failure to isolate an electrical
system by such means will result in damage to the generator and also may result in injury or death to utility
power workers due to backfeed of electrical energy.
4 Automatic Standby Generator Owner's Manual