I have been asked to provide an automatic transfer switch for a portable generator. Specifically a 8 circuit, 100A (rated) Generac ATS. The purpose being , that during a power outage the customer could plug their portable generator into a 30A inlet which is wired to this ATS, they would manually start their generator (located outside)and the transfer switch (located in basement next to panel) would automatically sense emergency power and transfer his 6 EM circuits on. This eliminates the customer from having to start his generator then walk thru his dark home down to the basement and manually switch his emergency breakers on. SO here lies my problem/ question. The ATS requires a 24 volt relay to fire/ energize in order to actuate the transfer switch. The 24 Volts is supposed to be provided by a stand alone generator that the customer does not have. He has this portable gen. So why could he not install a 120 volt primary to 24 volt secondary transformer inside his ATS, use the 120 volt power provided by his portable gen to power this transformer which will then power the 24V transfer relay and switch his emer circuits on. Besides voiding the manufacturers warranty for changing/ altering the product, can anyone think of any other code or safety reason why this can not be done?