Thank you for your inquiry!
The RDT-CFNA-0200A will be compatible with the 20RESA although this is not the most common installation. This configuration will require that the RDC2 controller (found in the generator) be disabled causing the system control to be performed by the MPAC 500 controller (located in the transfer switch).
We would strongly recommend utilizing the services of a certified Kohler installer to perform this configuration.