This interlock kit fits Square D HOMELINE 40/80 panels having a top-mounted, full width (spans both columns of branch breakers) horizontal-throw main breaker with approx. 1 3/8" spacing from the bottom of the main breaker to the top of the branch breakers.
 
The interlock will toggle between these two sates:
 
1. The gen breaker is locked out and either one or both the main breaker and solar breaker may be turned on or off.
 
2. The main AND solar breakers are BOT locked out and the gen breaker may be turned on.