Just remember, the interlock is the legitimate way to prevent back feeding the utility co service from your generator. To accomplish that, it must be in the main panel. Not in the sub. The point of that interlock is to make it impossible to back feed the utility. That only happens when it's is the right spot (main) panel.
If you put it in the sub panel, your generator can power the garage (everything on the sub). But a generator on an interlock on a sub can't power the main panel upstream.
Gotcha. I understand the first part. I don't think I need an interlock on the sub panel. The sub panel has a 100a (probably redundant) that I can open and lock somehow to power the workshop and prevent feeding back to the house main panel. I saw a "snap on" lock but haven't found one for a main breaker. I'll have to search for that again if that idea makes sense. I understand the concept of interlocking the breakers. I'm hoping that won't be needed.
So let me try to summarize what I currently have and then what I hope to do.
First, so your head isn't spinning wondering how I plan to feed my home panel off grid... This scenario involves powering the workshop and home with my off grid solar/inverter/battery system which is in the process of being built. So far it's a 1.8kw solar array (to be enlarged to 3kw this summer), 6.5kw off grid inverter (two phase) and 32kWh battery bank. We conserve, it's in my blood, so I'm not concerned about it's size. Yes the battery bank is large. I hope to have plenty of cushion for weather challenges. The entire solar system will handle all or most of our daily needs except possibly in mid November through January or early Feb.
Our main panel is a 200a service. At the moment there are two SP breakers just under and on the right leg of the 200a home panel. I can move those down to another location fairly easily to make room for a Gen DP breaker (amperage tbd) with an interlock. However, if I want to power my home main panel without feeding back into the grid I can just open the main 200a breaker and lock it open; again with a "snap on" locking device. Then the 100a breaker which was feeding the sub panel can back feed to the main without connection to the grid.
Adding an interlock on the 200a main breaker and locking the 100a feed to the workshop would mean I couldn't power the workshop with the grid. Leaving the 100a where it is and adding a separate Gen breaker in the interlock position would be complicated.
If anyone is concerned how the solar feeds works check out some youtube videos about off grid solar "backup" systems.
Here's an example
