We have a small electrical subpanel with only 12 breaker slots. It's rated 125A, which is enough for all the circuits we want, but we will probably run out of slots when adding solar, induction range, electric dryer, heat pump water heater, etc. We can mitigate this to a large extend by using half-width breakers in quad arrangements, but will probably still run 1-2 slots short.
This subpanel also has a main breaker slot that is unused (the subpanel is protected by a breaker further upstream). So my question is, could we put a breaker in the main breaker slot and use it to feed a normal circuit or accept power from the solar array? We would also label it as "Range" or "Solar (not Main)" or something similar.
My guess is that this is not OK due to potential confusion, but I wonder what the code says about it? Electrically it seems like it should work fine.
UPDATE: for those who asked what model of panel this is: It's a Murray LW1224L1125 Model 2 outdoor load center. I've also attached a photo of the existing circuit breaker setup and subpanel label.
UPDATE 2: I've attached a photo of what I think is the same model of load center from here. As @jay613 pointed out in the comments, it looks like I made a faulty assumption that the subpanel had both lugs to connect the supply directly to the bus and also clips to attach a main breaker to, like the others. From the photo, it looks like you can only use one or the other approach, not both, so there's no way to add an additional breaker in the main breaker slot on this box.