I am considering solar and the easiest place to connect the system for grid-tie is into a subpanel. With the 2013 CEC, this is permitted. However, I have a slight problem. I don't have any free space for the additional 2 (240VAC) breakers to be back fed by my micro inverters. Fortunately, swapping out the subpanel for a new one is fairly easy to do. However, I don't know the specifics of the code here.
The issue is this... The panel is a 100A sub and all work was done with permits. Installation of solar will also be with permits. The question is what total amperage as a function of breaker specifications can I add to this sub panel. The question is further clouded by the fact that the two new breakers would never be loads. They will always be sources and hence would, I image, deduct from the aggregate value.
To be honest, I don't really see why there should be any load calc here (and possibly there isn't... I really don't recall that nuance of the code). When installed, the breaker in the main panel that feeds the sub was installed and is 100A; the wire to the sub was gaged to carry that load. So, the 100A breaker ultimately is protecting the sub. Even if there were some 500A worth of breakers in the sub, the sub's breaker in the main panel will trip at 100A. That comment applies to loads only. The solar only "helps" here.
So, how can I increase the the size of a subpanel so that I will have space for the additional breakers?