How can I increase the size of a subpanel to fit the breaker for a solar array?

Question

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?

Thanks!

Answer 1

That is correct. The 100A breaker in the main panel protects the wiring to the subpanel, and the subpanel itself. The "main" breaker in the subpanel is nothing but a shutoff switch (unless it is also a GFCI or something, as in the case of a hot tub panel).

The subpanel must be large enough to be protected by either its own main breaker (if it even has one) or the supply breaker in the main panel. In your case, a 60A subpanel would be wrong, but a 225A subpanel would be right.

The usual blunder people make is thinking the subpanel size must match the feed breaker, and thus they limit themselves to a panel with too few spaces.

So go for broke. 42 spaces is not too many. (Spaces are not circuits; a 40 “circuit" panel is often a 20-space). Good sign when spaces are a multiple of six, that indicates a panel of industrial quality (also sold in 3-phase).

Answer 2

Multiple feeds to a panel require care

When a panel is multiply fed (like the subpanel in your case), it's possible to overload the busbars on the panel even though no breakers are tripping. If you try to stick the two feeding breakers next to each other at one end, the results are problematic because then the current sum at the point in the panel just below them can exceed the busbar ampacity limits without causing any breaker to trip.

Avoiding this is simple though: just put the inverter breaker at the opposite end of the subpanel from the feeder shutoff breaker. This means that even if the panel is loaded to the maximum both feeders (utility and inverter) can provide, at no point will the sum of the provided currents exceed the busbar limits on your panel (in fact, it's the ideal scenario for current flow through the busbars -- the "up" current and the "down" current basically "meet in the middle" somewhere).