It was mentioned in another thread that, in some areas, municipalities and some POCOs are requiring a 22kA rating on residential services.
I've read Sq D's and C-H's documents on series ratings, and their panels are specified and tested such that a 22kA or 25kA main breaker in series with a 10kA branch-circuit breaker has an overall 22kA series rating.
But, these series ratings don't extend to three in a row. If you have a subpanel fed from a 10kA feeder breaker, those 10kA breakers in the subpanel aren't covered in the listing for the series rating; they only have their native 10kA rating.
To get to 22kA, you need to do one of two things: Either use a 22kA breaker for the feeder, or use 22kA breakers in the subpanel.
What I'm wondering is, what are people actually doing on subpanels in the areas that are requiring 22kA residential equipment? Are you installing 22kA feeder breakers, or just assuming that the 22kA main covers the whole thing?
Or is my understanding of three-in-a-row series ratings incorrect?
I'm not aware of any maximum distance requirements between series rated devices, or a requirement to be directly adjacent in the distribution path. If there is a fault downstream of the second 10kAIC breaker, the current will come from the first 22kAIC rated breaker and act appropriately. Be sure there is no significant motor load downstream, because then the fault current downstream may be different than that seen upstream.