Basically you have two systems to think about. Fully rated systems would have every breaker rated at or above the available fault current. "series rated" systems are where systems have been tested and rated where the combination of breakers meets the fault current requirement.
You can not assume "series ratings". They must be listed by the manufacturer. All breakers won't series rate and series ratings don't cross manufacturers.
If you are depending solely on ratings to meet requirements and your system rating is "x" then you would need to check any breaker rated below "x" to assure it "series rates".
For example, a two pole 100 amp breaker might not series rate with your main where a 3 pole 20 might.
Last edited by augie47; 12-11-09 at 02:36 PM.
At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.