Article 705 covers the interconnection of power production equipment (solar, wind, fuel cells, generators) with the utility grid, and the 120% rule from 705.12 is the one that every solar installer needs to have memorized. The rule: when backfeeding a panel via a solar breaker, the sum of the main breaker amperage plus the solar breaker amperage cannot exceed 120% of the panel’s busbar rating. On a 200A panel, your solar backfed breaker can be no more than 40A (200 x 1.20 = 240; 240 - 200 = 40).
Why 120%? The busbar is rated for the main breaker rating under the assumption that power flows in from the top and out through the branch circuits. When you backfeed solar through a breaker at the bottom of the panel, the busbar can see the sum of the main breaker plus the solar breaker. The 120% rule gives a safety margin.
Workarounds when the 120% rule limits your system size: use a line-side tap (connecting before the main breaker, per 705.12(B)), upsize the busbar or replace the panel, or use a subpanel for the solar interconnection. Line-side taps require coordination with the utility in some jurisdictions. Have your calculation ready and be able to explain it to the inspector.